Low-frequency constraints on the spectra of the lobes of the microquasar GRS 1758-258

0.11 MB·English

by  M. J. Hardcastle

Astronomy&Astrophysicsmanuscriptno.1735ms February2,2008
(DOI:willbeinsertedbyhandlater)
Low-frequency constraints on the spectra of the lobes of the
−
microquasar GRS 1758 258
M.J.Hardcastle
1 DepartmentofPhysics,UniversityofBristol,TyndallAvenue,BristolBS81TL,UK
2 School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
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([email protected])
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0
Thisversionsubmitted21stSeptember2004:accepted22ndDecember2004
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n Abstract.Ipresentlow-frequencyobservationsofthelobesofthemicroquasarGRS1758−258,which,togetherwithearlier
a
radio observations, provide the best constraints yet on the lobe spectra, and show that they are consistent with the electron
J
energyindicesexpectedfromstandardshockaccelerationmodels.Thereisnostatisticallysignificantevidenceforsynchrotron
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ageingalongthelengthofthelobes,whichisconsistentwithexpectationifthemagneticfieldstrengthisclosetotheminimum-

energyvalue.IftheenvironmentofthelobeshastypicalISMproperties,thelobeswilldissipateontimescalesofthousandsof
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v yearsorless,longbeforesynchrotronageingcanbecomesignificant.
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6 Keywords.stars:individual:GRS1758−258–stars:winds,outflows–radiocontinuum:stars
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1
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1. Introduction thelobedobjects,ithasbeendifficultsofartomakequantita-
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tive statements about their dynamics. In this paper I present
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Galactic‘microquasar’sources,drivenbyrelativisticoutflows
/ new observations that allow more definite statements to be
h fromclose to theaccretiondiscsofneutronstar orblackhole madeaboutGRS1758−258.
p
binaries(Mirabel&Rodr´ıguez1999)areofgreatinterest,not
-
o just for the insights they provide into the properties of ac-
r creting binary systems but as analogues of the much more GRS1758−258isabrightpersistenthardX-raysourcely-
t
s powerful and distant extragalactic radio sources, radio galax- ing 5 degrees away from the Galactic centre, originally dis-
a
: ies and radio-loud quasars. About 10% of known X-ray bi- covered with the SIGMA instrument on Granat (Sunyaev et
v nary systems have detected radio emission which may be as- al.1991).(ThroughoutthispaperIshallfollowpreviouswork
i
X sociated with jets, and a significant fraction of these show, inadoptingadistanceof8.5kpctothesource.)Becauseofthe
r or have at some stage shown, clear jet-related extended ra- highextinctiontowardsGRS1758−258intheoptical,searches
a dio emission (Mirabel & Rodr´ıguez). However, there is one for the companion star have been carried out in the infrared
crucialdifference between microquasarsand their extragalac- (e.g. Mart´ı et al. 1998,Rothstein et al. 2002):the most likely
tic counterparts. Essentially all powerful AGN-related radio counterpart,basedonradioandX-raypositions,isaK-III-type
sources,whenobservedatsufficientdynamicrange,showex- giant star. The associated extended radio structure was origi-
tended ‘lobes’ of radio emission, which are thoughtto be the nally discovered by Rodr´ıguez, Mirabel & Mart´ı (1992) and
repository for a significant fraction of the jet’s overall lumi- subsequentlyimagedinmoredetailattwofrequenciesbyMart´ı
nosityintegratedovertheradiosource’slifetime(e.g.Scheuer et al. (2002:hereafterM02).M02 used the fact thatthe spec-
1974)and which very often dominatethe radio luminosity of tralindicesbetween5and8.4GHzwereconsistentwithbeing
thesourcebymanyordersofmagnitude.Bycontrast,thereare steep (α > 0, where α is defined throughoutthe paper in the
only two known microquasars that have so far been found to sense thatfluxS ∝ ν−α) to arguethatthelobeemission was
exhibit persistent lobe-like radio emission: the two Galactic- definitely optically thin synchrotron radiation. The new radio
centre black hole candidates 1E 1740.7−2942(Mirabel et al. observations presented in this paper, combined with the data
1992) and GRS 1758−258, the subject of this paper. Heinz of discussed by M02, providemuchtighter constraintsonthe
(2002)hasarguedthatthisdifferencearisesbecauseofthe(rel- radiospectralpropertiesofthelobes,andallowmetodiscuss
atively) low-density environments generally inhabited by mi- theirimplicationsforthedynamicsoftheradiosource.
croquasars,whichmeanthatanylobesformedhaveshortlife-
times againstadiabaticlosses; this mightsuggestthatthere is
something special about the environmentsof the sources that Throughout the paper errors quoted are 1σ, unless other-
showlobes.However,becauseoftheverylowradiofluxfrom wisestated.
