Properties of the propagating shock wave in the accretion flow around GX 339-4 in the 2010 outburst
aa r X i v : . [ a s t r o - ph . H E ] S e p Astronomy&Astrophysicsmanuscript no. rnote˙aa˙rev3 c (cid:13)
ESO 2018November 20, 2018
Properties of the propagating shock wave in the accretion flowaround GX 339-4 in the 2010 outburst(Research Note)
Dipak Debnath , Sandip K. Chakrabarti , and Anuj Nandi , Indian Centre for Space Physics, Chalantika 43, Garia Station Rd., Kolkata, 700084, India S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata, 700098, India On deputation from Indian Space Research Organization-HQ, Bangalore, Indiae-mail: [email protected]; [email protected]; [email protected]
Received ; accepted
ABSTRACT
Context.
The black hole candidate GX 339-4 exhibited an X-ray outburst in January 2010, which is still continuing. We here discussthe timing and the spectral properties of the outburst using RXTE data.
Aims.
Our goal is to study the timing and spectral properties of GX 339-4 using its recent outburst data and extract information aboutthe nature of the accretion flow.
Methods.
We use RXTE archival data of the recent GX 339-4 outburst and analyze them with the NASA HEAsoft package, version6.8. We then compare the observed quasi-periodic oscillation (QPO) frequencies with those from existing shock oscillation modeland obtain the nature of evolution of the shock locations during the outburst.
Results.
We found that the QPO frequencies are monotonically increasing from 0 .
102 Hz to 5 .
69 Hz within a period of ∼
26 days.We explain this evolution with the propagating oscillatory shock (POS) solution and find the variation of the initial and final shocklocations and strengths. The model fits also give the velocity of the propagating shock wave, which is responsible for the generationof QPOs and their evolutions, at ∼
10 m s − . We observe from the spectra that up to 2010 April 10, the object was in a hard state.After that, it went to the hard-intermediate state. On April 18, it had a state transition and went to the soft-intermediate state. On May15, another state transition was observed and the source moved to the soft state. Conclusions.
As in the previously fitted outburst sources, this source also showed the tendency of a rapidly increasing QPO frequency( ν QPO ) in a viscous time scale, which can be modeled quite accurately. In this case, the shock seems to have disappeared at about ∼ Key words.
Black Holes, shock waves, accretion disks, X-Ray Sources, Stars:individual (GX 339-4)
1. Introduction
The source GX 339-4 is a well known stellar-mass Galacticblack hole candidate. This bright variable X-ray source was firstobserved during the survey period from October 1971 to January1973 by the MIT X-ray detector on-board the OSO-7 satellite inthe energy range of 1-60 keV. GX 339-4, a transient low-mass X-ray binary (LMXB) system located at ( l , b ) = (338 ◦ . , − ◦ . = h m s .
36 and Dec. = − ◦ ′ ′′ . M = . ± . M ⊙ and the distance D = ff er-ent wavebands to reveal the nature in multiple wavelengths (Liuet al. 2001, Homan et al. 2005). During the RXTE era (1996onward), this source exhibited frequent X-ray outbursts (1998,2002 / / / / hard state, which is quite common in other out-burst candidate of black holes (e.g., GRO J1655-40, XTE J1550-564).This general behavior is understood to be caused by sud- Send o ff print requests to : Dipak Debnath den variation of viscosity in the system (Mandal & Chakrabarti,2010), which in turn causes the accretion rate of the stan-dard Shakura-Sunyaev (1973) disk (hereafter referred to as theKeplerian rate) to rise and possibly makes the inner edge movein. These transient black hole candidates show low and inter-mediate frequency quasi-periodic oscillations (QPOs) in theirpower density spectra. In general, during the rising hard state ofthe outburst the frequency of the QPO increases, whereas duringthe declining phase, the QPO frequency is gradually decreased.The QPO evolution in these objects can be well understoodthrough the propagating oscillatory shocks (POS; Chakrabartiet al. 2008, 2009). Though several studies of the evolution ofthe temporal and spectral states of GX 339-4 during the previ-ous outbursts were carried out (Nowak et al. 1999, Belloni etal. 2005; Motta, Belloni & Homan 2009), the underlying phys-ical processes remained unclear. Our attempt here is to see ifthe POS solution of our group can also explain the present out-burst. Note that for the traditional soft-X-ray transients with fastrise and exponential decay (FRED) lightcurve with typicallylong recurrence times, there are so-called disk instability models(Cannizzo, 1993; Lasota, 1996) where matter also moves in ow-ing to viscous processes. However these models do not addressvariations of QPOs. (RN) Recently, after remaining in the quiescent state for three longyears (except for a short spell of very weak activity in 2009 asobserved in SWIFT / BAT), GX 339-4 became X-ray-active againon 2010 January 03, with a first detection by MAXI / GSC on-board HETE (Yamaoka et al. 2010). Immediately after the an-nouncement of the X-ray trigger, RXTE started monitoring thesource from 2010 January 12 (Tomsick, 2010). During the initialoutburst phase, the source was in the low-hard state without anysignature of QPO in the power density spectrum (PDS). In thisoutburst phase we first observed the QPO at 102 mHz on 2010March 22 (MJD 55277). After that, the QPO frequency mono-tonically increased to 5 .
