Quasar jet emission model applied to the microquasar GRS 1915+105

0.66 MB·English

by  M. Turler

Astronomy&Astrophysicsmanuscriptno.Fl182 February2,2008
(DOI:willbeinsertedbyhandlater)
Quasar jet emission model applied to the microquasar
GRS1915+105
M.Tu¨rler1,2,T.J.-L.Courvoisier1,2,S.Chaty3,4,andY.Fuchs4
1 INTEGRALScienceDataCentre,ch.d’Ecogia16,1290Versoix,Switzerland
4 2 ObservatoiredeGene`ve,ch.desMaillettes51,1290Sauverny,Switzerland
0 3 Universite´Paris7,2placeJussieu,F-75005Paris,France
0 4 Serviced’Astrophysique,DSM/DAPNIA/SAp,CEA/Saclay,F-91191Gif-sur-Yvette,Cedex,France
2
n Received18December2003/Accepted14January2004
a
J Abstract. Thetruenatureoftheradioemittingmaterialobservedtobemovingrelativisticallyinquasarsandmicroquasars

4 isstillunclear.Themicroquasarcommunityusuallyinterpretsthemasdistinctcloudsofplasma,whiletheextragalacticcom-
1 munityprefersashockwavemodel.HereweshowthatthesynchrotronvariabilitypatternofthemicroquasarGRS1915+105
observed on 15 May 1997 can be reproduced by the standard shock model for extragalactic jets, which describes well the

1 long-termbehaviourofthequasar3C273.Thisstrengthenstheanalogybetweenthetwoclassesofobjectsandsuggeststhat
v thephysicsofrelativisticjetsisindependentofthemassoftheblackhole.ThemodelparameterswederiveforGRS1915+105
5
correspondtoaratherdissipativejetflow,whichisonlymildlyrelativisticwithaspeedof0.60c.Wecanalsoestimatethatthe
7
shockwavesforminthejetatadistanceofabout1AUfromtheblackhole.
2
1
0 Keywords.radiationmechanisms:non-thermal–stars:individual:GRS1915+105–infrared:stars–radiocontinuum:stars
4
0
/ 1. Introduction 2002). This discovery, by strengthening the analogy between
h
p galacticandextragalacticobjects,risesthequestiononthetrue
Microquasars are high-energy binary systems exhibiting jets
- natureofthesynchrotronemittingmaterial.Isthemicroquasar
o with apparent superluminal motion suggesting that they are
plasmoid model or the quasar shock wave model the correct
r the miniature replicates in our Galaxy of the distant quasars
t interpretation? Atoyan&Aharonian (1999) showed that ex-
s (Mirabel&Rodriguez1998).GRS1915+105wasthefirstmi-
a panding clouds of relativistic plasma cannot describe the ob-
v: croquasardiscovered(Mirabel&Rodriguez1994)andcanbe servations unless there is an in situ electron acceleration pro-
considered as an archetypical object of its class. Its radio
i cess.Analternativemodelofavariablecontinuousjetalsofails
X emission can be divided into three distinct states: a plateau
to explain the observedflux oscillations in GRS1915+105as
r jet, pre- and post-plateau flares and radio oscillation events it leads to unphysically large electron densities (Collinsetal.
a
(Klein-Woltetal. 2002). The flares are the most powerful
2003). There is therefore growing evidence that the emitting
events. Their emission arises from two emitting blobs ob-
electronsmustbereacceleratedalongthejet,asitisthecasein
served to move relativistically in opposite directions up to
ashockwavemechanism(Kaiseretal.2000).
about 3000AU from the core (Fenderetal. 1999). This
Tofurthertesttheshockwaveinterpretation,weapplyhere
has been interpreted as evidence for pairs of plasma clouds
to GRS1915+105the same model used to describe the long-
symmetrically ejected on both sides of the accretion disc
termvariabilitybehaviourof3C273fromsubmillimetertora-
(Rodr´ıguez&Mirabel 1999). This plasmoid ejection model
diowavelengths(Tu¨rleretal.2000).Thismodelderivedfrom
is also used to explain the less powerful oscillation events
the model of Marscher&Gear (1985) describes analytically
arising in a shorter jet extending only up to ∼ 30–50AU
the evolution of synchrotronemission resulting from a shock
(Dhawanetal. 2000). This interpretation is supported by the
wave moving down a relativistic jet. Compared to the model
observationofsimultaneousX-raydips,suggestingthatmate-
of vanderLaan (1966) for an expanding plasma cloud it has
rialfromthedisappearinginneraccretiondiscisejectedfrom
the advantage to include the effects of both synchrotron and
the black hole vicinity (Bellonietal. 1997a,b; Mirabeletal.
inverse-Comptonenergylosses.