2 M.J.Hardcastle:LobesofGRS1758−258
Fig.1.GreyscaleimagesoftheGRS1758−258lobesatthethreefrequenciesusedinthepaper:1.4GHz(left),4.9GHz(centre)
and8.4GHz(right).Allthreeimageshavethesameresolution,9.2×5.8arcsec.Thegreyscalerangesfromthe2σlevelofthe
off-sourcenoiseto0.5mJybeam−1 (1.4GHz),0.2mJybeam−1 (4.9GHz)and0.15mJybeam−1 (8.4GHz).Boxesshowthe
extractionregionsusedforthefoursourcecomponentsdiscussedinthepaper.ComponentCisidentifiedwiththeX-raysource.
M.J.Hardcastle:LobesofGRS1758−258 3
2. Observationsandanalysis Table1.Fluxdensitiesmeasuredforthedifferentcomponents
ofGRS1758−358
GRS 1758−258 was observed twice with the NRAO Very
LargeArray(VLA)inBconfiguration,usingtwostandardob- Component Fluxdensity(mJy)
serving frequencies (1385.1 and 1464.9 MHz) with a band- (1.4GHz) (4.9GHz) (8.4GHz)
width of 50 MHz at each frequency. The first observation, A 1.51±0.23 0.80±0.09 0.59±0.10
madeon2002August11,wasbadlycompromisedbyradiofre- B 2.35±0.25 1.25±0.09 1.01±0.11
(Bnorth) 1.04±0.18 0.68±0.07 0.49±0.07
quencyinterference,andaccordinglyI re-observedthesource
(Bsouth) 1.31±0.18 0.56±0.07 0.52±0.07
with the same observational parameters on 2004 Jan 13. The
C 0.52±0.05 0.19±0.03 0.23±0.03
total usable on-source integration time was about 3h. 3C286
D 0.37±0.05 0.14±0.02 0.08±0.03
wasusedastheprimaryfluxcalibratorandthephasewascal-
ibratedusingobservationsofthenearbysource1751−253ev-
ery15minutes.Thedatawerecalibratedinthesoftwarepack-
tabulatedbyM02:thisseemslikelytobebecauseIhaveused
age AIPS in the standard manner, and the task IMAGR was
larger measurement regions, in an attempt to measure all the
usedtoproduceimagesofthesourceataneffectivefrequency
lobe-relatedflux.
of 1425 MHz. Because there was no evidence for significant
Thefluxdensitieswerethenfittedwithapowerlawinfre-
changesinthestructureofthesourceoverthe16-monthinter-
quency using standard least-squares fitting (Fig. 2). The lobe
valbetweenobservations(seebelowformorediscussionofthis
regions(A and B) are bothwell fitted with a powerlaw, with
point)Icombinedthetwoobservationstomakeafinalimage. spectralindicesαof0.52±0.12and0.49±0.08respectively,in
Theoff-sourcenoiseonthisimage(35µJybeam−1)wassig-
good agreementwith the expectationfromsynchrotronradia-
nificantlyabovethe expectedthermallevel, butwas sufficient
tionfromapopulationofparticlesacceleratedatastrongshock
forasignificantdetectionofallthepreviouslyknowncompo-
(α = 0.5).Thereisnoevidenceforanyhigh-frequencyspec-
nentsoftheradiosource(Fig.1).
tral steepening. The central source, C, is not well fitted with
Inordertomakereliablemulti-frequencymeasurements,I a power-law model because of the upturn in the flux density
retrieved the data discussed by M02 from the VLA archives. between5 and8 GHz,butthismaywell bedueto the effects
Thisconsistsof9observationsatCandXbandsintheCcon- ofvariability(seesection3.1below).ComponentDiswellfit-
figurationoftheVLAtakenbetween1997August03and1997 ted witha powerlawof slope0.81±0.15,whichwouldbea
August24 as partof a multi-wavelengthvariabilitycampaign plausiblevalueforabackgroundradiogalaxy.
