On the variability of quasars a link between Eddington ratio and optical variability
a r X i v :0711.4844v 1 [a s t r o -p h ] 29 N o v 2007
Mon.Not.R.Astron.Soc.000,1–??(2006)Printed 2February 2008
(MN L A T E X style ?le v2.2)
On the variability of quasars:A link between Eddington
ratio and optical variability?
Brian C.Wilhite 1,2?,Robert J.Brunner 1,2,Catherine J.Grier 1,Donald P.
Schneider 3,and Daniel E.Vanden Berk 3
1
The University of Illinois,Department of Astronomy,1002W.Green St.,Urbana,IL 61801USA
2National
Center for Supercomputing Applications,1205W.Clark St.,Urbana,IL 61801USA
3The Pennsylvania State University,Department of Astronomy and Astrophysics,525Davey Lab,University Park,PA 16802
2February 2008
ABSTRACT
Repeat scans by the Sloan Digital Sky Survey (SDSS)of a 278-deg 2stripe along the Celestial equator have yielded an average of over 10observations each for nearly 8,000spectroscopically con?rmed quasars.Over 2500of these quasars are in the red-shift range such that the C iv λ1549emission line is visible in the SDSS spectrum.Utilising the width of these C iv lines and the luminosity of the nearby continuum,we estimate black hole masses for these objects.In an e?ort to isolate the e?ects of black hole mass and luminosity on the photometric variability of our dataset,we create several subsamples by binning in these two physical parameters.By comparing the ensemble structure functions of the quasars in these bins,we are able to reproduce the well-known anticorrelation between luminosity and variability,now showing that this anticorrelation is independent of the black hole mass.In addition,we ?nd a correlation between variability and the mass of the central black hole.By combining these two relations,we identify the Eddington ratio as a possible driver of quasar variability,most likely due to di?erences in accretion e?ciency.
Key words:galaxies:active –quasars:general –techniques:photometric
1INTRODUCTION
The luminosities of quasars and other active galactic nuclei (AGN)have been observed to vary on time-scales from hours to decades,and from X-ray to radio wavelengths.The majority of quasars exhibit continuum variability on the order of 20per cent on time-scales of months to years (e.g.,Hook,McMahon,Boyle,&Irwin 1994;Vanden Berk et al.2004).In fact,variability has long been used as a selection criterion in creating quasar samples from photometric data (e.g.,Koo,Kron,&Cudworth 1986;Ivezi′c et al.2004;Rengstorf et al.2004).Many simple correlations between photometric variability and various physical parameters have been known for decades.These relationships are summarised by Helfand et al.(2001)and Giveon et al.(1999).Numerous studies (e.g.,Hawkins 2002;de Vries,Becker,&White 2003)have shown variability to correlate with time lag.Anti-correlations have been found between vari-ability and luminosity (e.g.,Uomoto,Wills,&Wills 1976;
?
E-mail:wilhite@https://www.wendangku.net/doc/0016281203.html, (BCW)
Cristiani,Trentini,La Franca,&Andreani 1997)and wave-length (e.g.,Giveon et al.1999;Tr`e vese,Kron,&Bunone 2001).Vanden Berk et al.(2004,hereafter VB04),using a sample of ～25,000quasars,con?rmed these known correlations,and parametrized relationships between vari-ability and time lag,luminosity,rest-frame wavelength and redshift.
Our understanding of the physics of the central black hole in quasars and active galactic nuclei (AGN)has long been tied to the variability of the quasar’s luminosity.Intra-day variability in X-ray and optical light (see,e.g.,Terrell 1967;Kinman 1968;Boller et al.1997)point towards a com-pact object,speci?cally a supermassive black hole,at the centre of an AGN.More recently,reverberation mapping techniques have been used to determine the radius of the broad line region and,indirectly,to measure the mass of the central black hole (Blandford &McKee 1982;Peterson 1993;Kaspi et al.2000).
Recently,Wold,Brotherton,&Shang (2007),presented evidence suggesting that the photometric variability of quasars is linked to the mass of the central black hole.Strength of variability for ～100quasars was approximated
c
2006RAS
2 B.C.Wilhite et al.
by?nding the greatest single-epoch R-band deviation from the mean,by using optical light curves from the QUEST1 survey(Rengstorf et al.2004).Black hole masses were es-timated from Sloan Digital Sky Survey(SDSS;York et al. 2000)spectra using the5100-?A continuum luminosity and Hβline width,as calibrated by Vestergaard&Peterson (2006).However,while a clear correlation between black hole mass and variability was found for time-scales greater than 100days,Wold,Brotherton,&Shang(2007)were unable to reproduce the well-known inverse relationship between lu-minosity and variability for their sample as a whole.This is likely due to the redshifting of the blue(and more variable) portion of the spectrum into the R-band at higher redshifts; this causes the high-luminosity quasars visible at higher red-shifts to appear more variable than one might expect,based on previous published results,such as Vanden Berk et al. (2004).With so few objects,it is di?cult to truly isolate the dependence of variability upon black hole mass,given the correlation of mass with luminosity,which is in turn tied to redshift and wavelength.Wold,Brotherton,&Shang(2007) do report a correlation between black hole mass and variabil-ity at constant luminosity;these intriguing results might be more convincing,however,if the variability-luminosity de-pendence were in line with expectations,or if the sample were larger.
