Monitoring of Transient Black Hole Candidates observed in the INTEGRAL survey
F. Capitanio, A. Bazzano, A. J. Bird, P. Ubertini, M. Federici
aa r X i v : . [ a s t r o - ph ] O c t MONITORING OF TRANSIENT BLACK HOLE CANDIDATES OBSERVED IN THEINTEGRAL SURVEY
F. Capitanio , A. Bazzano , A. J. Bird , P. Ubertini , and M. Federici IASF-roma INAF, Rome, Italy, Via Fosso del Cavaliere 100, I-00133 Rome, Italy School of Physics and Astronomy, University of Southampton, Highfield Southampton, SO17 1BJ, UK
ABSTRACT
The INTEGRAL/IBIS survey was performed collectingall the GPS and GCDE data together with all the availablepublic data . The second catalogue, published in 2006 byBird et al. [1], is dominated by detection of 113 X-raybinaries, with 38 being high–mass and 67 low–mass. Inmost systems the compact object is a neutron star, butthe sample also contains 4 confirmed Black Holes and 6LMXB black hole candidates (BHC). There are also, inadditional, 6 tentative associations as BHCs based sim-ply on spectral and timing properties. In the sample of12 sources (BHC and tentatively associated BHC) thereare 7 transient sources that went into outbursts during theINTEGRAL survey observations. We present here themonitoring of the time and spectral evolution of these 7outbursts.
1. BHC SAMPLE • IGR J17464-3213 is associated with H1743-322,a bright black hole candidate (BHC) observed byHEAO1 in 1977. After the start of the outburst in2003, the source remained bright in soft X-rays (E <
15 keV) for ∼ months and was regularly de-tected with the JEM-X monitor. At high energy, theIBIS telescope detected the source only on Septem-ber 9 (52891 MJD). The source, after the first andmost bright outburst, showed another two peculiaroutbursts. The results of the data analysis of thefirst and brightest outburst, were published by Cap-itanio et al. [5], Joinet et al. [9]. A Radio-band ob-servation campaign was also performed. A radioflare was seen by NRAO during the rising part ofthe first outburst (MJD=52728) and two jets weredetected between the first and the second outburst(MJD= 52955–53049) [7] For the second outburst,the analysed data show that the source had a shortertransition to hard state and then back to soft statewith a peculiar temporal and spectral behaviour asthe RXTE/ASM HR in figure 1 shows [6]. The dataof the third outburst are still proprietary. Interest-ingly, the time period between the peak of an out-burst and the subsequent one is equal and it has a Figure 1. IGR J17464-3212 RXTE/ASM light curves (en-ergy bands: A-B-C) and the hardness ratio of the outburstperiods (HR definition in the footnote n.1). value of about 400 days with a decreasing flux. Thisbehaviour suggests a fourth outburst could occur inmiddle of September 2006. Figure 1 shows theRXTE/ASM light curves in three energy bands (A-B-C) and the hardness ratio ( HR). The three peaksof the source outbursts are clearly evident. The firstpeak was observed by INTEGRAL quite continu-ously (Capitanio et al. [4]), while the second peakwas observed only in its declining part. Figure 2 HR = ( flux C − flux A ) / ( flux A + flux C ) Figure 2. IGR J17464-321 IBIS light curves and the cor-responding HR (defined in the footnote n.2). hows the IBIS light curves and the corresponding HR, of all the INTEGRAL public observations ofthe source. The source spectral evolution of the firstoutburst followed the typical behaviour of a tran-sient black hole: it was firstly detected in hard state,after passed thorough soft and very high soft stateand, at the end of the outburst, it was back to hardstate. Also the time evolution of the flux vs photonindex diagram followed the ”circular” or hysteresis-like behaviour expected for a transient BHC. Figure3 shows IBIS-JEMX spectrum of the Very high stateof the source : the soft power law (
Γ = 3 . ) with-out any cut-off up to 200 keV is a signature of a nonthermal comptonised emission spectrum [5]. Figure4 (top panel) shows the JEMX-IBIS light curves insix energy bands of the declining part of the sec-ond outburst. It is noticeable the presence of a smallpeak, not evident in the RXTE one-day averagedlight curves. The spectrum of this peak, that weshow in the bottom panel figure, indicates a soften-ing of the source with the presence of a moderateblack body component and an hard tail without anycutoff up to 200 keV suggesting the presence of nonthermal processes at work. Figure 3. IGR J17464-3213 2003 outburst: Soft state atthe very peak of the outburst. • IGR J17091–3624 was discovered in 2003 April by
INTEGRAL /IBIS during its Galactic Centre DeepExposure programme. Its flux reached 40 mCraband 25 mCrab in the 15–40 keV and 40–100 keVbands respectively.