69 Hz until 2010 April 17 (MJD 55303).Afterward, QPOs were sporadically on and o ff (e.g., 5 .
739 Hz,5 .
677 Hz and 5 . ν QPO started declining imme-diately after reaching maximum (CDP09), in the present case, ν QPO began to stall at about 5 . − . m s − . In both cases, a clear picture emerged about the sys-tem: it was found that while the low-angular momentum tran-sonic flow (hereafter referred to as the sub-Keplerian matter, see,e.g. Chakrabarti, 1990) was always present even in the quiescentstate, the Keplerian disk moved in closer to the black hole in therising phase and then receded far away in the declining phase.This picture was corroborated when even the hardness-intensitydiagram was reproduced (Mandal & Chakrabarti, 2010) with thisconsideration.We here examine the nature of the rising phase of GX 339-4 and show how it changed from hard state to soft state viaa short-lived hard-intermediate and soft-intermediate state. Wealso show that the evolution of QPOs can be understood by thePOS solution as in the case of other outbursts. In the next sec-tion, we present the observational results of GX 339-4 since itshowed evidences of the outburst in January, 2010. We also fitthe QPOs using POS solution presented in CDNP08 and CDP09.Finally, in Sect. 3, we present the concluding remarks.
2. Observational results and analysis
We now present the timing and spectral properties of the GX339-4 X-ray outburst using the archival data of the RXTE PCAand ASM instruments. We use the standard RXTE data anal-ysis software package HEAsoft 6.8. For RXTE / ASM (Levineet al. 1998), one day averaged archival data of the di ff erent en-ergy bands (2-3, 3-5, 5-12 & 2-12 keV) were downloaded andanalyzed. For the PCA data (Jahoda et al., 1996) analysis, we mainly use the most stable and well conditioned proportionalcounter unit 2 (PCU2) data (all six layers). Background spectrawere made using FTOOLS runpcabackest task and the most re-cent bright source model. The task pcarsp was used to generatethe PCA response file. For the timing analysis, we used the PCAEvent mode data with a maximum timing resolution of 125 µ s ,and for the spectral analysis we used the PCA ‘standard 2’ data.In the entire PCA data analysis, we did not include the deadtimecorrections because the counts are not very high, the maximumrate being around 1200 cts / s. We verified that the error caused bythis is at most 4%. In Fig. 1(a-b), we present the ASM lightcurveand the hardness ratio as a function of days. Sudden changes inslope on MJD 55296, MJD 55304, and MJD 55331 are indica-tions of the state transitions. From the hardness ratio (Fig. 1b) wesee that the spectrum was hard till MJD 55296. At MJD 55304the spectrum became softer very quickly. This is the so-calledhard-intermediate state (Homan & Belloni 2005). After that, theratio remained almost constant. Sporadic QPOs (see below) inthis state indicate that the object is in the soft-intermediate state.Around 2010 May 15 (MJD 55331), the source moved to the softstate, with a sharp fall in the count rate in the 4-15 keV energyrange. In Fig. 2 we show the total counts (2 −
20 keV) as a func-tion of the hardness ratio HR = (6 − keV ) / (2 − keV ) from2010 January 12 (MJD 55208) until August 14 (MJD 55422).Four phases are clear: in the range A to B, the object is inthe hard state, in the range B to C, the object is in the hard-intermediate state, C to D, the object is in the soft-intermediatestate and beyond D, the object is in the soft state. A detailedphysical picture will be discussed below in Sect. 2.2. We analyze 114 observational IDs from 2010 January 12 (MJD = = / flux variations.To generate the PDS, we used the “powspec” task of XRONOSpackage with a normalization factor of ‘-2’ to have the ‘white’noise subtracted rms fractional variability on 2-15 keV (0-35channels) PCU2 lightcurves of 0 .
