Steady jets from radiatively efficient hard states in GRS 1915+105

4 MB·English

by  A. Rushton

Astronomy&Astrophysicsmanuscriptno.14929 c ESO2011
(cid:13)
January27,2011
Steady jets from radiatively efficient hard states in GRS1915+105
A.Rushton⋆12,R.Spencer3,R.Fender45,andG.Pooley6
1 OnsalaSpaceObservatory,SE-43992Onsala,Sweden
2 EuropeanSouthernObservatory,Karl-Schwarzschild-Str2,85748Garching,Germany
e-mail:Anthony.Rushton at eso.org
3 JodrellBankCentreforAstrophysics,SchoolofPhysicsandAstronomy,UniversityofManchester,Manchester,M139PL
e-mail:Ralph.Spencer at Manchester.ac.uk
4 SchoolofPhysicsandAstronomy,UniversityofSouthampton,Southampton,SO171BJ
e-mail:rpf at phys.soton.ac.uk
1
5 AstronomicalInstitute‘AntonPannekoek’,UniversityofAmsterdam,Kruislaan403,1098SJAmsterdam,theNetherlands
1
6 TheUniversityofCambridge,MullardRadioAstronomyObservatory,CavendishLaboratory,J.J.ThomsonAvenue,Cambridge
0
CB30HE
2
e-mail:guy at mrao.cam.ac.uk

n
a Received2010–05–04;accepted2010–09–03
J
5 ABSTRACT
2
RecentstudiesofdifferentX-raybinaries(XRBs)haveshownaclearcorrelationbetweentheradioandX-rayemission.Wepresent

evidenceofacloserelationshipfoundbetweentheradioandX-rayemissionatdifferentepochsforGRS1915+105,usingobservations
]
E fromtheRyleTelescopeandRossiX-rayTimingExplorersatellite.Thestrongestcorrelationwasfoundduringthehardstate(also
H knownasthe‘plateau’state),whereasteadyAU-scalejetisknowntoexist.BoththeradioandX-rayemissionwerefoundtodecay
fromthestartofmostplateaustates,withtheradioemissiondecayingfaster.AnempiricalrelationshipofS Sξ wasthen
h. fittedtodatatakenonlyduringtheplateaustate,resultinginapower-lawindexofξ 1.7 0.3,whichissignriafidicoan∝tlyXh−igrahyerthanin
p otherblackholeXRBsinasimilarstate.Anadvection-flowmodelwasthenfittedto∼thisr±elationshipandcomparedtotheuniversal
- XRBrelationshipasdescribedbyGalloetal.(2003).Weconcludethateither(I)theaccretiondiskinthissourceisradiativelyefficient,
o
evenduringthecontinuous outflow ofacompact jet,whichcouldalsosuggest auniversal turn-over fromradiativelyinefficientto
r
st edffiomciiennattefdorbyallemstieslslaior-nmfarossmbtlhaeckbahsoeleosfathteajcertiatincdalnmotatshseaaccccrreetitoionnrdaitsek(m(˙ec.g≈.v1ia01i8n.v5egr/sse);Coorm(pIIt)onthsecXat-terariynsginfrothmetphleatoeuatuflostwat)e. are
a
[ Keywords.Accretion,accretiondisks–Blackholephysics–X-ray:binaries–ISM:jetsandoutflows

1
v
5 1. Introduction quickly become divorced from the central accretion region as
4 they moveaway and typicallylast for manydaysor weeks. To
9 It hasbeen well establishedthat the formationof astrophysical study the direct coupling between the accretion disk and jet,
4 jets is associated with accretion disks. Powerfuljets have been we must select observations of the compact ‘steady jet’ when
. observedfrom sizes that rangefrom the AGN of extra-galactic
1 it is close to the energy source. As VLBI observations have
quasars,tothemuchsmallerejectaaroundproto-planetarydisks.
0 shown this emission region to be only a few light hours in
Whilst the theory of accretion has been the focus of intense
1 size (Dhawanetal. 2000), changes in the accretion disk may
1 studyformanydecades,thereexistsnosatisfactoryexplanation therefore directly change the steady jet over the time-scale of
: forthisscale-free,invariantrelationshipbetweenaccretiondisks hours.
v
andjets.
i For the first time, observations taken over ten years have
X This paper presents a clear relationship between the accre-
made it possible to clearly study the repetitive trends between
r tion disk and jet formation in one of the most powerful black the X-ray and radio. The results presented in this paper select
a holesin the Galaxy,GRS1915+105.Using the high-resolution
radio and X-ray monitoring observations during the steady jet
resultsandconclusionspresentedinRushtonetal.(2010),acon-
(‘plateau’)state.Adirectrelationshipbetweentheinflowingac-
fident assumption regarding the ‘state’ of the jet can be made
cretionandoutflowjetcanbemadeforGRS1915+105.