1998;Eikenberryetal.1998).
Observational evidence for this phenomenon has recently
alsobeenobtainedfortheactivegalaxy3C120(Marscheretal. 2. DataandModel
Send offprint requests to: M. Tu¨rler, e-mail: The data we use here are the radio oscillation events of
[email protected] GRS1915+105 measured on 15 May 1997 (Mirabeletal.
2 M.Tu¨rleretal.:QuasarjetemissionmodelappliedtothemicroquasarGRS1915+105
Fig.1. Modelfittotheinfraredandra-
dio observationsof GRS1915+105on
15 May 1997. The points in the four
panels show the time dependence of
the emission at observed wavelengths
of2.2µm,2cm,3.6cmand6cm,from
top to bottom. The 2.2µm data are
dereddenedusinga K-bandabsorption
of 2.2mag (Chapuis&Corbel 2004).
The continuous red line is the best fit
model, it is the sum of the emissions
fromtheapproachingjet(blueline)and
therecedingjet(greenline)plusacon-
stant flux at 2.2µm fixed at 21.8mJy.
Red dotted lines separate the emission
from distinct events. In the top panel,
the onsettimesofthe outburstsarein-
dicated by arrows and the grey line is
the X-ray light curve in the 2–60keV
bandinunitsof500counts−1.
1998; Dhawanetal. 2000). The Very LargeArray (VLA) ob- metric in both jets. Starting as a sound wave, the disturbance
servationshavetheadvantagetobeverywellsampledatthree willbecomesupersonicatsomepointbecauseofthedecreas-
different radio wavelengths (see Fig 1). The data start with a ingpressurealongthejet.Theresultingshockwavewillprop-
fluxdecreasefollowedbysuccessiveoutburstsspacedbytypi- agate downthe jet and accelerateparticlescrossing the shock
cally1–2hours.Theseoscillationeventsareinterpretedasejec- front. Accelerated electrons will then emit synchrotronradia-
tions in a small jet that was observed simultaneously by the tionintheincreasedmagneticfieldofthecompressedplasma
Very Long Baseline Array (VLBA) (see Dhawanetal. 2000, behindtheshockfront.Theevolutionoftheemittedspectrum
Fig.9).Theirevolutionfromshorttolongwavelengthsisrem- dependson the dominantenergy loss mechanism of the elec-
iniscentoftheobservedbehaviourofthebrightquasar3C273 trons.Justaftertheonsetoftheshock,inverse-Comptonradi-
onatimescaleofyears(Tu¨rleretal.2000). ation is likely to be dominant,then synchrotronlosses should
Additional data from the United Kingdom Infrared become more important and finally adiabatic expansion, due
Telescope (UKIRT) in the 2.2µm infrared band show a flux tothewideningofthejet,willdominatetheenergylosses.As
enhancement which is very likely the synchrotron precursor a consequence, the turnover of the self-absorbed synchrotron
ofthelastradiooutburst(Mirabeletal.1998).Thesimultane- spectrumwillmovefromhighto lowfrequenciesfollowinga
ousX-raymeasurementsbytheRossi X-rayTimingExplorer characteristicthree-stagepath(seeFig.2).