01 sec time bins. The powerobtained has the unit of rms / Hz. Quasi-periodic oscillationsare generally of a Lorentzian type (Nowak 2000, van der Klis2005) and thus these are fitted with model Lorentzians. In Fig.3a we show the variation of the QPO frequency in this period.The monotonically increasing frequency (starting from 102 mHzobserved on March 22 to 5 .
69 Hz observed on April 17) as inGRO J1655-40 and XTE J1550-564 (CDNP08, CDP09) moti-vated us to fit it with the same POS solution as used in CDNP08and CDP09. In this solution, at the onset of the outburst, ashock wave moves toward the black hole, which oscillates ei-ther because of resonance (cooling time ∼ infall time; Molteni,Sponholz & Chakrabarti, 1996) or because of the fact that theRankine-Hugoniot relation is not satisfied (Ryu, Chakrabarti &Molteni, 1997) to form a steady shock. The QPO frequencyis obtained from the inverse of the in-fall time scale from thepost-shock region. The oscillation of the shock produces the os-cillation of hard X-ray intensity, because the post-shock flowbehaves like a Compton cloud that intercepts a variable num-ber of soft photons during oscillations. The governing equationsare in CDNP08 and will not be repeated here. By fitting theQPO frequencies with POS solution we find that the compres-sion ratio R = ρ − /ρ + monotonically goes down from R = (RN) (strongest shock) to ∼ / R → / R + α ( t d ) . , where, t d is the time in days (takingthe first day of the QPO observation as the 0 th day, i.e., fromMarch 22). Here, α is a constant that determines how rapidlythe shock strength decreases with time. The value of α is ob-tained by the constraint that on the last day (i.e., after t r ∼
26 onApril 17) the QPO was observed, the shock became the weak-est ( R ∼ R → t d = t r , weobtain α at 6 . × − in the present case. In Fig. 3b we showthe fitted shock strength and the shock location as a functionof day, where 0 is March 22 when the QPO was first detected.According to our fit, the shock started at r ∼ r g = GM / c ) and disappeared on the 26 th day (April17), when it was at 172 Schwarzschild radii. The shock waveis found to move toward the black hole at a constant velocityof ∼ m s − , somewhat slower that the other two members,namely, GRO J1655-40 and XTE J1550-564 where the velocitywas about twice as high. After that, the QPO is seen sporadically,but the frequency remains about the same. This is because whenthe shock is weakest, moving inward cannot reduce its strengthany more. During this period, the Keplerian matter also movesin, increasing the softness of the spectrum (Fig. 1). In the soft-intermediate state, the Keplerian rate rises to become compara-ble to the sub-Keplerian rate, while in the soft state (i.e., currentspectral state) the Keplerian rate dominates. The net durationof the soft-intermediate and soft states would therefore dependon how long the viscosity remains su ffi cient high to maintain aKeplerian flow. Thus the prediction of the net duration of theoutburst is not easy. Detailed modeling is required to predict thepossible duration of any outburst. For the spectral study we use 3 −
25 keV PCA ‘standard-2’ dataof PCU2 and XSPEC (version 12.5) package. For all observa-tions we kept the hydrogen column density ( N H ) fixed at 5 × (Mota et al., 2009), using the absorption model wabs . All thespectra were fitted with standard disk blackbody and power-lawmodels. For each spectrum one Gaussian line at ∼ . .