using radio and X-ray monitoring observations. As variations
intheX-rayemissionareassociatedwithchangesintheaccre-
tiondisk,couplingtheseradioobservationstosimultaneousX- 1.1.X-rayspectralstatesofGRS1915+105
rayobservationscandirectlyrelatetotheinflow-outflowmech-
anisms around the black hole (reviewed by Fender&Belloni TheX-rayspectraofGRS1915+105canbegeneralisedintotwo
2004). distinctcomponents:asoftblackbodydisk(kT 1 2keV)and
∼ −
It has been clearly established that changes in the X-ray a hard non-thermalpower-law extendingto 100 keV associ-
≥
spectralstate of GRS1915+105are associatedwith superlumi- atedwiththeveryinnerregionor‘corona’oftheaccretiondisk.
nal knots (Fenderetal. 1999); however, large-scale structures Bellonietal. (2000) initially identified 12 distinct X-ray
classeswiththeRXTE-PCAbasedonthelight-curveandcolour-
⋆ ESOfellow colourdiagramofthesource.Fromtheseclasses,theoverallX-
2 A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105
ray spectra can be reduced as a transition between three basic (2000) found a clear relationship in the low-hard state of the
statesknownasstate A, stateB andstate C;these statesrepre- XRBGX339-4betweentheX-rayandradioemission.Theob-
senttheinterchangebetweenthemulti-temperaturediskandthe servedflat(orslightlyinverted)spectrumwassuggestedtobea
power-lawcomponent.StatesA andB correspondtoa soften- compactjetassociatedwiththelow-hardstate.Discreteejections
ergyspectrum,withthedominantX-rayemissioncomingfrom of relativistic plasma are associated with transitional changes
the inner region of a thermal accretion disk with temperatures fromthishardstatetoasofterstateformostradioemittingblack
of 1.8 keV and 2.2 keV respectively. In-contrast, state C hole XRBs (Fenderetal. 2004, 2009). An empiricalnon-linear
∼ ∼
represents the near absence of this inner region and exhibits a relationshipbetweentheradioandX-rayluminositywasfound
dominantpower-lawcomponentintheX-rayemission. byCorbeletal.(2003)as
Migliari&Belloni (2003) performed an analysis of the X-
ray spectra with the RXTE-PCA, by isolating the observations Lradio ∝ LX0.71ra±y0.1, (1)
into states A/B/C. They fitted a blackbody component to the −
energy spectrum and directly related this to the inner radius whilst the source remained in the low-hard state. Galloetal.
(R = D√Ncosθ), where D is the distance of the source, θ (2003),thencompiledalargesampleofquasi-simultaneousra-
in
theinclinationangleofthediskandNthenormalisationofthe dio and X-ray observationsof stellar-mass black hole binaries,
thermalcomponent.Theinnerradiuswasthencomparedtothe confirmingthisrelationshipto beuniversalformostblackhole
overalltemperature,kT .ItwasfoundatthestartofstateCthat XRBs.
in
R wouldrapidlyincreaseinsizeandkT woulddroptoamuch Followingthis,Merlonietal.(2003)andFalckeetal.(2004)
in in
coolertemperature.Thiswasexplainedasthecollapseofthein- independently linked the existence of a ‘fundamental plane’
ner accretion region, leaving a truncated thin disk. During the associated with all accreting black holes, with the parameters
evolutionoftheRin kT relationshipinthestateC,thetempera- Lradio, LX ray and mass M. This relationship applied to a large
turewouldslowlyin−creaseagainandR wouldreduce.Thishas rangeofm−asses, fromsuper-massiveblackholesinextragalac-
in
beensuggestedtobethere-fillingoftheinnerregionofthethin ticAGNsandtheGalacticcentre,SgrA∗, tostellarmassblack
disk,untilthereturnofthethermallydominantstateoftheXRB. holes in XRBs. This fundamental plane was found to take the
form
A detailed spectral observation of the state C by XMM-
NewtonwasperformedbyMartocchiaetal.(2006).Theyfound
onlyasimplepower-lawofΓ∼1.7wasneededtofittotheover- Lradio ∝ L0X.−6rayM0.8. (2)
all spectra, which then gaveresiduals of a 1 keV excess, small
variations between 1.5 3 keV and a deficit above 8 keV. Theaimofthispaperistotestthefundamentalrelationshipbe-
Theoverallpower-la∼wwas−consistentwiththeRXTE-PCAob- tweentheinflowandoutflowingemission,withthesourcevari-
servations,andwasattributedtoeithera hotcoronaaroundthe abilityfoundinGRS1915+105.
accretion disk or to Comptonized emission from the base of a
jet (Rodriguezetal. 2004). The 8 keV deficit was explainedas
anopticallythickreflectorthatgaveevidenceofthepresenceof 2. Observations
athindisk.The1keVexcesswas(tentatively)explainedasthe
Datawereobtainedfrominstrumentsthathavecontinuallymon-
presenceofanopticallythincomponent(e.g.awind/jetorage-
itoredGRS1915+105to ensurenostatisticalbiasingfrompar-
ometrically thick disk). Martocchiaetal. (2006) noted that the
ticularperiodsactivityorobservationalinterest.Theonlyradio
relativelylargeamountofreflectioncomponentsimplythatthe
telescope to have constantly observed GRS1915+105over the
primaryX-rayemittingregionwouldhaveasizecomparableto
lastdecadewastheRyleTelescope(RT).Thesedatawerethen
theinnerdiskradius.