(see Lin et al. 2000 for the observational details). Following Finally,inviewofthedifferencesbetweentheimagesinL
M02,IcalibratedeachoftheCandX-banddatasetsseparately andCband,suggestingthatthesourceismorestronglyedge-
andconcatenatedthemtomakeimagesateffectivefrequencies brightened at higher frequencies, I measured fluxes for two
of4860and8460MHz.Theuv-planecoverageofmynewL- sub-regionsof componentB, obtained by dividing the region
banddataiswellmatchedtothatofthe1997C-banddata,but shown in Fig. 1 into two equal halves. The fitted spectral in-
theX-banddataareobviouslynotaswellmatched;atthetime dices for these two regions are 0.40±0.12 and 0.55±0.10.
ofwriting,therearenopubliclyaccessibleD-configurationX- Thereisthuslittlestatisticallysignificantevidenceforspectral
bandobservationsofusefullengththatcouldbeusedtoprovide indexvariationalongthelengthofthenorthernlobe.
short baselines at 8 GHz. Accordingly, I elected to image all
threedatasetswithashortestbaselineof0.7kλ,matchedtothe
shortestbaselineofthe8-GHzdataset,toensurethatthesame 3. Discussion
largestangularsizewassampledbyeach.Asthisshort-baseline
3.1. Source variability
cutoffhassignificanteffectsonspatialscalesabove2.5arcmin,
itshouldnotaffecttheimagesproduced.Theweightingofthe Animportantquestioniswhethertheradiosourcecomponents
uvplanewastaperedinimagingtheX-banddatatoproducea havevariedontimescalesofyears,and,inparticular,whether
similarbeamarea,andprimarybeamcorrectionwasappliedto therecouldhavebeensignificantvariabilitybetween1997and
boththeC-andX-bandimagesusingtheAIPStaskPBCOR. 2002/2004, the respective epochs of the C and X band mea-
I then measured flux densities from four regions of the surementsandmynewL-bandmeasurements.Strongvariabil-
source,correspondingto the componentsdenotedVLA A, B, itywouldinvalidatethespectralindexresultspresentedabove.
C and D by Rodr´ıguez et al. (1992). A is the south lobe, B The images show no evidence for motion of the lobes, and
the north lobe, C is coincident with the Chandra position of this is also the case for the 1992 and 1997 C-band images
theX-raysource(Heindl&Smith2002)andseemslikelytobe (M02). There is no detectable variability in the 1.4-GHz lobe
analogoustothe‘core’ofanextragalacticradiosource(seebe- fluxes (or the fluxes of the other components) between 2002
low)whileDisthoughttobeanunrelatedobject.Fluxdensities and 2004, but the errors are comparatively large because the
weremeasuredinrectanglesaroundeachcomponentusingthe 2002dataarepoor:M02commentthattheywereunabletode-
AIPSverbIMSTAT.Theregionsusedtoextractthefluxesare terminewhethertherewasvariabilitybetween1992and1997.
showninFig.1,andthefluxdensitieslistedinTable1.Errors WeknowthatcomponentCisvariableoncomparativelyshort
onthefluxdensitiesweredeterminedfromtheoff-sourcenoise timescales(Rodr´ıguezetal.1992)althoughtherewaslittleev-
ineachimageinthestandardway.Notethatthefluxdensities idenceforstrongvariabilityinthemonitoringcampaignofLin
for C and X bands are systematically higher than the values et al. (2000): if the L-band flux in 1997 was consistent with
4 M.J.Hardcastle:LobesofGRS1758−258
nT;theminimumlobeenergydensityis7×10−12Jm−3,and
theminimumpressurein thelobeis2×10−12 Pa.Themini-
mumtotalenergystoredinthelobeis2×1038 J,comparable
totheearlierestimatebyRodr´ıguezetal.(1992).
If we assume that the dominant loss mechanism for the
electrons is synchrotronradiation1, then standard results (e.g.
Leahy1991),assumingthatparticleaccelerationinitiallypro-
ducesan electronenergyspectrumthat extendsto well above
theenergiesbeingobservedatGHzfrequencies,showthat,for
theminimum-energymagneticfieldstrength,theeffectsofage-
ing will not become apparent at 8 GHz until 4 × 106 years,
afterparticlesin thatfield werelast accelerated.(Thesimilar-
ity between this and the ‘radiative loss’ timescale quoted by
Heindl&Smith(2002)iscoincidental.)Sincethesetimescales
are verymuchlongerthan the characteristictimescales ofthe
jet source, we cannotexpectto be able to use spectral ageing
at GHz frequencies as a probe of lobe dynamics, as is often
done for extragalacticradio sources (e.g. Alexander& Leahy
1987) unless the magnetic fields are very much stronger than
theminimumenergyestimate.Inextragalacticsources,thereis
independentevidencefrominverse-Comptonemissionthatthe
field strengths are close to the minimum-energy values (e.g.
