A Chandra detection of the radio hotspot of 3C123
a r X i v :a s t r o -p h /0101240v 2 7 J u n 2001
Mon.Not.R.Astron.Soc.000,000–000(0000)Printed 1February 2008
(MN L A T E X style ?le v1.4)
A Chandra detection of the radio hotspot of 3C 123
M.J.Hardcastle,M.Birkinshaw and D.M.Worrall
Department of Physics,University of Bristol,Tyndall Avenue,Bristol BS81TL 1February 2008
ABSTRACT
Chandra X-ray Observatory observations of the powerful,peculiar radio galaxy 3C 123have
resulted in an X-ray detection of the bright eastern hotspot,with a 1-keV ?ux density of ～5nJy.The X-ray ?ux and spectrum of the hotspot are consistent with the X-rays be-ing inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission (‘synchrotron self-Compton emission’)if the magnetic ?elds in the hotspot are close to their equipartition values.3C 123is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equiparti-tion.Chandra also detects emission from a moderately rich cluster surrounding 3C 123,with L X (2??10keV)=2×1044ergs s ?1and kT ～5keV ,and absorbed emission from the active nucleus,with an inferred intrinsic column density of 1.7×1022cm ?2and an intrinsic 2–10keV luminosity of 1044ergs s ?1.
Key words:galaxies:active –X-rays:galaxies –galaxies:individual:3C 123–radiation mechanisms:non-thermal
1INTRODUCTION
The magnetic ?eld strengths in the extended components of ex-tragalactic radio sources cannot be inferred directly from obser-vations of synchrotron emission,and so the energy densities and pressures in the radio-emitting components are poorly constrained.In order to make progress it is common to estimate ‘minimum en-ergy’?eld strengths (Burbidge 1956),which minimise the energy density required for a given synchrotron emissivity.This is roughly equivalent to the assumption that that magnetic and relativistic par-ticle energy densities are equal (‘equipartition’).But without mea-surements of magnetic ?eld strengths these assumptions,for which there is no physical justi?cation,may underestimate the true energy densities by arbitrary factors.
The magnetic ?eld strength may be measured by observa-tions of the ‘synchrotron self-Compton’(SSC)process,in which the synchrotron-emitting electrons inverse-Compton scatter syn-chrotron photons up to X-ray energies.Because the emissivity from this process depends on the photon number density (which is known from radio observations)and the electron number density as a function of energy,observations of SSC emission allow the elec-tron number density to be inferred,and so determine the magnetic ?eld strength.Such tests require observations of regions with well-measured volume and a well-de?ned synchrotron spectrum with a high photon energy density;these conditions exist in the hotspots of FRII radio sources.Direct evidence supporting the equiparti-tion/minimum energy assumptions in hotspots has come from only two X-ray observations.Harris,Carilli &Perley (1994)detected the hotspots of the powerful FRII Cygnus A (3C 405)with ROSAT and showed that the X-ray emission could be interpreted as being due to the SSC process,with a magnetic ?eld strength consistent
with the equipartition model.[This result was recently con?rmed
with Chandra by Wilson,Young &Shopbell (2000).]ROSAT was not sensitive enough to detect any other SSC hotspots,though it was used to put lower limits on the ?eld strengths in some sources (Hardcastle,Birkinshaw &Worrall 1998).More recently,Chandra veri?cation observations have detected the hotspots of 3C 295,an-other powerful radio galaxy,at a level which implies ?eld strengths fairly close to the equipartition values if the emission process is SSC (Harris et al.2000).Here we report a third detection,of the E hotspot of the radio galaxy 3C 123,based on our Chandra AO1guest observer (GO)observations.
3C 123is a z =0.2177radio galaxy,notable for its peculiar radio structure.Like normal classical double sources it has twin hot spots on either side of the active nucleus,but the lobes take the form of diffuse twisted plumes unlike those in any other well-studied object (e.g.Riley &Pooley 1978;Hardcastle et al.1997,hereafter H97).Like Cygnus A and 3C 295,its radio luminosity is unusually high for its redshift.For our purposes,its most important feature is its bright eastern double hotspot.With a ?ux density of ～6Jy at 5GHz,it is the second brightest hotspot complex known (after Cygnus A).The hotspots’structure and synchrotron spectrum are well known (H97;Meisenheimer et al.1989;Meisenheimer,Yates &R¨o ser 1997;Looney &Hardcastle,2000).