comparedto2 12keVX-raydatatakenbytheAllSkyMonitor
Klein-Woltetal. (2002)clearlyestablishedthatradioemis- onboardtheR−XTEsatellite.
sion is intimately related to the hard X-ray power-law compo-
Data from each of the two instruments were binned into
nent and implies a close physical connection between the in-
daily averages, and observations taken on the same day were
neraccretionregionandtheoutflowingsynchrotron-emittingjet. cross-correlated to form a comprehensiveradio-X-raycompar-
Theyfounda‘one-to-one’relationshipbetweenradiooscillation ison. Whilst intra-day variability in both X-ray (Bellonietal.
events, originally discovered by Pooley&Fender (1997), and 2000) andradio(Pooley&Fender1997) isknownto occurfor
quasi-periodicX-raydipsduringcertainepochsinstateC;inall variousstates,theaimoftheworkwasnottostudythedetailsof
othercasesthesourceshowedeither‘low-level’or‘high-level’ a particularflare; rather,the goalwas to study any longerterm
radio emission, but no radio oscillation (see Pratetal. 2010, correlation (i.e. longer than a few days) between the radio and
for a detailed characterisation of the radio oscillation events). X-rayinthesteadyjetstate.
Moreover, when the source stays in spectral hard state C for Moreover,GRS1915+105doesnotalwaysproducecompact
longperiodsofdaystomonths,the‘high-level’radioemission
self-absorbedemissionandlarge-scale,opticallythinstructureis
(i.e.non-oscillatingemission)hasbeenshowntooriginatefrom
frequentlyobservedaftera state change.Onlywhenacompact
the compact flat-spectrum jet. This state is also known as the
jet,with nosuperluminalpropermotionispresent,canadirect
‘plateau’stateandisalsoreferredtoasclassχbyBellonietal.
comparison between the radio and X-ray be made. Therefore,
(2000).
it was important to identify the states that were not contam-
inated by large scale extended knots, such as those observed
byMirabel&Rodriguez(1994).
1.2.AuniversalX-raybinariesrelationshipandthe
fundamentalplane
2.1.RyleTelescope
X-raybinaries(XRBs)areidealobjectstostudythedisk-jetcou-
plingrelationship,astheydisplaybrightX-rayandradioemis- The RT is a linear east-westradiointerferometerlocatedat the
sion from an accretion disk and jet, respectively. Corbeletal. MullardRadioAstronomyObservatory,UK.Thearrayoperates
A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105 3
atafrequencyof15GHzwithassociatedbaselinesbetweenap- (5 12keV), with the total X-ray intensity (2 12 keV). This is
2−5keV −
proximately18metresand4.8kilometres,althoughforthema- sho−wninFigure1asahardnessintensitydiagram(HID).
jorityoftheseobservationsonlyasubsetofthebaselines,typi- The diagram in Figure 1 can broadly be divided into three
callyupto150metres,wereused. dominantmodesofX-rayemission:
An extensive monitoring campaign began with the RT
– Weak/verysoftfluxof<20c/swithaHR2<1
in 1996 of a few radio-bright X-ray binaries (including
– Highlyvariable/softflux between20 200c/s with a soft
GRS1915+105), which coincided with the launch of the −
HR2 1 1.5
RXTEsatellite.Observationsoftargetsourceswereinterleaved ≈ −
– Persistent/slightlyharderfluxatapproximately30 50c/s
with a nearby phase calibrator (B1920+154 in the case of −
witharelativelyharderspectraofHR2 1.5 2
GRS1915+105)andtheflux-densityscalewassetbyshortscans ≈ −
of 3C 48 and 3C 286. The data were sampled every eight sec- These are akinto the X-raystates B, A and C respectively,de-
ondsandaveragedintofiveminutedatapointswithanRMSof scribedbyMigliari&Belloni(2003).Resolvedjetsareobserved
2mJy. when the source transitions from state C to A (Fenderetal.
∼
Near daily observations of GRS1915+105 were collected 1999;Rushtonetal.2010).
betweenMay 1995(MJD 49856)and June 2006(MJD 53898) Next, the X-ray emission associated with little or no radio
at 15 GHz. Pooley&Fender (1997) describe the details of flux was identified.This allowedareas of the HID that showed
this programme, indicating the detection of a 20 – 40 minute no radio correlation to be discounted.It was found that during
quasi-periodicvariationofGRS1915+105atafrequencyof15 the softer X-ray states of HR2< 1.5), there was no associated
GHz,apparentlycoupledwithvariationsinthesoftX-rays(see radioemissionthatlastedmorethanoneortwodays.Likewise,
Pratetal.2010,foracomprehensivestudy). the strongX-ray flares of > 50 counts/secondwere notassoci-
atedwithanylongperiodsofstrongradioemission.Asubstan-
tial fraction of the X-ray emission in the HID is therefore not
2.2.RossiX-rayTimingExplorersatellite
associatedwithanystrongradiofluxanddemonstratesnoclear
TheAllSkyMonitor(ASM)instrumenton-boardtheRXTEhas connectionbetweenthetwoemissionmechanisms.