With this in mind,we have examined the variability properties of a much larger sample of quasars from the Equa-torial Stripe(see§2.2)of the SDSS.With this sample,we are able to reproduce all of the well-established dependences of photometric variability,including the inverse correlation with luminosity,as well as the recently measured correlation with black hole mass.We brie?y describe the quasar sample in§2and the statistics used to measure variability in§3. In§4,we describe the methods used to estimate black hole masses,including the continuum-and line-?tting techniques, as well as the dependence of variability on black hole mass and luminosity.Finally,we interpret our results in terms of physical models for quasars in§5,and we conclude in§6.
Throughout the paper we assume the standard concor-dance cosmology with parameter values?Λ=0.7,?M= 0.3,and H0=70km s?1Mpc?1,consistent with Spergel et al.(2006).
2THE QUASAR DATASET
2.1The Sloan Digital Sky Survey
The Sloan Digital Sky Survey(York et al.2000)is de-signed to image～10,000deg2and obtained follow-up spectra for roughly106galaxies and105quasars.All imaging and spectroscopic observations are made with a dedicated 2.5-metre telescope(Gunn et al.2006)at the Apache Point Observatory in the Sacramento Moun-tains of New Mexico.Imaging data are acquired by a 54-CCD drift-scan camera(Gunn et al.1998)equipped with the SDSS u,g,r,i and z?lters(Fukugita et al. 1996);the data are processed by the PHOTO soft-ware pipeline(Lupton et al.2001).The photometric sys-tem is normalised such that SDSS magnitudes are on the AB system(Smith et al.2002;Lupton,Gunn,&Szalay 1999).A0.5-metre telescope monitors site photometricity and extinction(Hogg,Finkbeiner,Schlegel,&Gunn2001; Tucker et al.2006).Point source astrometry for the survey is accurate to less than100milliarcseconds(Pier et al.2003). Ivezi′c et al.(2004)discuss imaging quality control.
Objects are targeted for follow-up spectroscopy as can-didate galaxies(Strauss et al.2002;Eisenstein et al.2001), quasars(Richards et al.2002)or stars(Stoughton et al. 2002).Targeted objects are grouped in3-degree diameter ‘tiles’(Blanton et al.2003)and aluminum plates are drilled with640holes whose locations on the plate correspond to the objects’sky locations.Each plate is placed in the imag-ing plane of the telescope and plugged with optical?bres, which run from the telescope to twin spectrographs and are assigned to roughly500galaxies,50quasars and50stars.
SDSS spectra cover the observer-frame optical and near infrared,from3900?A–9100?A at a spectral resolution of ～1900.Spectra are obtained in three or four consecutive 15-minute observations until an average minimum signal-to-noise ratio is met.The spectra are calibrated by observations of32sky?bres,8reddening standard stars,and8spec-trophotometric standard stars.Spectra are?at-?elded and ?ux calibrated by the Spectro2d pipeline.Next,Spectro1d identi?es spectral features and classi?es objects by spec-tral type(Stoughton et al.2002).Ninety-four percent of all SDSS quasars are identi?ed spectroscopically by this au-tomated calibration;the remaining quasars are identi?ed through manual inspection.Quasars are de?ned to be those extragalactic objects with broad emissions lines(full width at half maximum velocity width of>1000km s?1),re-gardless of luminosity.Vanden Berk et al.(2005)found that the SDSS targeting algorithm is95per cent complete to i=19.1,the limiting magnitude of the low-redshift survey.
2.2The SDSS equatorial stripe quasar dataset During the autumn months,when the Northern Galactic Cap is unavailable for observation,the survey continually re-images a stripe centred on the celestial equator,as well as two‘outrigger’stripes roughly10degrees North or South of the equator.The equatorial scan,identi?ed as Stripe82,con-sists of a2.5-degree wide stripe ranging from309.2?to59.8?Right Ascension,covering a total of278deg2.The SDSS Fifth Data Release(Adelman-McCarthy et al.2007)con-tains the57survey-quality imaging runs that cover Stripe 82,which were observed as part of regular SDSS-I operations through2005June.
We study only those objects observed spectroscopically, as they have been con?rmed as quasars,and information about their black hole masses can be extracted directly from their spectra.In this region,7886objects have been spec-troscopically observed by the SDSS and con?rmed to be quasars.The majority of these quasars have been imaged by the SDSS between8and12times each,with an average of9.5(and a maximum of27)observations per object.
c 2006RAS,MNRAS000,1–??