Throughout this letter we use H 0=50km s ?1Mpc ?1and q 0=0.At the redshift of 3C 123,1arcsec corresponds to 4.74kpc.
2OBSERV ATIONS
We observed 3C 123with the Chandra X-ray Observatory for 46.7ks on 2000March 21.The source was near the aim point for the
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04 37 06.5
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05.004.504.003.503.0
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39 45Figure 1.Exposure-corrected 0.5–7.0keV Chandra image of the central region of 3C 123.Linear greyscale:black is 4×10?7photons cm ?2s ?1per standard Chandra pixel (0.492arcsec on a side).Superposed is the 3mJy beam ?1contour from an 8.4-GHz Very Large Array (VLA)map with 0.6-arcsec resolution (H97),showing the position of the radio lobes.As discussed in section 4,the radio map has been shifted by ～3arcsec so that the radio core aligns with the X-ray nucleus.
S3ACIS chip.After ?ltering for intervals of high background,the usable exposure time was 38.5ks.We considered events in the en-ergy range 0.5–7.0keV ,as the spectral response of the instrument is uncertain outside this range.Fig.1shows the exposure-corrected Chandra image of 3C 123in this band.Diffuse cluster emission,an X-ray nucleus and the eastern hotspot are all detected in X-rays.We discuss each component in turn.In each case,spectra were extracted using CIAO ,with the best available responses being constructed for each extraction region,and analysed using XSPEC .Spectra were binned such that every bin had >20net counts.
3THE CLUSTER
The X-ray counts from 3C 123are dominated by diffuse cluster-scale emission,which is detectable above the background more
than an arcminute away from the central source.The source was known to be extended from ROSAT images (Hardcastle &Worrall 1999),and there are many faint optical objects in the ?eld of 3C 123which may be cluster members (e.g.Longair &Gunn 1975),though strong galactic reddening (the source is at b ≈?12?,and see be-low)means that this cluster has not been studied in detail in the optical.We detect around 5,000counts in a 75-arcsec radius circle centred on the nucleus,excluding the core and hotspot.The over-all spectrum of this region is well ?tted with an absorbed MEKAL
spectrum with kT =5.0+0.6?0.4keV ,abundance of 0.47+0.12
?0.11solar
and a galactic hydrogen column density of 4.3+0.2?0.3×10
21
cm ?2(errors are 1σfor one interesting parameter).3C 123lies behind a well-known molecular cloud system in Taurus (e.g.Ungerechts &Thaddeus 1987),and our derived column density is consistent with the total hydrogen column inferred from radio observations of the molecular cloud system.From HI absorption against 3C 123,Col-
gan,Salpeter &Terzian (1988)derive N HI =1.97×1021cm ?2at the velocity of the cloud system,while the molecular hydrogen column can be inferred from CO measurements [W CO ≈10K km s ?1,Megeath (private communication)]to be N H 2≈1.6×1021cm ?2,with a large systematic uncertainty [we use a recent esti-mate of the conversion factor averaged over the galactic plane,due to Hunter et al.(1997),but this may not be appropriate for the Tau-rus region].We adopt a galactic absorption column of 4.3×1021cm ?2,unless otherwise stated,from now on.
The X-ray spectral ?t implies a rest-frame 2–10keV lumi-nosity from the cluster within a radius of 75arcsec (360kpc)of 2×1044ergs s ?1,consistent with the ?tted temperature on the temperature-luminosity relation (determined largely for Abell clus-ters)of David et al.(1993).Fig.1shows a plateau of X-ray emis-sion on scales comparable to those of the radio source,with clear structure in the emission (note particularly X-ray voids to the E and SW of the nucleus)although there is no evidence for interaction be-tween the X-ray gas and the radio lobes.The voids may represent large-scale inhomogeneity in the cluster gas;if so,they would help to explain the peculiar radio structure.The central cooling time is a few ×109years,so we might expect to see a cooling ?ow around the source.But there is no strong evidence for cooling in the tem-perature ?ts;the best-?tting temperature for the material within 15arcsec of the nucleus (?xing the abundance to the value obtained for the whole cluster)is 4.4±0.3keV .Since gas with temperatures below ～3keV is not observed even in the centres of well-studied cooling ?ows (e.g.Fabian et al.2000,Peterson et al.2000)this is perhaps not surprising.Our preliminary analysis implies parti-cle densities around the lobes which are similar to those reported by Hardcastle &Worrall (2000)using ROSAT data,and the mea-sured temperature implies comparable,but slightly larger,external pressures.