been monitoring the sky since March 1996 and the data pre- The clearest correlation between radio and X-ray flux ob-
sented here covers the period from March 1997 to 2007. With served in GRS1915+105 was during the persistent/slightly
each orbit of the RXTE, the ASM surveyed 80% of the sky harder X-ray state (as marked as triangles in Figure 1). When
toadepthof20 100mCrab,makingapprox∼imatelytenobser- theX-rayfluxispersistently(i.e.morethanafewdays)between
vationsof a sour−ce per day.A more detailed descriptionof the 30 50 counts/secondandhasa hardnessradioofHR2 > 1.5,
−
RXTE-ASMcanbefoundinLevineetal.(1996). thesourceisalwaysassociatedwithstrongradioflux.Thisstate
was first identified by Pooley&Fender (1997) as the ‘plateau’
Each individual pointing, or dwell, was a 90 second inte-
state and later by Bellonietal. (2000) with the RXTE-PCA as
grationofthesource,with intensitiesmeasuredinthreeenergy
classχ.Itisalsothestatewhereacompactjetisalwayspresent
bandsof1.5 3, 3 5,and5 12keV. TheCrab Nebulaflux
between1.5 −12ke−Vcorrespo−ndstoabout75ASMcountss 1. (Dhawanetal.2000;Fuchsetal.2003).
−
−
Tocalculatethespectralhardnessratio(HR2),individualdwells
were averaged into daily points and the ratios between the
3.3.Radio/X-rayrelationship
5 12keVand1.5 3keVenergybandsweretaken.
− − TheX-rayandradiolightcurvesofGRS1915+105overtheten
year period between 1996 and 2006 are shown in Figure 2.
3. Resultsandanalysis Marked red are the points selected in the persistent/slightly
harderX-raystate(i.e.thecompactjetstateidentifyastriangles
3.1.Absorptioncorrection inFigure1).Itisclearthatthelongflaringperiodsofradioac-
tivityarecoincidentwiththisX-raystate.Furthermore,boththe
The observed X-ray flux density from the RXTE-ASM, mea-
X-rayandradioemissionappeartodecayduringeachflaringpe-
sured in counts per second, is partially absorbed by hydrogen
riod.Table1listseightperiodsofflaringactivity(rangingfrom
within the line-of-sight. The data were therefore compensated
4to63days)identifiedbetweenJanuary1996(MJD50110)and
for the level of absorption using previous high-spectral reso-
May2006(MJD53883).Itthusappearsthatthe‘plateau’state
lution observations.Galloetal. (2003) modelled the spectra of
occursapproximatelyevery1.3years.
hardstateBHs,withhydrogencolumndensities(N )ofzeroup
to12.5 1022cmusingChandraobservations.ByfiHxingtheflux Furthermore, the radio/X-ray correlation appears particu-
× larly strong during a period between April 2001 (MJD 52000)
correspondingtonoabsorption,itispossibletomodelthedata
and February 2006 (MJD 53767), as shown in Figure 3. Five
pointsusingasimpleexponentialfunction:
periodsof strong radiooutburstare clearlyidentified separated
by approximately one year. During each period both the radio
F (N /1022cm 2)
abs. =exp − H − . (3) andX-rayemissionappeartodecayfromthebeginningofeach
Funabs. " 18.38 # outburst.AstherelationshipbetweenradioandX-rayhasbeen
shown as non-linear for XRBs (Galloetal. 2003), the ratio of
We then used a line-of-sight hydrogen column density for log(radio):log(X-ray)isalsoshowninFigure3(top).Thedata
GRS1915+105ofNH = 5 1022 cmasfoundbyGreineretal. showsastrongcouplingbetweenthetwowave-bandsduringthe
×
(1994). plateaustateandthereisalsotentativeevidencethatthecoupling
extendsbeyondthisstate,butthelatterresultisnotconclusive;
however,itisclearthattheradioemissiondecaysmorequickly
3.2.HardnessIntensityDiagram
thantheX-rayfromthestartofmostplateaustates.
To identify the differentX-ray spectral states, the RXTE-ASM Theratiobetweenthebandsappearsquasi-periodicoverout-
dataweresortedbycomparingthespectralhardnessratio,HR2 bursts in a saw-tooth pattern: there is a short period of radio
4 A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105
220000
119900
118800
117700
116600
115500
114400
113300
d)d)
onon 112200
cc
sese 111100
s/s/
ntnt 110000
uu
coco 9900
M (M ( 8800
SS
AA 7700
Fig.1. Hardness intensity diagram of 6600
GRS1915+105 between 1996 and 2008, 5500
showingtheonedayaverageintensity,be- 4400
tween2-12keV,plottedasafunctionofthe 3300
hardness radio, HR2 (52−152kkeeVV). The trian- 2200
glesrepresentpersistent−radioemissionas- 1100
sociated with the X-raysknownas class χ 00
00 00..55 11 11..55 22 22..55
by Bellonietal. (2000) and the ‘plateau’
HHaarrddnneessss rraattiioonn ((HHRR 22))
statebyPooley&Fender(1997).