RXTE observed the source si-multaneously on 2003 April 20, with an effectiveexposure of 2 ksec. Spectral and temporal evolu-tion of the source, with a transition between the hardand soft states has been reported by Capitanio et al.[4]. Data collected by
INTEGRAL and
RXTE showsa Comptonised spectrum of the hard state as well asthe JEM-X detection of a blackbody component dur-ing the source softening. From archival data search,IGR J17091–3624 appears as a moderately bright HR = ( flux − − flux − ) / ( flux − + flux − ) Figure 4. IGR J17464-3213 second outburst. Toppanel:JEMX (4-10, 10-20 and 20-35 keV) and IBIS (20-40, 40-80 and 80-100 keV) light curves of the final partof the outburst; bottom panel: IBIS-JEMX spectrum ofa small secondary peak showed in the light curves (toppanel). transient source with flaring activity in 1994 Oc-tober (
Mir /KVANT/TTM), 1996 September (
Bep-poSAX /WFC), 2001 September (
BeppoSAX /WFC[8]), and 2003 April (
INTEGRAL /IBIS, [10]). IGRJ17091–3624 is often below the detection limitsof IBIS and JEM-X in a single 2000-s SCW ( ∼ ∼
23 mCrab in the 3–20 keV JEM-X energy range).During MJD 52860–52924 (2003 August–October),the source was rather faint, especially in the 40–100energy range, and the IBIS 20–40/40–100 keV hard-ness ratio shows a weak indication of softening cor-responding to 25% [4]. The 3 spectra in figure 5represent the spectral evolution of the source duringits outburst:1) revs. 61–63 (2003 April 15–21, ∼
15 ks), which isthe first
INTEGRAL observation combined with the
RXTE /PCA data;2) revs. 100–119 (2003 August–October, ∼
77 ks),during which the IBIS spectrum softened, and whichincludes the only JEM-X detection.3) revs. 165–179 (2004 April, ∼
77 ks), containingonly the IBIS data.e clearly see a transition from the hard to the softstate from the first to the second epoch. Then, thesource, after about one year, returns to the hard state,as shown by the IBIS data for revs. 164–179, (blackspectrum). An intriguing feature of our hard-statespectra is the rather low electron temperature of theComptonising plasma, ∼
20 keV ( τ ∼ ). Thislow temperature is obtained in both COMPPS and
COMPTT models, and for both occurrences of thehard state, in the rise and decline. The detailed re-sults on this source were published by Capitanio etal. [4].
Figure 5. Spectral evolution of IGR J17091-3624 duringits outburst. • XTE1908+094 has been detected for the first timeduring a RXTE/PCA scanning of the soft-gamma-ray repeater SGR 1900+14 [11] The source spec-trum (2-30 keV) can be best fit with a power-lawfunction including photoelectric absorption (columndensity N h = . × , photon index = 1.55). Aniron line emission is present, but may be due to theGalactic ridge. The maximum source flux (2-10keV) reached was from 64 mCrab, no coherent pul-sations are seen between 0.001 and 1024 Hz. XTEJ1908+094 is classified as a possible black hole can-didate. Figure 6 (top panel) reports the A,B and CRXTE/PCA bands light curves and the hardness ra-tio of the outburst periods. While the bottom panelshows the IBIS light curves in three energy bands;unfortunately the INTEGRAL data covers only thefinal part of the outburst, when the sources is comingback to hard state. • IGRJ18539+0727 , this faint transient source wasdetected during a routine scan of the Galactic Planeand deep observations of GRS1915+105 field onApril 17-18 2003. It is classified as a probably BHCbecause of its hard spectrum. The spectral analysisof this source is still in progress. Figure 7 showsthe IBIS light curves and hardness ratio between thefirst and the second energy band. The HR shows aslight softening probably due to a spectral transitionof the source.