4THE NUCLEUS
The point-like nucleus contains 517±360.5–7.0keV counts,mea-sured in a 2.5-arcsec region about the centroid,with the back-ground being taken from a 3–5arcsec concentric annulus.Pileup is not signi?cant.The X-ray core position (J2000.0)is measured to be 043704.30+294011.2.The core position measured from the VLA radio map of H97is 043704.375+294013.86,and this is expected to be accurate to within about 0.05arcsec.The X-ray core position is therefore offset from the (true)ra-dio position by about 3arcsec.We attribute this to uncertain-ties in aspect determination in the early version of the pipeline software (R4CU4UPD7.4)used to process the Chandra data (see URL:
The spectrum of the nucleus is well ?tted with an absorbed,?at-spectrum power law model.Fitting with free galactic absorp-tion,the best-?t values of photon index Γand N H are 1.16±0.14
and 1.48+0.32?0.24×10
22
cm ?2,respectively.If we ?x galactic ab-sorption at the value derived from the cluster ?ts and require the absorber to be at the redshift of the galaxy,the best-?t N H for the
intrinsic absorber is 1.69+0.51?0.41×10
22
cm ?2,with the photon in-dex unchanged.This implies a rest-frame 2–10keV luminosity (as-sumed isotropic)of 1044ergs s ?1,comparable to the luminosity of the nuclear component in 3C 295(Harris et al.2000).Fits in which the absorbing column is constrained to the galactic value are much poorer and require an inverted nuclear spectrum.The column den-sity required for the intrinsic absorber is considerably lower than
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Chandra detection of 3C 123
3
Figure 2.Joint con?dence contours for the spectrum of the nucleus of 3C 123using the model described in the text.Intrinsic absorbing column is plotted on the x -axis,power-law photon index on the y -axis.The con-tours are at 1,2and 3σfor two interesting parameters.The cross marks the best-?t values.
the 4×1023cm ?2inferred for Cygnus A by Ueno et al.(1994),and more comparable to that inferred for the much lower luminos-ity nucleus of Hydra A by Sambruna et al.(2000).The best-?t pho-ton index is ?atter than might be expected from,for example,the photon indices of radio-loud quasars (https://www.wendangku.net/doc/2615331244.html,wson &Turner 1997)or other radio galaxies (Sambruna,Eracleous &Mushotzky 1999)although the errors are large;as shown in Fig.2,more reasonable values of Γare allowed in conjunction with somewhat higher in-trinsic absorbing columns.The inferred absorbing column in front of the nucleus explains the non-detection of this core component in the ROSAT HRI image (Hardcastle &Worrall 1999).
5THE HOTSPOT
The E hotspot complex is detected with 145±320.5–7.0keV counts,using a 2.5-arcsec source circle and concentric 3–5arcsec background annulus.The X-ray hotspot is positionally coincident with the larger,‘secondary’hotspot of the eastern hotspot pair in the radio (after the X-ray and radio cores have been aligned).The X-ray emission appears to be slightly elongated in an east-west di-rection,matching the radio.(The X-ray structure will be discussed in more detail in a subsequent paper which will include results of further radio observations now in progress.)The X-ray spectrum of the hotspot is well ?tted with a power law with Γ=1.6±0.3,with the absorbing column ?xed at the galactic value.The corresponding unabsorbed 1-keV ?ux density is 4.6±0.9nJy.