22..66
22..44 XX--rraayy:: CCoolloouurr//HHaarrddnneessss rraattiioo
22..22
22
11..88
R2R2 11..66
HH 11..44
11..22
11
00..88
118800 XX--rraayy:: RRXXTTEE--AASSMM ((22--1122 kkeeVV))
116600
Counts/second)Counts/second) 111111 8802402400000000
SM (SM ( 6600
AA
4400
2200
2255 0000
RRaaddiioo:: RRyyllee tteelleessccooppee ((1155 GGHHzz))
220000
mJy)mJy)
Density (Density ( 111105050000
Flux Flux 5500
00
5500000000 5500550000 5511000000 5511550000 5522000000 5522550000 5533000000 5533550000
DDaattee ((MMJJDD))
Fig.2. X-ray and radio lightcurves of GRS1915+105 over a ten year period with the red marks representing persistent radio
emissionassociatedwithclassχorthe‘plateau’state(i.e.ASMcountsof30 50persecondandHR2> 1.5);thebottomgraph
−
shows15GHzradioobservationstakenbytheRyleTelescope,themiddleandtopgraphsshow2 12keVX-rayandHR2hardness
ratio(5 12keV)respectivelyobservationstakenwiththeRXTE-ASM. −
2−5keV
−
quenchinglasting10-20days,followedbytheplateauhardstate 4. Accretionandjetformationmodels
lasting 100dayswheretheradiodecreasesmorerapidlythan
∼
the X-ray emission, a period of 200 days where the radio Understanding the processes connecting in-falling matter with
∼
is weaker and the X-ray are highly variable, returning to the outflowingjets requiresmeasuringthe massaccretionrate (m˙),
quenchedstateagain. which fundamentallyconnects the mechanisms responsible for
radio andX-ray emission. The followingderivesa relationship
assuminganunderlininggeometryandphysics,basedontheac-
cretionprocessandpowerofthejet.
A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105 5
Startdate Enddate ChangeinX-rays Changeinradio Duration
† ‡
(MJD) (MJD) (counts/second) (mJy) (Days)
50286 50297 42 40 110 100 11
→ →
50732 50736 36 33 94 87 4
→ →
50926 50989 45 34 154 31 63
→ →
51522 51546 39 33 78 50 24
→ →
52315 52354 43 36 122 73 41
→ →
52729 52783 46 33 163 68 54
→ →
53077 53116 39 31 122 70 39
→ →
53416 53466 52 42 197 112 50
→ →
Table1.Listofthehardstates(i.e.‘plateau’state/classχ)ofGRS1915+105betweenJanuary1996(MJD50110)andMay2006
(MJD53883).TheX-rayobservationsweremeasuredasonedayaveragesoftheRXTE-ASMobservationsbetween2-12keV.Radio
observationsweretakenat15GHzwiththeRyletelescope(RT). RXTE-ASMerrorsweretypicallylessthan1count/second.
† ‡
RTerrorsareestimatedatapproximately 6mJy.
±
1
Ratio of log(Radio):log(X-ray)
Radio (Jy)]/logX-ray(c/s)]-- 021
log[-3
-4
X-ray: RXTE-ASM (2-12 keV)
140
M (Counts/second) 11 68020000
AS 40
20
0
Radio: Ryle telescope (15 GHz)
200
mJy)
Density ( 110500
Flux 50
0
52000 52250 52500 52750 53000 53250 53500 53750
Date (MJD)
Fig.3.TheradioandX-raylightcurves(bottomandmiddlerespectively)offivelongperiodsofoutburst.Thetopgraphrepresents
theratiooflog(radio)/log(X ray),clearlyshowingdirectrelationshipbetweenthetwobands.
−
Accretiondiskscanbeeitheranefficientorinefficientmech- thickdisk,producinga blackbodyspectrum.TheX-rayspectra
anismforloweringmaterialintoagravitationalpotentialandex- ofXRBsinthesoftstate,havebeensuccessfullymodelledasthe
tracting energyas radiation.The vitalprocessis convertingor- thermalemissionfromtheinneraccretingregionofsuchathin
bital kinetic energy into heat. As the accretion around a black disk.
holeinevitablyinvolvesrotatinggasflows,wemustconsidered Inabinarysystemtheoutflowingmatterfromanormalstar
hydrodynamicequationsofviscousdifferentiallyrotatingflows. willhavemuchhigherangularmomentumthanthecompactob-
Chen(1995)describesfourknownself-consistentsolutions,two ject.Theparticlesinthediskwillloseangularmomentumdueto
ofwhich(athindiskanadvection-dominatedflow,asshownin interactionsbetweenthe variouslayers. A compactobjectwith
Figure4)areconsideredinSections4.1and4.2. mass M will thenaccretewith a luminosity(atanefficiencyη)
of
4.1.Thindisk
GMm˙
L =2η , (4)
disk
Inmanysituationstheaccretionflowontoacompactobjectcan R
∗
be approximated by a two-dimensional gas flow. A thin disk
modelaroundacompactobjectcanbeusedtomodelthethermal whenthematterhasfallentoradiusR (Franketal.2002).Such
emissionfromabinarysystem(Shakura&Sunyaev1973).The anaccretionprocessisthereforecons∗ideredradiativelyefficient
accretinggasisassumedtoformageometricallythin,optically andonecanestimatem˙ directlyfromtheX-raybolometriclumi-
6 A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105
nosity(L ).Assumingadiskefficiencyη,theaccretingratefor
X
luminosityisgivenby
L L 0.75 0.1 g
m˙ = X =1.5 1018 X , (5)
fηc2 × 1038ergs 1! f ! η ! s
−
where f is the fractionof the total accretion thatis notejected
viajetsorwinds.