On the variability of quasars:a link between Eddington ratio and optical variability?3
3V ARIABILITY PROPERTIES OF
EQUATORIAL STRIPE QUASARS
3.1Construction of the structure function
To measure the strength of the variability of our full sample
and various subsamples,we use a standard formulation of
the structure function(di Clemente et al.1996):
V= 2) |?m(?τ)| 2? σ2n ,(1)
where?m(?τ)is the di?erence in magnitude between any
two observations of a quasar,separated by?τin the quasars
is the square of the uncertainty in that
rest frame,andσ2
di?erence(which is equal to the sum of the two individual
observations’errors in quadrature).The units of the struc-
ture function are magnitudes.The means of these quantities
are taken over10bins,ranging from7days to700days,of
equal width in the logarithm of the time lag.
The structure function can be a useful tool,espe-
cially in comparing the relative variability of two sub-
samples of quasars,which is the primary approach em-
ployed in this paper.However,the structure function is
not an ideal measure in a statistical sense.It assumes each
point is statistically independent from all others,which
is clearly not the case,as most quasars contribute more
than one data point to each bin;this makes a true mea-
surement of the error quite di?cult.We follow the lead
of Cristiani,Trentini,La Franca,&Andreani(1997)and
Rengstorf,Brunner,&Wilhite(2006)in estimating the er-
ror,by making the(known incorrect)assumption that the
individual data points are independent,and ignoring covari-
ance between points.We then apply standard error prop-
agation to Equation1,using the statistical error in the
mean as the uncertainties for |?m(?τ)| and σ2n in each
bin.This leads to a slight overestimation of the uncertainty
in the structure function(Rengstorf,Brunner,&Wilhite
2006),which does not change any of our results;we defer
a complete treatment of the covariance to a later paper.
3.2Structure function of the entire sample
Figure1shows the structure function in all?ve SDSS pho-
tometric bands for the full sample of7,886quasars.A com-
parison of these?ve structure functions shows that quasars
are most variable in the u band,and least variable in the z
band.This is as one would expect,since it is well known that
quasars vary more at blue wavelengths in the ultraviolet and
optical(see,e.g.,Wilhite et al.2005).To characterise these
structure functions,we?t a power low to these data of the
form:
V= ?τ
,(3)
G
where f is a dimensionless factor of order unity that depends
upon the precise geometry of the broad line region.In this
scenario,the width of a given emission line is related to the
gravitational potential of the central source;thus the line
width serves as a proxy for the gas’s orbital velocity.Though
plausible other scenarios exist in which the line width is
not dominated by gravity,but some other factor such as c 2006RAS,MNRAS000,1–??
4 B.C.Wilhite et al.
radiation pressure,we here assume that these line widths
provide information relating to the mass of the central black hole.
Considerable work has been done recently to calibrate the radius-luminosity relationship,applying reverberation mapping techniques to a collection of nearby Seyfert galax-ies (e.g.,Peterson et al.2004;Kaspi et al.2005;Bentz et al.2006).Once a reliable calibration has been determined,a single-epoch measurement of the luminosity may be used to estimate the radius of the broad-line region.Here,we follow the prescription ?rst described by Vestergaard (2002)and later re?ned by Vestergaard &Peterson (2006):
log M BH (C iv )=log σ(C iv )
1044erg s
?1 0.53 +(6.73±0.01).This particular estimate for M BH employs the non-parametric dispersion (σ)of the C iv line.To measure the
dispersion,we use the techniques developed previously by
Wilhite et al.(2006).A linear ?t is applied to the local con-tinuum,using only the portion of the spectrum correspond-ing to the rest-frame intervals 1472?A
–1487?A and 1685?A –1700?A .This ?t is subtracted from the full spectrum to
isolate the ?ux in the C iv line.The median of the line is
calculated and the dispersion in the line is calculated around
the median measurement of the line centre.Uncertainties
are estimated by a Monte Carlo technique which involves
repeatedly adding Gaussian noise to the spectrum and re-measuring the dispersion.A full description of this technique
can be found in Wilhite et al.(2006).
To measure the continuum luminosity at 1450?A
,we simply take the mean ?ux in a 10-?A region centred on 1450
?A and use it to calculate the intrinsic luminosity in our as-sumed cosmology.The uncertainty in λL λ(1450?A )is esti-mated by calculating the error in the mean ?ux for the
region and using standard error propagation.The median
value for this uncertainty in λL λ(1450?A )is 2.2×1044erg
s 1,indicating the uncertainties are at the roughly 5%level.
The uncertainty in black hole mass is estimated by stan-dard propagation of the uncertainties in λL λ(1450?A )and
σ(C iv ),which yields a median M BH uncertainty of 9×10
7M ⊙,at the 10%to 15%level.
It should be noted that these mass estimates su?er from
large systematic and random uncertainties.Baskin &Laor
(2005)demonstrated that black hole mass estimates involv-ing C iv line width may be biased,perhaps with systematic
over or underestimates of mass by a factor of a few.Addi-tionally,Kelly &Bechtold (2007)?nd that the distribution
of single-epoch mass estimates is likely too broad,relative
to the presumed intrinsic distribution,while Shang et al.
(2007)suggest that out?ows may play a signi?cant role
in broad line widths.Vestergaard &Peterson (2006)state
that UV-based single-epoch mass estimates,based on com-parisons with their reverberation-mapping counterparts,are
good to within a factor of a few.As we are binning our ob-jects in black hole mass and comparing the variability ampli-tudes of these subsamples (see §4.2),rather than studying
individual objects,our results should be robust against both
random and systematic uncertainties,provided that the C iv
line width is related to the mass of the central black hole.