Fig.2.Thefluxdensitiesandbestpower-lawfitsofthecompo-
nentsofGRS1758−358.Thesolidlinesshowthebest-fitting Hardcastleetal. 2002)butwehavenosuchindependentcon-
straintsinmicroquasars,noraretheylikelytobeeasytoobtain.
linesandthe90%confidencelimitsontheslope.
However, the long synchrotron lifetimes are consistent with
theobservedlackofanysignificantspectralgradientalongthe
northernlobe.
theflatspectralindextheymeasuredbetween4.9and8.4GHz,
however,thenitmusthaveincreasedbya factor2–3between
1997and2002/4.Variabilitywouldcertainlyexplainthepecu- 3.3. Dynamics andexpansion
liarspectrumofthiscomponent.ComponentDappearstohave
Ibeginthissectionbynotingthatitisnotclearwhetherthere
varied between 1992 and 1997 (Lin et al.), which is perhaps
is continuing energy supply to the lobes of GRS 1758−258.
surprisinginviewofitssteepspectrum.Intheabsenceofany
TheobservedradiovariabilityincomponentCsuggestsongo-
detectedvariabilityinthelobes,Iwillassumeinwhatfollows
ing jet activity, but this may be at a very low level. The long
that the measured spectra for the lobes are valid. Data (at the
lifetimesofthelobesderivedabovemeanthatwe cannotrule
timeofwriting,stillproprietary)existintheVLAarchivethat
outthepossibilitythattheyaretheresultofanoutburstofef-
shouldallowadefinitiveanswertothisquestion.
ficient jet activity at some time in the past (to be determined,
but<∼106yearsago).AsHeindl&Smith(2002)pointout,the
3.2. Synchrotron lobes entirestoredenergyofthelobescanbesuppliedinarelatively
shorttime(afewyears)iftheaccretionluminosityistappedef-
Theagreementbetweenthemeasuredlobespectralindicesand
ficiently.Forthisreason,Iconsiderbelowmodelsofthelobes
the standard spectral index for shock-acceleratedsynchrotron
in which the energysupply fromthe jet can be neglectedand
emissionisstriking.Itallowsustoassumeasynchrotronmodel
thedynamicsofthelobesaredrivenbytheirinternalenergy.In
withagooddealofconfidence,butitalsosuggestsstronglythat
thiscasetheevolutionoftheradiofluxdensityofthelobesis
the lobes are generated in a manner similar to that of power-
determined by adiabatic expansion;we expect flux density to
fulextragalacticradiosources.Alow-frequencyspectralindex
goasr2+4α (e.g.Longair,Ryle&Scheuer1973).
of 0.5 is often observed in the hotspots of such objects (e.g.
The minimumpressure in the lobesis very similar to that
Meisenheimeretal.1989).
of the dense molecular material found near the Galactic cen-
If we adopt a synchrotron model, then we can obtain re-
tre (e.g. Martin et al. 2003), so that the lobes could be con-
vised versions of the standard derived quantities. I model the
fined(andthereforenotevolving)iftheylayinsideamolecular
electronspectrumasapowerlawwithenergyspectralindexof
cloud. A molecular cloud is known to lie in the direction of
2,correspondingtoα = 0.5,andassumethattheelectronen-
thesource(Linetal.2000)butthecomparativelylowabsorb-
ergiesextenddowntoγ =1anduptoγ =105,andthatthere
are no relativistic protons.The derivedenergydensity is only
1 The magnetic field energy density, assuming minimum-energy
logarithmicallydependentontheassumedlowandhigh-energy
conditions,appearstodominateoverthephotonenergydensityfrom
cutoffsintheelectronspectrum.TheNlobe(B),whichImodel thecentralstarandX-raysourceatthedistance(∼2pc)ofthelobes.
asanellipsoidwithmajoraxis55arcsecandminoraxis25arc- An estimate of the overall photon energy density near the Galactic
sec,thenhasaminimum-energymagneticfieldstrengthof2.7 centreishardtoobtain.