We used the synchrotron-self-Compton code described by Hardcastle et al.(1998)to predict the SSC ?ux density expected at this frequency from the hotspots.The basic model for the hotspots is described by Looney &Hardcastle (2000).The larger hotspot is treated as a cylinder of 1.14×0.54arcsec (length times ra-dius);the fainter ‘primary’(more compact)hotspot is a cylinder
of 0.74×0.14arcsec,based on the MERLIN maps of H97.Ra-dio ?ux densities of the two components are taken from Looney &Hardcastle.In addition to these,we have used infra-red and opti-cal upper limits and a 231-GHz data point from Meisenheimer et al.(1989,1997)and archival HST observations,and low-frequency radio data from Readhead &Hewish (1974)and Stephens (1987).As these data do not resolve the two hotspot components,we have approximately corrected them by scaling by the appropriate factors measured from the 5-GHz data.Looney &Hardcastle showed that the radio-to-mm spectra of the two hotspots are well modelled as broken power laws,and we adopt the break energies they found.The apparent low-frequency turnover in the spectrum observed by Stephens (1987)requires a low-energy cutoff in the electron energy spectrum corresponding at equipartition to a minimum Lorentz fac-tor γmin ≈1000,and we adopt this value,although Stephens’?ux densities are inconsistent with a larger ?ux at a lower fre-quency derived from the scintillation measurements of Readhead &Hewish.If we were to adopt the scintillation measurements as our low-frequency constraint,we would obtain γmin ≈400,which is more consistent with the value inferred for the hotspots of Cygnus A by Carilli et al.(1991);but this would not signi?cantly affect our conclusions.(Scheduled low-frequency VLBA observations should give a de?nitive answer.)An upper limit on the maximum Lorentz factor is given by the non-detection in the IR,γmax <3.6×105;a lower limit is given by the detection at 231GHz,γmax >8×104.The SSC emissivity at 1keV turns out to be insensitive to γmax if it lies between these two values,and so we ?x γmax at its largest value.With these parameters,the equipartition ?eld strengths of the two eastern hotspot components,assuming no contribution to the energy density from non-radiating particles such as protons,are 24nT (primary)and 16nT (secondary),and the predicted SSC ?ux densities at 1keV are respectively 0.44and 2.6nJy.The pre-dicted photon index at this frequency is 1.55(of course,this is sim-ply a function of the electron energy spectrum,and so is true for any inverse-Compton process).Fig.3shows the synchrotron ?uxes and SSC prediction for the secondary hotspot.The predicted SSC ?ux density for the much weaker western hotspot pair is negligible,～0.07nJy,corresponding to 2Chandra counts in this observation.
These predictions are relatively insensitive to cosmological parameters;for example,using a cosmology where ?matter =1.0gives a 3per cent decrease in the expected ?ux density from the secondary,while using H 0=70km s ?1Mpc ?1,?matter =0.3,?Λ=0.7gives a 2per cent decrease.The hotspots may be pro-jected,but this also has only a weak effect:if the projection angle is 45?
then the true long axis is larger by a factor
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Figure3.The spectrum of the secondary hotspot of3C123at equipartition.
Points are from data as described in the text.Symbols indicate the source
of the data,as follows:?lled circles,Looney&Hardcastle(2000);crosses,
Meisenheimer et al.(1989);squares,Stephens(1987);triangle,Readhead
&Hewish(1974);star,optical limit from HST data;open circle,Chandra
data point.Arrows denote an upper limit.Error bars are smaller than the
symbols in most cases.The solid line is the model synchrotron spectrum. The dash-dotted line shows the predicted SSC spectrum at equipartition
and the dotted line shows inverse-Compton emission due to scattering of
microwave background photons.The two solid bars through the X-ray data
point show the0.5–7.0keV band of the Chandra data and the1σrange
of photon indices permitted by the data.Frequencies are plotted in the rest
frame of the radio source.
the effect of reducing the SSC?ux density;to increase it we must reduce the magnetic?eld strength or?nd an additional(external)
source of photons.If the magnetic?eld in the secondary hotspot is
reduced by25per cent to12nT,the secondary can produce all the ?ux seen in X-rays.Neglecting the small SSC contribution from the
primary hotspot,the Chandra data with their associated uncertain-
ties imply within the SSC model that the magnetic?eld strength in the secondary hotspot is12±2nT(1σstatistical errors only).