0% systematic error to thefull spectrum. In Fig. 4(a-d) we show the evolution of photoncounts and the photon index. We show the results from March 5onward. Panel (a) shows very hard photons in the 15 −
30 keVrange, which could come only from Comptonization, (b) showsthe photon counts of the intermediate energy range 4 −
15 keV,which is monotonically increasing, and the panel (c) shows verysoft photons (2 − Γ . Note that QPOs are observed immediately aftera ‘kink’ occurring on MJD 55274, the physics which is still un-clear. Γ remains less than 2, and the very hard photons increaseuntil MJD 55296. We may assume that the object is in the hardstate. After MJD 55296, the hard photon count rapidly dimin-ishes and the soft photon count rapidly increases. In this phase,the Keplerian rate increases and becomes comparable to the sub-Keplerian rate. As Chakrabarti & Titarchuk (1995) pointed out,the spectral index becomes very sensitive to the Keplerian ratewhen the rates are comparable at around ∼ . − . - e V C oun t R a t e ( c t s / s ) Day (MJD) H a r dn e ss ( - k e V / - k e V ) (a)(b) Hard Soft-IntermediateHard-Intermediate Soft
Fig. 1. (a) 2-12 keV ASM lightcurve and (b) hardness ratio (5-12keV vs. 2-5 keV count ratio) as a function of the MJD of the ob-servation. The vertical dashed lines at MJD 55296, MJD 55304,and MJD 55331 indicate the state transitions from hard to hard-intermediate state, then to soft-intermediate state and finally tosoft state.
Hardness Ratio (6-20 keV/2-6 keV) P C A - k e V C oun t R a t e ( c t s / s ) ABCD
Fig. 2.
Hardness intensity diagram (HID) of GX 339-4 observedwith RXTE / PCA from 2010 January 12 to August 14, as it ap-proaches the soft state from the hard state via hard-intermediateand soft-intermediate spectral states. The total count rates in the2-20 keV energy band along Y-axis and the ratio of the countrates in the 6-20 keV and 2-6 keV bands are given on the X-axis. The points A, B, C, and D are on MJD 55208, MJD 55296,MJD 55304, and MJD 55331 respectively. Point A indicates ourfirst observation day and points B, C, and D indicate the statetransitions.rate, the inner part of the disk evaporates, which is equivalent tosaying that its inner edge has receded.
3. Discussions and concluding remarks
We analyzed the recent outburst of the black hole candidate GX339-4. We studied the evolution of the spectral and timing prop-erties since 2010 January 12 till 2010 July 8 and showed thatthere was no signature of QPO till March 21. After this, un-til April 17, QPO was continuously observed. The frequencywas monotonically increasing in a very similar way to what wasshown in GRO J1655-40 and XTE J1550-564. Until April 9, thespectral photon index was less than 1 .
6, the source was at a purehard state. Then the object moved to the hard-intermediate state (RN)
Time (day) Q P O F r e qu e n c y ( H z ) Time (day) S ho c k L o ca ti on (r g ) , R x (a) (b) (0, 1500) (26.13, 172)R = 4 R = 1 Fig. 3. (a) Variations of the QPO frequency with time (in day)of the rising phase of the outburst with the fitted POS model(dotted curve) are shown. The diamonds indicate the last twoincidents of observed QPOs (on 2010 April 18 and 22) and theyare not included in our model fitting, because the shock alreadyachieved its weakest strength of R ∼ r g ) and shock strength ( R ).The shock seems to be stagnating at around 172 Schwarzschildradii. C n t - k e V C n t - k e V C n t - k e V Day (MJD) P ho t on I nd . ( Γ ) (a)(b)(c)(d)Hard Soft-IntermediateSoftHard-Intermediate Fig. 4.
Variation of the RXTE / PCA count rates in (a) 15-30 keV,(b) 4-15 keV and (c) 2-4 keV energy bands and (d) the power-law photon index Γ with day (in MJD) are plotted. The verticaldashed lines are on MJD 55296, MJD 55304.7 and MJD 55331,indicating the state transitions from hard to hard-intermediate, tosoft-intermediate and to soft states respectively.as the Keplerian rate started increasing when the photon spec-tral index changed from ∼ . ∼ .
0. This short durationstate was continued until April 17. Then as the Keplerian ratebecame comparable to the sub-Keplerian rate, the spectral indexrapidly became soft ( Γ ∼ .
5) and the object remained in thesoft-intermediate state for ∼
26 days until 2010 May 14. Heresporadic QPOs were observed and the QPO frequency remainedat around 6 Hz. After that the Keplerian rate dominates and thespectrum becomes softer with a high spectral index ( Γ > . T ∼ / r while the pre-shock Keplerianflow was heated up as r − / (Shakura & Sunyaev, 1973). Thismeans that so-called ‘soft-photons’ in 2 − − − −
15 keV and 15 − ∼ r g , r g and244 r g respectively with compression ratios of 2 .