M.J.Hardcastle:LobesofGRS1758−258 5
ingcolumn(2×1022 cm−2:Linetal.andreferencestherein) molecular clouds, and so the lobes could be confined by
would require the source to lie relatively close to the surface suchasystem.Ifthelobeslieinalessdenseenvironment
of the cloud. This is by no means impossible, and the simi- theywillhavealifetimeoftheorderofatmostthousands
larity between the minimum pressure and the available exter- of yearsbeforeadiabaticlosses renderthem undetectable,
nalpressureisinteresting.However,iftheexternalpressureis assumingthereisnofurtherenergysupplyfromthejet.
lowerthanthis,thelobescannotbeconfined.Indiscussingthis
case we take (forsimplicity, ratherthan outof anybelief that Acknowledgements. I thank the Royal Society for a Research
Fellowship,andChristianKaiserfororiginallypointingouttomethe
theseconditionsobtainneartheGalacticcentre)afiducialex-
ternal density n0 ≈ 1 cm−3. A lower limit on the expansion existenceofdouble-lobedmicroquasars.J.Mart´ıkindlysuppliedme
withaversionofthemapofMart´ıetal.(2002)insupportofthiswork.
speed at the presenttime, using ram pressure balance,is then
IamgratefultoJohnBensonforhelpinretrievingthearchivalVLA
around 35(n/n0)−1/2 km s−1. Assuming expansion accord- data.Ithanktheanonymousrefereeforcommentsthatallowedmeto
ing to a Sedov law, this would give a timescale for a signifi-
improvethepapersignificantly.
cantdropinfluxdensity,saybyafactor∼2,of104(n/n0)1/2 The National Radio Astronomy Observatory is a facility of the
years—lessifthelobepressureexceedstheminimumvalue. National Science Foundation operated under cooperative agreement
Adiabaticexpansion,notsynchrotronlosses,woulddetermine byAssociatedUniversities,Inc.
the timescale on which the lobe can be detected in this sce-
nario(andthisistrueforanyplausiblemagneticfieldstrength
References
in the lobes). This would again be in contrast to the situa-
tionforextragalacticsources,wheretheminimum-energypres- Alexander,P.,Leahy,J.P.,1987,MNRAS,224,1
suresinlobesroutinelyfallbelowtheexternalthermalpressure Croston,J.H.,Birkinshaw,M.,Hardcastle,M.J.,Worrall,D.M.,2004,
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Crostonetal.2004).
Heindl,W.A.,Smith,D.M.,2002,ApJ,578,L125
Thelobesdonotappeartobemovingrelativistically;M02
Heinz,S.,2002,A&A,388,L40
estimate an expansion speed < 0.3c. If we assume that the
Leahy,J.P.,1991,inHughesP.A.,ed.,BeamsandJetsinAstrophysics,
lack of detectable variability between 2002 and 2004 implies
CambridgeUniversityPress,Cambridge,p.100
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give rise to a decrease in flux density of a factor 1.5, which Mart´ı, J., Mereghetti, S., Chaty, S., Mirabel, I.F., Goldoni, P.,
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4. Conclusions
Mirabel,I.F.,Rodr´ıguez,L.F.,1999,ARA&A,37,409
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Rothstein, D.M., Eikenberry, S.S., Chatterjee, S., Egami, E.,
theaccuratemeasurementofthespectralindexofthelobesof
GRS1758−258presentedhereareasfollows: Djorgovski,S.G.,Heindl,W.A.,2002,ApJ,580,L61
Scheuer,P.A.G.,1974,MNRAS,166,513
Sunyaev,R.,Gilfanov,M.,Churazov,E.,Pavlinsky,M.,Babalyan,G.,
1. The good agreement between the spectral indices and
Dekhanov,I.,Kuznetsov,A.,Grebenev,S.,Yunin,S.,Yaburenko,
the expected value for synchrotron emission from shock-
N., Cordier, B., Lebrun, F., Laurent, P., Ballet, J., Mandrou, P.,
accelerated electrons strongly reinforces earlier conclu-
Roques,J.P.,Vedrenne,G.,1991,SovietAstron.Lett.,17,50
sions that the radio emission process is synchrotron radi-
ation.
2. The lack of significantdifferencebetween the spectralin-
dices at differentregions in the N lobe suggests that syn-
chrotronlosses arenotimportantatGHz frequencies:this
isconsistentwithexpectationifthemagneticfieldstrengths
areclosetotheminimum-energyvaluesandthesourcehas
anage<∼106years.
3. The lobe minimum pressures on a synchrotron model are
comparable to the internal pressures in Galactic centre