One possible external source of photons is the active nucleus of3C123.Inverse-Compton(IC)scattering of photons from the
active nucleus will make a signi?cant contribution to the X-ray
emission if such photons(at frequencies around1011Hz,since γmin≈1000)are comparable in number density to the synchrotron photons.At this frequency,the number density of synchrotron pho-
tons is approximately0.08m?3Hz?1;since the hotspots are a pro-jected37kpc from the nucleus,a similar number density would be produced from the nucleus if its luminosity at this frequency as seen by the hotspot were>～2×1027W Hz?1sr?1,which would corre-spond to a?ux density of>～100Jy at1011Hz.(This condition is equivalent to S core=(3D/2R)2S hs,where D is the core-hotspot distance and R is the hotspot radius,and the two?uxes are mea-sured at the required frequency.)The observed core?ux density of3C123at1011Hz is about40mJy,though it may be variable (Looney&Hardcastle2000),so that isotropic radiation from the core cannot provide the required photon density.However,if the core emission at this frequency is beamed,the hotspot will see the core as having a higher luminosity than the one we observe.We assume the most favourable case for this model of no misalignment
between the pc-and kpc-scale jet,though such good alignment is
not often observed,and we neglect effects due to the?nite angle subtended by the hotspot at the nucleus,which may be signi?cant.
The ratio of required to observed?ux,R(R≈2500for rough
equality of predicted SSC and IC?ux densities),then constrainsβ, the bulk speed in the nucleus,andθ,the angle of the core-hotspot
vector to the line of sight:
(1?β)?(2+α)+(1+β)?(2+α)
sin2θwhere the term(1?cosθ)?(1+α)approximately corrects for the
anisotropic nature of the resulting IC emission(e.g.Jones,O’Dell &Stein1974),and the core is treated as a two-sided jet with a
power-law spectrum;the sin2θterm incorporates the effects of pro-jection.If we assume a core spectral indexα=0.5,then we can obtain R≈2500for plausibleβ,corresponding to bulk Lorentz
factors～4–10,if the source is within50degrees of the plane of
the sky.(In reality,this value forαis probably an overestimate, since the1011Hz photons seen by the nucleus will be blueshifted
from lower frequencies where core spectra are typically?at,α≈0.
Lowerαrequires higherβ.)As no jet has been detected in3C123 and its optical emission lines are weak,the angle to the line of sight
is not constrained,and so we cannot rule out a contribution to the
X-ray emission from nuclear IC scattering.However,a contribution at approximately the same level as the SSC emission would not af-
fect the conclusion that the hotspot is close to equipartition;in fact,
it might account for some of the difference between the equipar-
tition prediction and the observed1-keV?ux density.But if nu-clear IC emission dominates the observed X-rays,then the hotspot
could have a magnetic?eld higher than the equipartition value,and
this cannot be ruled out by the present observations.For example, R=2.5×104,corresponding to an IC emissivity ten times the SSC value,can be obtained for a bulk Lorentz factor≈6if the
source is close to the plane of the sky andα=0.5,and would require B≈4B eq.Such a model requires a coincidence to explain
the similarity of the observed emissivity to that predicted by the
simple SSC model with a near-equipartition?eld.
In carrying out the SSC calculations we have assumed that the
hotspots are homogeneous,that they contain no relativistic protons,
and that the small-scale?lling factor is unity.From the MERLIN maps of H97we know that the hotspots do have internal structure on100-pc scales,although the variations in surface brightness are not very large;there is no evidence for?lamentary structures of the kind seen in radio lobes.As discussed in Hardcastle&Wor-rall(2000),the general effect of a?lling factor less than unity is to increase the SSC emissivity,but the results are dependent on the ge-ometry of the synchrotron-emitting regions,particularly if the low ?lling factor is a result of a spatial variation in electron density in a relatively constant magnetic?eld.Our present results may be taken as evidence against low?lling factors in the hotspots,as such?ll-ing factors would require coincidences to produce X-ray emission at the observed levels.Similarly,if the particle population is ener-getically dominated by non-synchrotron-emitting particles such as relativistic protons,it is a coincidence that the energy density in magnetic?eld corresponds so closely to that in the synchrotron-emitting electrons.