4.2
Binning in black hole mass and luminosity
Figure 2displays the distribution in continuum luminosity versus the estimated black hole mass for the majority of the 2,531quasars with measured C iv emission lines from the SDSS Equatorial Stripe.For continuum luminosity,we simply use the value for λL λ(1450?A )determined in §4.2.To investigate the dependence of variability on black hole mass,we subdivide the luminosity-black hole mass plane into the six bins as marked on the distribution of quasars in Figure 2.The median continuum luminosity,redshift and black hole mass for each bin are listed in Table 2.Also listed are the boundaries in black hole mass and luminosity for each bin.
To keep quasars with unreasonably low estimates of black
hole mass from a?ecting the results,we do not include any of the 226quasars with M BH <106M ⊙.The vast majority of these low estimates are due to broad absorption of either the
continuum or the emission line itself,and are unlikely to be
accurate estimates of the true black hole mass.Additionally,
no bins include those 322quasars with estimated masses
above 2×109M ⊙,as there are simply too few in any region of
λL λ(1450?A )—M BH parameter space to allow for a reliable
measurement of their ensemble variability.The quasars with
estimated black hole mass less than 106M ⊙or greater than 2×109M ⊙are not shown in Figure 2.For each bin,we calculate the structure functions for all ?ve bands of the quasars in that bin.All thirty structure functions (six bins times 5bands)are shown in Figure 3.Each structure function demonstrates the familiar relation between wavelength and variability;the u band in each bin shows the largest amplitude in its structure function,while the z -band measurements show the least variability.The structure functions shown in Figure 3have only nine points in ?τ,rather than the ten seen in Figure 1;the high-redshift nature of these quasars (which is necessary to observe C iv )results in the largest rest-frame time lag bin containing no observations,after one translates from the observed frame to the quasar’s rest frame.One quickly notices the large level of uncertainty in vir-tually all of these 30structure functions in the ?fth bin in ?τ,which is at approximately 60days.This is due to the lack of observations separated by 180days in the observed frame;this bin spans 180days /(1+ z ),where z is the mean redshift at which C iv is observable (i.e.,z ≈2.5).Ad-ditionally,in certain time-lag bins,a reliable measurement of the variability cannot be made,as the average uncertainty is greater than the average variability.This is seen most often in u -and z -band structure functions,as those bands have the lowest signal-to-noise ?ux determinations.By comparing the structure functions of quasars from adjacent bins in Figure 2,we can isolate the dependences of variability upon luminosity and black hole mass.For ex-ample,the left-hand panel of Figure 4shows the g -band structure functions for the quasars from bins 1,2and 3.Bin 1quasars are clearly more variable than those in bin 2,which are,in turn,more variable that those in bin 3.Table 3shows the results of the power-law ?ts to these structure functions (as well as those representing the quasars in bins 4,5and 6).The progression from high to low variability,as one travels from bin 1to bin 3,seen in Figure 2is re-?ected in the values for V (?τ=100)for those bins.In the right-hand panel of Figure 2,the same relation is observed c
2006RAS,MNRAS 000,1–??
On the variability of quasars:a link between Eddington ratio and optical variability?5
for quasars at higher black hole mass.Quasars in bin4are of lower luminosity than those in bin5,and are also more variable.
These results are not surprising,in that an anticorrela-tion between luminosity and variability has been known for decades.However,this shows,for the?rst time,that this de-pendence exists independent of black hole mass,a property known to be correlated with luminosity.