(RXTE)inthe2–60keVbandpresenttwoflaresapparentlyas-
sociated with two consecutiveradiooutbursts.ThisX-ray be-
3. Method
haviourisindeedquitedifferentfromthelightcurveobserved
on 9 September 1997 which displays very rapid oscillations To applythe modeloutlinedaboveto the observedvariability
followed by a clear X-ray dip (Mirabeletal. 1998). A possi- of GRS1915+105 we use a methodology similar to the one
ble interpretation of this difference is that the X-ray flares of usedpreviouslyfor3C273(Tu¨rleretal.2000).Therearehow-
15May1997arenotemittedbytheaccretiondisc,butarisein ever several modifications mainly implied by the specificity
the jet because of inverse-Comptonscattering of synchrotron of GRS1915+105,but includingalso some more generalim-
photons. provements.Animportantdifferencebetweenthemodellingof
The Marscher&Gear (1985) shock model assumes a 3C273 and GRS1915+105 is that for the latter the emission
steadyrelativisticjetflowthatcanbedisturbedbyanyincrease from the receding jet is well observed (Mirabel&Rodriguez
ofthepressureofthejetplasmaorofitsbulkvelocity.Ifthis 1994;Fenderetal.1999;Rodr´ıguez&Mirabel1999)andthus
disturbanceisrelatedtotheaccretiondisc,itislikelytobesym- cannot be neglected. We take this into account by modelling
M.Tu¨rleretal.:QuasarjetemissionmodelappliedtothemicroquasarGRS1915+105 3
To have a more realistic model we replaced by progres-
sivetransitionsthelow-andhigh-frequencybreaksofthesyn-
chrotron spectrum (see Tu¨rleretal. 2000, Fig. 2) and we
smoothedoutthesharpedgesofthethree-stageevolution.The
normalization of the synchrotron spectrum was redefined as
the extrapolationof the optically thin slope, as in the original
Marscher&Gear(1985) model.Finally,weintroduceda new
parameter to allow the frequency ratio ν /ν of the low fre-
h m
quencybreaktotheturnoverofthespectrumtovarywithtime.
Thisallowstodescribeasynchrotronsourcewhichisfirstin-
homogeneous and becomes progressively homogeneous near
thetransitiontothefinaladiabaticexpansionstage,asitseems
tobethecaseinGRS1915+105(seeFig.2a,b).
The model presented here has a total of 34 free parame-
ters. 13 parametersare used to describe completelythe shape
and the evolution of the synchrotronspectrum for an average
outburstintheapproachingandtherecedingjets.Theremain-
ing 21 parameters are used to define the start times and the
specificity of each distinct event. This model is the simplest
jetmodelreproducingwelltheobservations.Itassumesacon-
stantDopplerfactoralongthejetandalineardecreaseB∝R−1
Fig.2. Evolution of the synchrotron emission of an average of the magneticfield B with the jet half-width R, as expected
model outburst in GRS1915+105 on 15 May 1997. The ob- if B is perpendicular to the jet axis (Begelmanetal. 1984).
servedevolutionisdifferentfortheapproachingjet(a,c) and Themodelparametersareconstrainedbyasimultaneousleast-
therecedingjet(b,d).Thegreylinesaresynchrotronspectraat squarefittothe795radioandinfraredmeasurementsofFig.1.
timesspacedby0.2dexina,b,andequalfluxdensitycontours This is achievedby manyiterative fits of small subsets of the
spacedby0.3dexin c,d. Thethick blackline isthe pathfol- 34parameters.TheoverallfitisshowninFig.1.Althoughthe
lowedbytheturnoverofthespectrum.Thetrianglesshowthe adjustmentisnotperfect(reducedχ2 of6.4),theshapeofthe
positionofthetwostagetransitionsforeachoutburstascom- lightcurvesisverywellreproducedbythemodel.
paredto their averageposition (circles).The frequencyof the
fourlightcurvesinFig.1areindicatedbyverticallines. 4. ResultsandDiscussion
TheoverallevolutionofanaverageoutburstinGRS1915+105
eachoutburstasbeingthesumoftwinoutbursts,onearisingin as derivedfromthe observationsof15 May1997isshownin
the approachingjet and the other in the receding jet. The ob- Fig.2a,cfortheapproachingjetandinFig.2b,dforthereced-
servedemissionfromthetwooppositejetsishoweverdifferent ingjet.Theoutburstisassumedtobeintrinsicallythesamein
duetoorientationandrelativisticeffects.Wemodelthisbytwo bothjets,buttheobservedevolutionisdifferentbecauseofori-
parameters,onebeingtheratiobetweentheDopplerfactorsof entation effects. The differentviewing angle changesthe nor-
the emitting materialin the two jets and the second beingthe malization, as well as the slopes of the three-stage evolution.
observeddelaybetweentheonsetofthetwinoutbursts.Inaddi- Thefirsteffectisduetoa differentrelativistic Dopplerfactor,
tion,theangleof66◦betweenthejetaxisandthelineofsight while the second is due to the assumption that the approach-
(Fenderetal.1999)issuchthattheshockislikelytobeviewed ingshockwaveisviewedsideways,whiletherecedingoneis
sideways for the approaching jet and from behind for the re- viewedfrombehind.
cedingjet(Marscheretal.1992).Thiswastakenintoaccount Themodelparametersdescribingthejetpropertiessuggest
inthemodellingandresultsinadifferentobservedevolutionof thatthejetwhenobservedwasratherdissipativeandthiscould
thetwojets(seeFig.2). explainwhyitdidnotpropagateveryfar(Dhawanetal.2000).