4.2.Advection-dominatedflows
Radiatively efficient thin-disk models have successfully de-
scribed the basic accretion properties of a thermally-dominant
disk; however, this disk model has only been suggested over
a certain range of accretion rates and breaks down when the
gas pressure is below a critical value. In recent years, al-
ternative solutions for radiatively inefficient accretion flows
have therefore become popular mechanisms for explaining the
low-hard/quiescent state in stellar black holes (reviewed by
McClintock&Remillard 2006). The inflow of plasma forms a
geometricallythickdiskandtherateofgascoolingissuchthat
mostofthedissipatedenergyisnotradiated.
Fig.4. A schematic drawing of advection accretion. Accretion
Within advection-dominated accretion flow (ADAF;
onto a black hole forms a thin disk until a critical radius, R ,
in
Reesetal. 1982; Abramowiczetal. 1995) the accreted gas
where upon the accretion flows in geometrically thick disk,
is held in a low density quasi-spherical ‘corona’ around the
known as an advection flow. Powerful bipolar jets are formed
compact object. The lower accretion rates of the thick disk
intheadvectionzoneandaresensitivetothemassaccretion,m˙
inhibitCoulombcouplingbetweenelectronsandions,trapping
andthesizeoftheadvectionzone,R .
in
part of the viscous energy as heat within the gas. Transport
of angular momentum then occurs as the viscous energy is
“advected”ontotheblackhole,ratherthanbeingradiatedaway.
wherem˙ andL arenormalizationfactors(assumingaflat
0 Radio,0
Withoutthe viscosity,noaccretionwouldoccur,as theangular
spectral index of α = 0 and an energy distribution of dN
e
momentum would preventdirect radial in-fall. For black holes E p where p = 2). Ko¨rdingetal. (2006) then showed that b∝y
−
with no physical surface, part of the advected material will
assuming the accretion rate does not vary much during a state
cross the event horizon and not escape. This results in some
change, it is possible to estimate m˙ from the soft state X-ray
of the accreting power becoming lost and making the ADAF 0
luminosity(i.e.usingEquation5).Theyfoundthatforasample
radiatively inefficient (i.e. the bolometric luminosity will be
of neutronstar (NS) and black hole (BH) sources, with known
lower than expected from a given mass accretion rate). This
accretionratesthat
is perhaps evidence of the existence of an event horizon, and
predictsanX-rayluminosityofblackholesthatscalesas m˙NS =7.7 1017g/s and m˙BH =4.0 1017g/s, (9)
0 × 0 ×
LX ray m˙2. (6) whensetting L = 1030 ergsasthisistheradioluminosity
− ∝ Radio,0
wherethe accretiondisk arounda 10M blackholechangesit
state. ⊙
4.3.Jetpower
The precise details of how a scale-free relativistic jet can be 5. Modelfitting
formed from an accretion disk remain unknown;however,it is
possible to calculate the total power of a partially absorbed jet ThedataselectedinSection3.2areplottedinFigure5tosearch
andcanbeusedasa‘tracer’forthemassaccretion.Equation56 for an empiricalrelationship between the X-rays and radio. To
inFalcke&Biermann(1995)showsunderthesameconstraints, determine if any statistical relationship existed, a Spearman’s
theobservedluminosityoftheself-absorbedjetdependsonthe rank correlation test (Barlow 1989) was used to quantify the
poweras level of correlation between the X-rays and radio, without as-
suminganyparametrizedmodel.TheSpearman’srankcorrela-
L Q17/12, (7) tion coefficient was found to be 0.81 (i.e. statistically signifi-
Radio ∝ jet cant) and the analysis thus clearly shows a strong relationship
ifmodelledbyasimpleconicaljet(asimilarresultwaspredicted between the X-rays and radio. This indicates a close coupling
byBlandford&Konigl1979).Assumingthatthejetformsalin- betweenthemechanismsproducingtheradioandX-rayswhilst
earinter-dependencywiththeaccretiondisk,thepowerofthejet GRS1915+105isinthehardstate.
willthenbeaconstantfractionoftheoutflowrateQjet = qm˙c2, WethenfittedafunctionofSradio ∝SXξ ray,withanindexof
withan efficiencyq = 10 3 10 1 (Falcke&Biermann1995). ξ 1.7 0.3,whichismuchsteeperthan−thepreviouscorrela-
Wethereforefind − − − tio∼ns tha±t have shown ξ 0.7 for XRBs in the low-hardstate.