By comparing bins with quasars of similar luminosity, but di?erent black hole mass,one can isolate the dependence of variability on black hole mass.This is seen with bins2 and4,as they cover the same range in luminosity,but bin2 contains objects with M BH<5×108M⊙,while bin4con-tains quasars with between5×108M⊙ This same trend can be seen by comparing the three highest-luminosity bins:3,5and6.In the right-hand panel of Figure5and Table3,it can be seen that variability ap-pears to increase with increasing black hole mass.The in-crease is especially clear when one compares bin3with bin 6,the highest-black-hole-mass bin in our sample. 5DISCUSSION By isolating the dependence of variability upon luminos-ity and black hole mass,we are,in e?ect,able to probe the dependence of variability upon the Eddington ratio, L bol/L Edd.The Eddington ratio of a quasar is a comparison of the actual bolometric luminosity,L bol,to the Eddington luminosity,L Edd,which is the maximum stable luminosity at which accretion can occur.However,as we are measuring the optical luminosity,we can recast this as: L opt=εL bol,(4) whereεrepresents the fraction of the bolometric luminos-ity emitted in the optical.This is likely to be a function of the bolometric luminosity;however,recent measurements for quasars with L bol>1010L⊙have shown this value to be approximately0.1(Hopkins,Richards,&Hernquist 2007;Richards et al.2006).Furthermore,since the Edding-ton luminosity is directly proportional to black hole mass (Rees1984),we have that L bol/L Edd～L opt/M BH. Characteristic Eddington ratios have been calculated for each bin and are provided in Table2.These values do not represent an average L bol/L Edd for the bin,but rather the Eddington ratio one obtains from the average values for λLλ(1450?A)and M BH also given in Table2.The black hole mass is converted to an Eddington luminosity through the familiar L Edd=1.3×1038(M/M)⊙erg s?1.To get the Bolo-metric luminosity,we use the L bol～9×λLλ(5100?A)rela-tion used in Kaspi et al.(2000)and Kollmeier et al.(2006) and combine it with theαν=0.44quasar spectral slope of Vanden Berk et al.(2001)to get a new relation for the continuum near the C iv line:L bol～5×λLλ(1450?A).Five of the six bins have L bol/L Edd between0.1and1,as did the vast majority of objects in Kollmeier et al.(2006).Even Bin3,with a value of L bol/L Edd greater than1is not un-reasonable;a number of objects studied in Kollmeier et al. (2006)were calculated to have super-Eddington luminosi-ties.At any rate,the Eddington ratios calculated in Table 2should primarily be used as a means for comparing the relative Eddington ratios of the quasars in di?erent bins. By combining the established(and herein reproduced) inverse dependence of variability upon optical luminosity with the newly demonstrated correlation of variability with black hole mass,we?nd that variability appears to be in-versely related to the Eddington ratio.Quasars with higher Eddington ratios are less variable than those with lower Ed-dington ratios.This suggests that the well-known anticorre-lation of variability with luminosity may in fact simply be a side e?ect of a primary anticorrelation between variability and the Eddington ratio. In Figure1,lines of constant Eddington ratio are simply lines with intercept zero.In this plane,a higher Eddington ratio corresponds to a line with smaller positive slope.We have avoided binning objects by their Eddington ratio in this paper,simply because the shapes of those bins would not lend themselves to easy comparisons.We would,how-ever,point out that bin3is the bin with the highest mean Eddington ratio.As seen in Table3,the quasars in bin3are also seen to be the least variable,with the lowest value for V(?τ=100). To interpret our hypothesised relationship between op-tical variability and the Eddington ratio,we use the theo-retical relationship between the luminosity of a quasar and its accretion rate: L bol=η˙Mc2,(5) whereηis a measure of the radiative e?ciency of the quasar and is dependent on the speci?c physical parameters used to model the black hole(see,e.g.,Krolik1998,for detailed calculations).The two canonical values correspond to the Schwarzschild black hole,which hasη≈0.06,and the Kerr black hole,which hasη=0.42.Given our lack of knowledge about the physical parameters of the supermassive black holes that power quasars,the general practice is to adopt a value that lies between these two extremes,i.e.,η～0.1. By combining Equations4and5,we have the simple model in which the optical luminosity is related to the accre-tion rate(˙M),the radiative e?ciency(η)and the fraction of the bolometric luminosity that is emitted in the optical (ε): L opt=εη˙Mc2(6) In light of Equation6,changes in the optical luminosity of a quasar can be driven either by a change in?,η,or˙M.A varying value ofεwould require radical changes of a quasar’s spectral shape across multiple wavelength regimes.A vary-ingηwould require the nature of an individual black hole to change with time.On the rest-frame time-scales of our observations,it is unlikely that either of these two would be comparable to variations in the accretion?ow,which should naturally occur due to the dynamics of the entire accretion process. If we assume that variations in the optical luminosity of the quasar are tied to variations in the accretion rate,this can be interpreted as a link between the optical variability of a quasar and its‘age’.In the cocoon model(see,e.g., c 2006RAS,MNRAS000,1–?? 