Todefinethespecificityofeachoutburst,weallowtovary Afirstindicationforthisisthenon-linearincreaseoftheradius
fromoneeventtotheotheronlytwooutofthethreeparameters R ∝ Lr of the jet opening with distance L along the jet. The
weusedfor3C273(Tu¨rleretal.2000).Thesetwoparameters bestfitvalueofr = 1.52accountsfortherapidchangeofthe
are the normalization K of the electron energy distribution spectral turnover frequency with time (see Fig. 2c,d). It sug-
on
(N(E) = KE−s)andthemagneticfieldstrength B attheon- geststhattheinnerjetofGRS1915+105ismorelikeatrumpet
on
set ofthe shock.The bulkDopplerfactorD of the emitting than a cone. This shape could result from a pressure gradient
on
materialisassumedheretoremainconstant.Anothersimplifi- of the material surroundingthe jet or from divergingexternal
cationisthattheredoesnotseemtobeaconstantradioemis- magnetic field lines channeling the jet plasma. Alternatively,
sioncomponentinGRS1915+105similartothehotspotatthe itmightbethesignatureofadeceleratingjetflow.Totestthis
farendof3C273’sjet.Furthermore,thestartoftheradiolight hypothesis,werelaxtheconstraintofaconstantDopplerfactor
curvesofGRS1915+105canbesimplymodelledbythefinal alongthejet.Thisresultsinamodelwithaslighttendencyto-
decayofanoutburstwithaverageproperties. wardsdeceleration,buttheimprovementofthefitisnotsignif-
4 M.Tu¨rleretal.:QuasarjetemissionmodelappliedtothemicroquasarGRS1915+105
icantduetothelackoffurtherinfraredorsubmillimeterobser- tionssuchthattheiremissionisshiftedbyordersofmagnitude
vations.Therearehoweverotherindicationssuggestingadis- towards lower frequencies, longer time scales and enhanced
sipative jet flow. In particular the fact that the obtained value fluxes.
of s = 2.02 for the index of the electron energy distribution
N(E) = KE−s is close to 2. This meansthat there is roughly
5. Conclusion
asmuchtotalenergycarriedbyhigh-energyelectronsthanby
low-energyelectrons.For s < 2,thestrongradiativelossesof This work shows thatthe microquasarGRS1915+105has, at
thedominanthigh-energyelectronswouldleadtoasubstantial leastduringsomeepochs,avariabilitybehavioursimilartothe
pressuredecreasealongthejetandpreventtheshocktopropa- quasar3C273,whichcanbemodelledbyshockwavespropa-
gatefar(Marscher&Gear1985).Thatsuchradiativelossesare gatinginarelativisticjet.Thisanalogysuggeststhatthephys-
notnegligibleisalsosuggestedbytheslightlysteeperdecrease ical nature of relativistic jets and their emission is notdepen-
(k=3.10)ofthenormalizationK ∝R−k oftheelectronenergy dentonthemassoftheblackholeattheoriginoftheejection.
distribution than expected if the jet flow was really adiabatic Thestudyofjetsinbothquasarsandmicroquasarsistherefore
(k=2(s+2)/3=2.68). complementary. While quasars are bright and their jet struc-
FromtheratiooftherelativisticDopplerfactorsinthe re- ture is relatively well resolved, microquasarshave the advan-
ceding and the approachingjet, we can derive the bulk speed tage of shortervariabilitytime scales and of a less relativistic
oftheemittingregion.Weobtainaratioof0.61,whichcorre- flowallowingustoobservealsotheircounter-jetemission.The
sponds,byassuminganangleof66◦ betweenthejetaxisand ability shownhere to constrain jet emission modelsby multi-
thelineofsight(Fenderetal.1999),toaspeedof0.60c,cbe- wavelength observationsopens new perspectivesfor studying
ing the speed of light. This mildly relativistic value contrasts relativisticjetsinquasars,microquasarsandtentativelyevenin
withthespeedsexceeding0.9cderivedforthegianteruptions gamma-raybursts.
(Fenderetal. 1999; Rodr´ıguez&Mirabel 1999), but is com-
patiblewiththehypothesisofashockviewedsidewaysinthe Acknowledgements. WethankV.Dhawanforkindlyprovidingusethe
approaching jet provided that the jet opening half-angle is of VLBAdata.Y.F.acknowledgesfinancialsupportfromtheCNES.