Figure 6. XTE J1908+094. Top panel: A,B and CRXTE/PCA bands light curves and the hardness ratio;bottom panel: IBIS light curves in three energy bands(20–40, 40–60, 60-100 keV).Figure 7. IGR J18539+0727: IBIS light curves in threeenergy bands (20–40, 40–60, 60-100 keV) and the hard-ness ratio. • XTE J1720-318 was discovered on 2003 January 9with the ASM monitor onboard RXTE as a transientsource in outburst. The source flux increased to themaximum value of ∼
430 mCrab in 2 days, and thenstarted to decay slowly. Follow up observations ofRXTE/PCA have shown the presence of a 0.6 keVthermal component and a hard tail. The spectralparameters and the source luminosity suggested aBHB in a High/Soft State. Soon after, a radio coun-terpart was identified with the VLA and ATCA radiotelescopes. XTE J1720-318 was observed by XMM-Newton, RXTE and INTEGRAL in February dur-ing dedicated Target of Opportunity (ToO) observa-ions. It was then observed by INTEGRAL duringthe surveys of the GC region performed in Marchand April and again from August to October 2003.Even if the data coverage is not complete, the resultsobtained from all these observations present a weakand steep tail in the hard energy range, typical of aBHC in High/Soft State. The source became activeagain at the end of March 2003. During this periodit showed a possible transition to hard state, and thendecayed to a quiescent state after April. In the hardstate, the source was detected up to 200 keV witha power law index of ∼ ∼ × erg s − . A detailed analysis of theRXTE1720-318 outburst was published by Cadolleet al. [3]. Figure 8 (top panel) shows the RXTE-ASM light curves and the hardness ratio during thesource activity periods, while the bottom panel ofthe same figure shows the IBIS light curves in threeenergy bands. The outburst of the source and itssoftening are clearly visible in both RXTE and IBISlight curves and hardness ratios. Figure 8. XTE J1720-318. Top panel: A,B and CRXTE/PCA bands light curves and the hardness ratio;bottom panel: IBIS light curves in three energy bands(20–40, 40–60, 60-100 keV) and the hardness ratio • is one of the most active transient BHC.Its last outburst was very long covering more thentwo years (2002-2004). Moreover the source wentagain in outburst at the end of 2005. The 2002-2004 outburst, probably the longest observed forthis source, was followed continuously by RXTEand INTEGRAL. The analysis of INTEGRAL and Figure 9. IGRJ17285-2922: IBIS light curves in threeenergy bands (20–40, 40–60, 60-100 keV).
RXTE 2002-2004 outburst data was published byTomsick et al. [12]. • IGRJ17285-2922 had a single period of activity inSeptember 2003 when it was detected by INTE-GRAL. The spectral characteristics and its positionin the sky, very close to the galactic bulge, are con-sistent with a LMXB containing a black hole or aneutron star. Even if the nature of the compact ob-ject is still not constrained, its hard spectrum and thelack of type-I bursts suggests that the source couldbe a BHC. Figure 9 shows IBIS light curves. Adetailed analysis of the IGRJ17285-2922 data, col-lected by INTEGRAL, is published by Barlow et al.[2].
Acknowledgements
We acknowledge the ASI finan-cial/programmatic support via contracts I/R/046/04. Wealso thank Catia Spalletta for careful logistical support.