Although SSC emission is a required process,we cannot rule
out the possibility that the magnetic?eld strength is much greater than the equipartition value and that some other process happens to produce X-ray emission at a level consistent with the SSC model.
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Chandra detection of3C1235
One such process which we have already discussed is the IC scat-tering of nuclear photons.Some other simple models can be re-jected.Thermal bremsstrahlung can be ruled out by the compact structure seen with Chandra and the?at X-ray spectrum,while,as shown in Fig.3,IC scattering of the microwave background is two orders of magnitude too weak to be responsible for the observed emission.But more speculative models remain possible.For ex-ample,we cannot rule out the possibility that the X-ray emission is synchrotron radiation from an arbitrarily chosen population of electrons.This model has been invoked to explain some other X-ray hotspots(e.g.Harris et al.1999)in which the IC or SSC models do not seem to work well.In the case of3C123,it requires a coin-cidence to explain the close similarity between the observed X-ray emission and the predictions of the SSC model.
One model which makes quantitative predictions about the origin and properties of this second population of electrons is the proton-induced cascade(PIC)model of Mannheim,Kr¨u lls&Bier-mann(1991).In this model,the high-energy electrons which pro-duce X-ray synchrotron emission are the end result of photome-son production on a population of ultra-relativistic protons,through pion decay and pair production.If protons are present in the jet, they should undergo shock acceleration in the hotspot;there is some evidence for high-energy protons in the lobes of FRII sources (Leahy&Gizani1999,Hardcastle&Worrall2000).Mannheim et al.considered the hotspot of3C123and predicted that the PIC pro-cess would dominate over SSC if protons were highly energetically dominant in the hotspot and in equipartition with magnetic?elds at the level inferred by Meisenheimer et al.(1989),somewhat higher than our equipartition?elds.Their predicted X-ray?ux at1keV is～30nJy,nearly a factor10higher than the observed value, so a model with extreme proton dominance does not seem to be consistent with the data.However,if protons have energy densities closer to those of the electrons,we cannot rule out an origin from PIC-generated electrons for some or all of the observed X-rays; the spectra of the two processes are not distinguishable with our data.Once again,though,a moderate contribution from the PIC process would not affect our conclusions regarding the closeness of the hotspot to equipartition,while a model in which PIC was responsible for all the emission would require?ne-tuning of the energy fraction in?elds,electrons and protons,and so seems less plausible than the simple SSC model.
6CONCLUSIONS
The X-ray emission from the E hotspot of3C123is consistent with a SSC model,with the inferred magnetic?eld strength close to the value predicted from equipartition of energy between the magnetic ?eld and the synchrotron-emitting electrons.Other models are pos-sible,but require coincidences to explain the closeness of the X-ray ?ux density and(in some cases)the photon index to the SSC predic-tion.This reinforces a conclusion already drawn from observations of Cygnus A and3C295that the magnetic?eld strengths in typ-ical hotspots are near their equipartition values.Although there is still no a priori reason to expect equipartition,it is now very likely that it is achieved in the hotspots of at least some fraction of the source population.Observations of inverse-Compton scattering of microwave background photons from radio lobes may tell a simi-lar story(Feigelson et al.1995;Tsakiris et al.1996;Tashiro et al. 1998).Even the deviations from equipartition reported by Tashiro et al.require a magnetic?eld strength only a factor～2below the equipartition value.However,some X-ray detections of hotspots (e.g.3C120,Harris et al1999;Pictor A,Wilson,Young&Shop-bell2001)are at a level much too bright to be consistent either with synchrotron emission(from the electron population responsible for the radio and optical synchrotron radiation)or with SSC at equipar-tition.More observations are necessary to demonstrate that Cygnus A,3C295and3C123are typical of most radio sources.We will report on the results of scheduled Chandra observations of further hotspot sources in a future paper. ACKNOWLEDGEMENTS
We thank all those involved with the design and operation of Chan-dra for doing such an excellent job,and particularly the staff of the Chandra X-ray Center for their help with data analysis.We are grateful to an anonymous referee for a careful reading of the pa-per and constructive suggestions.The National Radio Astronomy Observatory Very Large Array is operated by Associated Universi-ties Inc.,under co-operative agreement with the National Science Foundation.
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