6 B.C.Wilhite et al. Haas2004;Hopkins et al.2005),quasars become observable in the optical at high accretion rate(after feedback‘blows away’enshrouding gas and dust),and fade away when the accretion rate drops.The Eddington ratio,therefore,could be construed as a proxy for the age of the quasar,or more precisely,the time since the quasar became observable in the optical portion of the spectrum.Martini&Schneider(2003) describe one possible test for measuring quasar lifetimes in models such as this,employing large,multi-epoch surveys. At constant black hole mass,optical luminosity could provide a measure of the gas that is available for accretion onto the black hole.Therefore,we might expect that younger quasars are more luminous because they have a greater fuel supply.Similarly,when comparing two quasars with the same optical luminosity,the quasar with the larger black hole mass would be older–its lower Eddington ratio is in-dicative of it having burned through much of its once-larger fuel supply.Thus,when comparing populations of quasars (as in our bins in L opt and M BH),the greater variability seen in the lower luminosity objects would be a consequence of a dwindling fuel supply.As less gas is available,the rate at which the gas is supplied to the black hole varies more, much like the?ickering of a dying?re.Either way,the pos-sibility that variability is tied to the Eddington ratio,which is in essence a measure of the e?ciency of a quasar,is an intriguing one. Both panels of Figure5appear to demonstrate that black hole mass is related to variability at larger time lags. This is also seen in Table3,which shows that the high black hole mass bins not only have smaller values of V(?τ=100), but also larger power law slopes,indicating that the di?er-ences in variability will be more prominent at longer time lags.This agrees with Wold,Brotherton,&Shang(2007), who saw little correlation between variability and black hole mass for a sample of observations with time separations less than100days,but a clear correlation between the two for ?τgreater than100days.This apparent increase in the ef-fect of back hole mass on longer time-scale variability clearly indicates the need for longer observed time baselines.The results presented herein only use data from the completed SDSS-I survey.The ongoing SDSS-II will ultimately add an-other three years to this baseline,for an average increase in the maximum rest-frame?τof roughly one year for each quasar. The analysis in this paper focused on the C iv sample, which consists only of quasars with z>1.69,as C iv is blueward of the SDSS spectral response at lower redshifts. The remaining,lower redshift quasars can be analysed in a similar manner,however,by utilising other emission lines, such as Mg ii or Hβ.Not only would this analysis nearly triple the number of quasars studied,but it would also ex-tend the redshift baseline of our sample,thereby allowing us to test the hypothesised relationship between optical vari-ability and accretion rate at other cosmic epochs. 6CONCLUSIONS In this paper,we have studied the ensemble variability prop-erties of almost8,000spectroscopically identi?ed quasars from the Sloan Digital Sky Survey Equatorial Stripe.These objects have been observed an average of over ten times each.By using their C iv line dispersions and nearby con-tinuum luminosities,we have estimated black hole masses for approximately2,500of these quasars.We have binned these quasars in luminosity and black hole mass and exam-ined the variability properties of the quasars in each bin.We have been able to: (1)Reproduce the well-known anticorrelation between luminosity and variability,and (2)Detect a correlation between variability and black hole mass. By combining(1)and(2),it appears that variability is inversely related to the Eddington ratio in quasars.This points to variability being related to the quasar’s accretion e?ciency.Given that the relation with black hole mass is more evident at longer time lags,we believe future studies involving longer time baselines will shed more light on this new result. B.C.W.and R.J.B.would like to acknowledge sup-port from Microsoft Research,the University of Illinois,and NASA through grants NNG06GH156and NB2006-02049. The authors made extensive use of the storage and com-puting facilities at the National Center for Supercomputing Applications and thank the technical sta?for their assis-tance in enabling this work. Funding for the SDSS and SDSS-II has been provided by the Alfred P.Sloan Foundation,the Participating In-stitutions,the National Science Foundation,the U.S.De-partment of Energy,the National Aeronautics and Space Administration,the Japanese Monbukagakusho,the Max Planck Society,and the Higher Education Funding Council for England.The SDSS Web Site is https://www.wendangku.net/doc/0016281203.html,/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions.The Partic-ipating Institutions are the American Museum of Natu-ral History,Astrophysical Institute Potsdam,University of Basel,University of Cambridge,Case Western Reserve Uni-versity,University of Chicago,Drexel University,Fermilab, the Institute for Advanced Study,the Japan Participation Group,Johns Hopkins University,the Joint Institute for Nuclear Astrophysics,the Kavli Institute for Particle As-trophysics and Cosmology,the Korean Scientist Group,the Chinese Academy of Sciences(LAMOST),Los Alamos Na-tional Laboratory,the Max-Planck-Institute for Astronomy (MPIA),the Max-Planck-Institute for Astrophysics(MPA), New Mexico State University,Ohio State University,Uni-versity of Pittsburgh,University of Portsmouth,Princeton University,the United States Naval Observatory,and the University of Washington. 