∼
Previousmodelshavethenassumedthatthebolometricluminos-
L 12/17 ityisdominatedbyanadvectionflowandthustheobservedX-
m˙ =m˙ Radio , (8)
0 LRadio,0! rayemissionisLX−ray ∝m˙2(i.e.Equation6).Astheradioemis-
A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105 7
180
S α S ξ
radio X-ray
160 ξ=0.7
ξ=1.4
ξ=1.7
y) 140
J
m
z ( 120
H
G
5
o: 1 100 Fig.5. X-ray/radio correlation in the hard
adi state of GRS1915+1915 (i.e. plateau
R 80
state).Thedottedlinerepresentsa‘best-fit’
of ξ 1.7 0.3, the solid line represent
60 ∼ ±
a modelof ξ 1.4 and the dashed-dotted
∼
40 line represents a model of ξ 0.7, where
30 32 34 36 38 40 42 44 46 48 50 S Sξ . ∼
X-ray: 2-12 keV (counts/second) radio ∝ X ray
−
sionrelatestotheaccretionrateasL m˙17/12 (Equation8), Theestimatedaccretionratewasthencomparedtothebolo-
radio
∝
theradioemissionshouldcoupletotheX-rayemissionas metric X-ray luminosity, as shown in Figure 7. The solid line
representsthepredictedX-raybolometricluminosityofaradia-
tivelyefficientXRB.ThedashedlinerepresentstheexpectedX-
L L0.7, (10) ray bolometric luminosity for different rates of advection flow
rad ∝ X
accretion. As shown on the graph, most black hole XRBs are
which is therefore a radiatively inefficient coupling. However, far below the expected X-ray luminosity if all of the heat en-
the fit has a much steeper slope, that can be better fitted with- ergyreleased fromangularmomentumlosswas radiatedaway.
outassuming an advectionflow. If the X-rayemission coupled However, variations in GRS1915+105 do not show this rela-
linearly,L m˙ (Equation5),withtheaccretionratethen tionship;instead, there appearsto be a ‘turn-over’aroundm˙
X ray
− ∝ 1018.5g/s. ∼
L L1.4, (11)
rad ∝ X 6. Discussion
which is therefore a radiatively efficient coupling and gives a It is apparent from the results presented here, that the X-
muchcloserfittotheobservedrelationship. ray/radio variations between GRS1915+105 and other black
This trend can be compared to other XRBs also whilst in hole XRBs do not exhibit the same scaling relationship. Two
the low-hard state (e.g. Galloetal. 2003, 2006). This showed possibleexplanationsarethereforesuggested:
asimilarpower-lawrelationshipbetweenX-raysandradio,but
with a much steeper slope. GRS1915+105and the other XRB
6.1.Radiativelyefficientaccretion
luminositieswerethenscaledto1kpcandassumedtohaveap-
proximately the same black hole mass. The X-ray/radio corre- The evidence presented by Migliari&Belloni (2003) suggests
lation was therefore compared for two similar black hole ac- thatduringstateC(includingthesteadyjetstate)thereisaloss
cretors, in a similar X-ray spectral state, as shown in Figure 6 of matter in the most inner region (r ) of an optically thick
in
and in more detail for GRS1915+105 in Figure 5. Note that and geometrically thin disk. The collapse of such an inner re-
correlation in Figure 5 is equivalent to the smooth part of the gion is believed to be due to a reduction in the mass accretion
log (radio)/ log (X-ray)plots in Figure 3 duringthe hardstate rate, thus formingan advectiondominatedregion(as shown in
(stateC).Thedashed-dottedanddottedlines,representthedif- Figure4),thatcanproduceasteadyoutflowofhigh-energypar-
ferentmodels:radiativelyefficientandradiativelyinefficient,as ticles;however,theX-rayemissionisafactorof 100brighter
in Equations 10 and 11. The solid and dashed lines represent in GRS1915+105 (and relatively softer) than m∼ost XRBs ob-
the empirical ‘best-fits’ to the data. It is therefore suggested servedinthelow/hardstate.Furthermore,XMM-Newtonobser-
that despite the similarities in the fundamental parameters of vations(Martocchiaetal.2006)suggestthepresenceofanopti-
thesources,theradiativemechanismsscalingthebolometriclu- callythickreflector,thatprovidesevidenceofthepresenceofa
minosities of GRS1915+105, to that of the Galloetal. (2003) thindiskintheX-rayspectrum.
blackholesources,isphysicallydifferent. It is therefore suggested that the domiante mode of accre-
tion is efficient accretion, where the X-ray emission is emitted
byaradiativelyefficientthindisk,whereL m˙,ratherthanan
5.1.Estimatesoftheaccretionrate X ∝
advectiondominated flow. This linear dependencecomes from
ThebolometricluminosityofaXRBinthelow/hardstatedoes thepropertiesofasurroundingcorona,ratherthanthethindisk
notgivea reliable estimate of the massaccretion.Itwas there- andmayonlybevalidforspecificgeometriesandphysicalcon-
fore suggestedby Ko¨rdingetal. (2006) thatthe radioemission ditions. One must assume that the fraction of gravitational en-
of XRBs, with known accretion rates, can be used to scale ergy(A)dissipatedfromthethindiskintothecoronaisconstant
Equation 8. For GRS1915+105 this has been found as m˙ andthepowerofthecoronaisQ = Aηm˙c2.Theenergystoredin
c
1019 g/s. Using the scaling relationship, L m˙17/12, thi∼s thecoronacanthenberadiatedawaybyinverseComptonscat-
radio
∝
accretion rate was extrapolated to the lower radio luminosities tering of the disk photons such that L m˙, before reaching
X
∝
observedintheotherblackholeXRBs. r .