2, 1 .
27 and 1 . χ and the number of degrees of freedom (dof) in the spec-tral fits are also included. In Table 2 we note that the rms am-plitude of the fundamental QPO decreased with time in 2 − − −
15 keVrange, the rms initially decreased and then increased. The latterincrease is because the entire Comptonized photons now belongto this energy bin. In high-energy channels (15 −
30 keV) the rmsamplitude decreased because of the paucity of photons.Clearly Table 2 shows that the rms amplitude was highest inall energy ranges when the shock was farther out. This showsthat the oscillation of the Compton cloud, which was caused bya stronger shock at a large distance, was high enough to signifi-cantly modulate the outgoing flux. This is particularly surprisingin the 2 − −
15 keV. At lower energybins QPO was not seen because they were emitted from the pre-shock flow, and at higher energy bins, the number of photons wasstatistically insignificant. This analysis indicates that the generaltwo-component solution of the outburst sources as developed inCDNP08, CDP09, and Mandal & Chakrabarti (2010) can ex-plain most of what is seen in GX 339-4 so far. Assuming thatthis outburst is similar to that in GRO J1655-40 (CDNP08) weexpect that once the object goes to the low-hard state, it will takeabout 32-38 days to return to quiescence and the QPO frequencywould monotonically decrease from ∼ ff et al. (2007) recently argued that the presence of a thermal emis-sion even in the low-hard state of some outburst sources pointsto a Keplerian disk with an inner edge very close to the blackhole. The objects exhibit a behavior similar to a classical out-burst with FRED-type lightcurve. In these cases, the data in the (RN) rising phase are scarce and these authors used the data from theday when the outburst was near its peak and had a softer spec-trum. The conclusions of these authors do not necessarily meanthat the Keplerian disk cannot move in within the viscous timescale as in our paradigm. According to our picture, the Kepleriandisk has already moved in during the rising part and was presentwhen these authors commenced their analysis. Table 1.
Spectral properties during the initial outburst phase
Obs. Id ∗ UT Photon Flux † χ / dof ∗∗ Date Index( Γ ) 3-10 keV 10-25 keVX-10-05 2010-03-17 1.473 2.533 2.868 39.92 / / / / / † Flux in unit of 10 − ergs cm − s − ∗ Here, X = ∗∗ dof means no. of degrees of freedom Table 2.
Timing properties during the initial outburst phase
Obs. Id ∗ UT ν QPO (Hz) and rms amp.(%)Date 2-4 keV 4-15 keV 15-30 keVX-10-05 2010-03-17 — — —X-13-02 2010-04-05 0.309, 15.190 0.316, 16.048 0.313, 11.893X-14-06 2010-04-13 2.420, 7.095 2.424, 8.258 2.430, 7.2924.846, 6.056 4.813, 5.585 —7.139, 4.241 7.310, 4.987 —X-15-00 2010-04-16 — 4.153, 10.293 —X-16-04 2010-04-28 — — — ∗ Here, X = Black hole accretion is a complex process, and it is abun-dantly clear that a simple standard disk (Shakura & Sunyaev,1973) is not capable of explaining most of the observations. Outof all the observations, the outbursting sources are extremely im-portant, because they exhibit the changes in spectral states inrapid succession. Similarly they also generally exhibit a system-atic variation of QPO frequencies. These enable us to study thedynamics of matter close to a black hole. A number of outburst-ing sources, whether they exhibit FRED lightcurves as in the softX-ray transients or slow-rise and slow-decay (SRSD) lightcurvesas in GX 339-4 or GRO J1655-40 show similar and timing prop-erties. Our two-component flow paradigm seems to be capableof explaining these sources quite naturally if one assumed thatthe outburst is caused by a rapid rise of viscosity, which drivesKeplerian flows towards the black hole and / or converts some ofthe low angular momentum flows into Keplerian flows. Becausequantifying viscosity is not easy, a prediction of a detailed be-havior has not been possible so far. However, analyses of thesesources are very useful in advance our quest for a general solu-tion of this di ffi cult problem. Acknowledgments
D. Debnath acknowledges the support of CSIR-NET scholar-ship.
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