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C.,Vanden Berk D. E.,Brunner R.J., Brinkmann J.V.,2006,ApJ,641,78 Wilkes,B.J.1984,MNRAS,207,73 Wold M.,Brotherton M.S.,Shang Z.,2007,MNRAS,1496 York,D.G.et al.2000,AJ,120,1579 c 2006RAS,MNRAS000,1–?? 8 B.C.Wilhite et al. Figure1.Variability as a function of time(the Structure Function)for the full sample of7,886quasars in all?ve SDSS photometric bands. c 2006RAS,MNRAS000,1–?? On the variability of quasars:a link between Eddington ratio and optical variability?9 Figure2.Distribution in luminosity and black hole mass for the2,531quasars for which black hole masses have been estimated.The lines overdrawn separate the objects into six bins,which are marked,used for the study of ensemble variability. c 2006RAS,MNRAS000,1–?? 10 B.C.Wilhite et al. Figure3.Variability as a function of time(the Structure Function)in all?ve photometric bands and all six bins in luminosity and black hole mass.In each panel,the curves represent,from most to least variable,the SDSS u,g,r,i,and z bands. c 2006RAS,MNRAS000,1–?? On the variability of quasars:a link between Eddington ratio and optical variability?11 Figure4.Variability as a function of time(the Structure Function)in the g band for the quasars in bins1,2and3(left),and4and 5(right).Both panels demonstrate the well-known anticorrelation between luminosity and variability.Luminosities listed in the legend are all λLλ(1450) for than bin in units of1044erg s?1. c 2006RAS,MNRAS000,1–?? 12 B.C.Wilhite et al. Figure5.Variability as a function of time(the Structure Function)in the g band for the objects in bins2and4(left),and3,5and6 (right).Masses listed in the legend are M BH for that bin in units of108M⊙. c 2006RAS,MNRAS000,1–?? On the variability of quasars:a link between Eddington ratio and optical variability?13 Table1.Results of Power-Law?ts to structure functions for full sample. u56100.4350.173±0.001 g54380.4790.147±0.001 r77020.4860.121±0.001 i164900.4360.108±0.001 z334000.4110.091±0.001 Bin Number of L low L high M low M high L z M BH L/L Edd Objects Table3.Results of Power-Law?ts to g-band structure functions for bins in luminosity and black hole mass. 13274.890.5138460.167±0.002 25486.220.4880120.142±0.002 35008.550.5433260.119±0.002 42811.680.5613190.153±0.002 54957.320.5238270.129±0.001 62940.300.5782940.141±0.002 On the contrary Onthecontrary, I have not yet begun. 正好相反，我还没有开始。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, the instructions have been damaged. 反之，则说明已经损坏。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I understand all too well. 恰恰相反，我很清楚 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I think this is good. ⑴我反而觉得这是好事。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I have tons of things to do 正相反，我有一大堆事要做 Provided by jukuu Is likely onthecontrary I in works for you 反倒像是我在为你们工作 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, or to buy the first good. 反之还是先买的好。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, it is typically american. 相反，这正是典型的美国风格。 222.35.143.196 Onthecontrary, very exciting. 恰恰相反，非常刺激。 https://www.wendangku.net/doc/0016281203.html, But onthecontrary, lazy. 却恰恰相反，懒洋洋的。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I hate it! 恰恰相反，我不喜欢！ https://www.wendangku.net/doc/0016281203.html, Onthecontrary, the club gathers every month. 相反，俱乐部每个月都聚会。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I'm going to work harder. 我反而将更努力工作。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, his demeanor is easy and nonchalant. 相反，他的举止轻松而无动于衷。 https://www.wendangku.net/doc/0016281203.html, Too much nutrition onthecontrary can not be absorbed through skin. 太过营养了反而皮肤吸收不了. https://www.wendangku.net/doc/0016281203.html, Onthecontrary, I would wish for it no other way. 正相反，我正希望这样 Provided by jukuu Onthecontrary most likely pathological. 反之很有可能是病理性的。 https://www.wendangku.net/doc/0016281203.html, Onthecontrary, it will appear clumsy. 反之，就会显得粗笨。 https://www.wendangku.net/doc/0016281203.html, 一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991 ●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the 高中英语~词性~句子成分~语法构成 第一章节：英语句子中的词性 1.名词：n. 名词是指事物的名称，在句子中主要作主语.宾语.表语.同位语。 2.形容词;adj. 形容词是指对名词进行修饰~限定~描述~的成份，主要作定语.表语.。形容词在汉语中是（的）.其标志是: ous. Al .ful .ive。. 3.动词：vt. 动词是指主语发出的一个动作，一般用来作谓语。 4.副词：adv. 副词是指表示动作发生的地点. 时间. 条件. 方式. 原因. 目的. 结果.伴随让步. 一般用来修饰动词. 形容词。副词在汉语中是（地）.其标志是：ly。 5.代词：pron. 代词是指用来代替名词的词，名词所能担任的作用，代词也同样.代词主要用来作主语. 宾语. 表语. 同位语。 6.介词：prep.介词是指表示动词和名次关系的词，例如：in on at of about with for to。其特征： 介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词 第二章节：英语句子成分 主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。 谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。 宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。 定语：只对名词起限定修饰的成分，一般由形容 M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition. 意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机 突袭 空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上. 英语造句大全English sentence 在句子中，更好的记忆单词！ 1、(1)、able adj. 能 句子：We are able to live under the sea in the future. (2)、ability n. 能力 句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地 句子：His calculation is accurately. 3、(1)、act v. 扮演 句子：He act the interesting character. （2）、actor n. 演员 句子：He was a famous actor. (3)、actress n. 女演员 句子：She was a famous actress. (4)、active adj. 