in
8 A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105
100000
S α S ξ
radio X-ray
GRS1915+105
Black hole XRBs
ξ=1.4
ξ=1.7
10000 ξ=0.7
ξ=0.58
y)
J
m
y ( 1000
sit
n
e
d
o
di
Fig.6. Scaled X-ray/radio correlation in d ra 100
the hard state (i.e. plateau state) of ale
GRS1915+1915 and other known black Sc
holeXRBs.Eachsourcehasbeenscaledto
adistanceof1kpc.Thedashed-dottedand
10
dotted lines represent the efficient and in-
efficient models of ξ 1.4 and ξ 0.7,
respectively, where S∼ Sξ ∼. The
radio ∝ X ray
solidanddashedlinesrepresentan−empiri- 1
0.0001 0.001 0.01 0.1 1 10 100 1000
cal‘best-fit’toeachXRB.
Scaled X-ray flux density (Crab)
GRS 1915+105
Black hole XRBs
2 Efficient accretion model
In-efficient accretion model
s)] 1
b
a
Cr
y (
sit
o
n 0
mi
u
y l
a
X-r
c -1
Fig.7. The bolometric luminos- metri
o
ity of known black hole XRBs and ol
B
GRS1915+105 as a function of accretion og[ -2
L
rate m˙. The solid line represents the pre-
dicted X-ray bolometric luminosity of a
radiatively efficientXRB. The dashed line
-3
represents the expected X-ray bolometric
luminosity for different rates of advection
16 17 18 19
flowaccretion.
log [Mass accretion rate (g/s)]
These argument have profound consequences on the phys- 6.2.Jetdominatedemission
ical conditions required to form an outflowing jet; there exists
a rate which the nature of accretion fundamentally changes. Atveryhigh-massaccretionrates,therecouldbeaturn-overin
Figure 7 suggests, for stellar-mass black holes, the dominant thedominanceofthe X-raybolometricluminositybetweenthe
modeofaccretionchangesfromradiativelyinefficienttoradia- accretion disk and the jet. The close relationship between X-
tively efficient around a mass-accretion rate of m˙ 1018.5 g/s. rays and radio emission shown in Figure 3 and Figure 5 could
∼
This would imply that at high m˙ that advection flow accretion beexplainediftherespectiveradiationfieldsoriginatefromthe
doesnotdominatethebolometricluminosity,despitestrongevi- same outflowing particles (as suggested for XTE J1118+480
denceofacontinuousoutflowofparticles(i.e.thecompactjet). by Markoffetal. 2001). This argumentis further supported by
Furthermore,this argumentcan be scaled with the mass of the Rodriguezetal. (2008), who used INTEGRAL and RXTE ob-
blackhole,meaningadvectiondominatedflowsarenotsustain- servations of GRS1915+105, during class χ/plateau state, to
able at a very high fraction of the Eddington accretion rate of modeltheX-rayspectraintotwocomponents:athermalcomp-
about m˙ > 0.1 m˙ . It is therefore speculated that the change tonizedcoronaandanadditionalpower-lawtailbetween2 200
edd
−
inaccretionmodeisafundamentalphysicalinvariantthatcould keV. They found a direct correlation between variability in the
occurfromstellar-massblackholestosupermassiveblackholes hardtailandthestrongradioemissionfromthecompactjet(us-
inAGN. ing the RT at 15 GHz). The X-ray bolometric luminosity may
thereforenotbedominatedbytheaccretiondiskandtheunder-
lyingradiativeefficiencyofthedisk(andhencemassaccretion
A.Rushtonetal.:SteadyjetsfromradiativelyefficienthardstatesinGRS1915+105 9
rate) would notsignificantlycontributeto the bolometriclumi- of Cambridge and supported by STFC. The X-ray data was provided by the
nosity;rathertheoverallX-rayluminositywouldsimplybecome ASM/RXTEteamsatMITandattheRXTESOFandGOFatNASA’sGSFC.
somefunctionoftheoutflowrate.
Representative parameters for the radio jet in the long
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2611
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Acknowledgements. ARacknowledgessupportfromanSTFCstudentshipdur-
ingthisresearchandpartofthisworkwasalsosupportedbytheEXPReSproject.
Thanks are also given to Tom Maccarone and Elena Gallo foruseful discus-
sionthroughoutthiswork.EXPReSisanIntegratedInfrastructureInitiative(I3),
fundedundertheEuropeanCommission’sSixthFrameworkProgramme(FP6),
contract number 026642. The Ryle Telescope is operated by the University