13◦ at least (Marscheretal. 1992). We can also estimate the
distance from the black hole at which the shock wave forms
References
basedonthetimeintervalbetweentheonsetoftheoutburstin
the two jets. This interval is found to be ∼ 400s and corre- Atoyan,A.M.&Aharonian,F.A.1999,MNRAS,302,253
sponds to a distance from the black hole measured along the Begelman, M. C., Blandford, R. D., & Rees, M. J. 1984,
jetof1.471013cm,byassumingthesameorientationasabove. ReviewsofModernPhysics,56,255
Thisisroughly1AUandisonlyabouttwicetheseparationof Belloni,T.,Mendez,M.,King,A.R.,vanderKlis,M.,&van
thebinarysystem(Greineretal.2001). Paradijs,J.1997a,ApJ,488,L109
Concerningthespecificityofindividualevents,represented Belloni,T.,Mendez,M.,King,A.R.,vanderKlis,M.,&van
bythepointsinFig.2,wenotethattheydomostlydifferinin- Paradijs,J.1997b,ApJ,479,L145
tensityfromtheaverageoutburst.Accordingtothemodel,this Chapuis,C.&Corbel,S.2004,A&A,inpress
isdueto changesofthenormalization K of theelectronen- Collins,R.S.,Kaiser,C.R.,&Cox,S.J.2003,MNRAS,338,
on
ergydistributionattheonsetoftheshock.Theeffectofsignifi- 331
cantchangesofthemagneticfieldstrengthB fromoneevent Dhawan,V.,Mirabel,I.F.,&Rodr´ıguez,L.F.2000,ApJ,543,
on
to the other would lead to a clear scatter of the points along 373
thefrequencyaxis,whichisnotobserved(seeFig.2a,b).The Eikenberry, S. S., Matthews, K., Morgan, E. H., Remillard,
fact that the specificity of individual outbursts is mainly due R.A.,&Nelson,R.W.1998,ApJ,494,L61
to differencesin the normalization K of the electron energy Fender, R. P., Garrington, S. T., McKay, D. J., et al. 1999,
on
distributionistobeexpectediftheoutburstsaredueto anin- MNRAS,304,865
creasedinjectionrateatthebaseofthejet.Thisfindingthere- Greiner,J.,Cuby,J.G.,&McCaughrean,M.J.2001,Nature,
foresupportsthe idea thatthe shockwaves formingin the jet 414,522
ofGRS1915+105aretheresultofinstabilitiesintheinnerac- Kaiser, C. R., Sunyaev,R., & Spruit, H. C. 2000,A&A, 356,
cretiondisc. 975
It is difficult to assess whether a scaled-up version of the Klein-Wolt, M., Fender, R. P., Pooley, G. G., et al. 2002,
shockmodelpresentedherecouldapplytothegianteruptions MNRAS,331,745
of GRS1915+105. Their radio light curve with its very fast Marscher,A.P.&Gear,W.K.1985,ApJ,298,114
rise compared to its slow decay (Dhawanetal. 2000) resem- Marscher,A.P.,Gear,W.K.,&Travis,J.P.1992,inVariability
blestheinfraredmodellightcurvefortheapproachingjet,but ofBlazars,85–101
expandedabout1000timesindurationand100timesinampli- Marscher,A.P.,Jorstad,S.G.,Go´mez,J.,etal.2002,Nature,
tude.Theopticallythinsynchrotronspectrumobservedinthe 417,625
radioduringthedecayofmajoreruptionshasalsoroughlythe Mirabel, I. F., Dhawan, V., Chaty, S., et al. 1998, A&A, 330,
sameslopethantheinfraredspectrumdeducedhere.Thisanal- L9
ogy suggests that giant eruptions could be qualitatively sim- Mirabel,I.F.&Rodriguez,L.F.1994,Nature,371,46
ilar events than those studied here, but with physical condi- Mirabel,I.F.&Rodriguez,L.F.1998,Nature,392,673
M.Tu¨rleretal.:QuasarjetemissionmodelappliedtothemicroquasarGRS1915+105 5
Rodr´ıguez,L.F.&Mirabel,I.F.1999,ApJ,511,398
Tu¨rler,M.,Courvoisier,T.J.-L.,&Paltani,S.2000,A&A,361,
850
vanderLaan,H.1966,Nature,211,1131