积极的 句子：He is an active boy. 4、add v. 加 句子：He adds a little sugar in the milk. 5、advantage n. 优势 句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。 【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early 《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high 一、翻译 1. The idea of consciously seeking out a special title was new to me., but not without appeal. 让我自己挑选自己最喜欢的书籍这个有意思的想法真的对我具有吸引力。 2.I was plunged into the aching tragedy of the Holocaust, the extraordinary clash of good, represented by the one decent man, and evil. 我陷入到大屠杀悲剧的痛苦之中，一个体面的人所代表的善与恶的猛烈冲击之中。 3.I was astonished by the the great power a novel could contain. I lacked the vocabulary to translate my feelings into words. 我被这部小说所包含的巨大能量感到震惊。我无法用语言来表达我的感情（心情）。 4，make sth. long to short长话短说 5.I learned that summer that reading was not the innocent(简单的) pastime(消遣) I have assumed it to be., not a breezy, instantly forgettable escape in the hammock(吊床)，( though I’ ve enjoyed many of those too ). I discovered that a book, if it arrives at the right moment, in the proper season, will change the course of all that follows. 那年夏天，我懂得了读书不是我认为的简单的娱乐消遣，也不只是躺在吊床上，一阵风吹过就忘记的消遣。我发现如果在适宜的时间、合适的季节读一本书的话，他将能改变一个人以后的人生道路。 二、词组造句 1. on purpose 特意，故意 This is especially true here, and it was ~. (这一点在这里尤其准确，并且他是故意的) 2.think up 虚构，编造，想出 She has thought up a good idea. 她想出了一个好的主意。 His story was thought up. 他的故事是编出来的。 3. in the meantime 与此同时 助记：in advance 事前in the meantime 与此同时in place 适当地... In the meantime, what can you do? 在这期间您能做什么呢？ In the meantime, we may not know how it works, but we know that it works. 在此期间，我们不知道它是如何工作的，但我们知道，它的确在发挥作用。 4.as though 好像，仿佛 It sounds as though you enjoyed Great wall. 这听起来好像你喜欢长城。 5. plunge into 使陷入 He plunged the room into darkness by switching off the light. 他把灯一关，房 The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design. M1 U1 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： add up (to) be concerned about go through set down a series of on purpose in order to according to get along with fall in love (with) join in have got to hide away face to face 1 We’ve chatted online for some time but we have never met ___________. 2 It is nearly 11 o’clock yet he is not back. His mother ____________ him. 3 The Lius ___________ hard times before liberation. 4 ____________ get a good mark I worked very hard before the exam. 5 I think the window was broken ___________ by someone. 6 You should ___________ the language points on the blackboard. They are useful. 7 They met at Tom’s party and later on ____________ with each other. 8 You can find ____________ English reading materials in the school library. 9 I am easy to be with and _____________my classmates pretty well. 10 They __________ in a small village so that they might not be found. 11 Which of the following statements is not right ____________ the above passage? 12 It’s getting dark. I ___________ be off now. 13 More than 1,000 workers ___________ the general strike last week. 14 All her earnings _____________ about 3,000 yuan per month. 二.用以下短语造句： 1.go through 2. no longer/ not… any longer 3. on purpose 4. calm… down 5. happen to 6. set down 7. wonder if 三. 翻译： 1.曾经有段时间，我对学习丧失了兴趣。（there was a time when…） 2. 这是我第一次和她交流。（It is/was the first time that …注意时态） 3.他昨天公园里遇到的是他的一个老朋友。（强调句） 4. 他是在知道真相之后才意识到错怪女儿了。（强调句） M 1 U 2 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： play a …role (in) because of come up such as even if play a …part (in) 1 Dujiangyan(都江堰) is still ___________in irrigation(灌溉) today. 2 That question ___________ at yesterday’s meeting. 3 Karl Marx could speak a few foreign languages, _________Russian and English. 4 You must ask for leave first __________ you have something very important. 5 The media _________ major ________ in influencing people’s opinion s. 6 _________ years of hard work she looked like a woman in her fifties. 二.用以下短语造句： 1.make (good/full) use of 2. play a(n) important role in 3. even if 4. believe it or not 5. such as 6. because of English sentence 1、(1)、able adj. 能 句子：We are able to live under the sea in the future. (2)、ability n. 能力 句子：Most school care for children of different abilities. (3)、enable v. 使。。。能 句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的 句子：We must have the accurate calculation. (2)、accurately adv. 精确地 句子：His calculation is accurately. 3、(1)、act v. 扮演 句子：He act the interesting character.（2）、actor n. 演员 句子：He was a famous actor. (3)、actress n. 女演员 句子：She was a famous actress. (4)、active adj. 积极的 句子：He is an active boy. 4、add v. 加 句子：He adds a little sugar in the milk. 5、advantage n. 优势 句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 句子：He is in America. 10、appear 出现v. He appears in this place. 11. artist 艺术家n. He is an artist. 12. attract 吸引 He attracts the dog. 13. Australia 澳大利亚 He is in Australia. 14.base 基地 She is in the base now. 15.basket 篮子 His basket is nice. 16.beautiful 美丽的 She is very beautiful. 17.begin 开始 He begins writing. 18.black 黑色的 He is black. 19.bright 明亮的 His eyes are bright. 20.good 好的 He is good at basketball. 21.British 英国人 He is British. 22.building 建造物 The building is highest in this city 23.busy 忙的 He is busy now. 24.calculate 计算 He calculates this test well. 25.Canada 加拿大 He borns in Canada. 26.care 照顾 He cared she yesterday. 27.certain 无疑的 They are certain to succeed. 28.change 改变 He changes the system. 29.chemical 化学药品on the contrary的解析
英语造句
学生造句--Unit 1
英语句子结构和造句
六级单词解析造句记忆MNO
base on的例句
英语造句大全
(完整版)主谓造句
初中英语造句
The Kite Runner-美句摘抄及造句
翻译加造句
改写句子练习2标准答案
用以下短语造句
英语造句