ap-6-11.dvi Acta Polytechnica Vol. 51 No. 6/2011 GRS1758–258: RXTE Monitoring of a Rare Persistent Hard State Black Hole M. Obst, K. Pottschmidt, A. Lohfink, J. Wilms, M. Böck, D. M. Smith, J. A. Tomsick, I. Kreykenbohm Abstract GRS1758–258 is the least studied of the three persistent black hole X-ray binaries in our Galaxy. It is also one of only two known black hole candidates, including all black hole transients, which shows a decrease of its 3-10keV flux when entering the thermally dominated soft state, rather than an increase. We present the spectral evolution ofGRS1758–258 from RXTE-PCAobservations spanning a time of about 11 years from 1996 to 2007. During this time, seven dim soft states are detected. We also consider INTEGRAL monitoring observations of the source and compare the long-term behavior to that of the bright persistent black hole X-ray binary Cygnus X-1. We discuss the observed state transitions in the light of physical scenarios for black hole transitions. Keywords: black hole binaries: general, black hole binaries: individual: GRS1758–258. 1 Introduction GRS1758−258 (Figure 1) is an intermediatemassX- ray binary harboring a black hole and a companion consistent with an early A-type main sequence star, butwithunusual colors [2]. Mass transfer is probably driven byRoche lobe overflow. Among such systems, usually transients, GRS1758−258 is one of only few persistent sources. Generally it can be found in the hard state. However, in some respects it still displays a behavior typical for transient sources [3–6, hystere- sis, rare decay-type soft states]. Fig. 1: INTEGRAL-ISGRI count rate mosaic image in the 20–40keV band obtained during Galactic Center Re- gion Key Programme observations performed in spring 2007 [1] 2 RXTE Monitoring GRS1758–258 was monitored by RXTE in 1–1.5ks pointed snapshots monthly in 1996, weekly through 2000 and twice a week from March 2001 to October 2007. Every year there is a gap from November to January as the sun is too close to theGalactic center and RXTE cannot observe the source. The spectra were modeled taking into account the Galactic ridge background (see Sect. 2.1 for details). The flux has been corrected for the contribution of the Galactic ridge emission (Figure 2 top). Error bars are shown, typical errors are in the range of 1.0–1.5 %. The photon index varies between 1.5 and 3. Most of the time, GRS1758–258 is in the hard state. How- ever, seven dim soft states, during which the flux de- creases and the spectrum softens, appear clearly in the data (Figure 2 bottom). However, no periodic- ity is detected in the occurrences of such soft states. During the 2001 soft state (highlighted in dark red ), the source almost turned off completely. This strongdecline in fluxmakesGRS1758–258especially interesting as it is typical for transient, not for per- sistent sources (see also hardness intensity diagram, Sect.3). 2.1 Background Modeling As GRS1758–258 is a rather faint source located close to the Galactic Center (Figure 1), the RXTE- PCA spectra contain not only source counts but also a strong background component caused by the Galactic ridge emission (Figure 3). In order to distinguish between these, a 13ks background ob- 49 Acta Polytechnica Vol. 51 No. 6/2011 Fig. 2: Top: Flux in keVs−1 cm−2 in the 3–20keV band, fitted to the spectra taken by RXTE. Bottom: Photon index obtained from modeling. Soft states are highlighted for episodes reaching a photon index above 2 Fig. 3: Spectrum for theApril 2003 GRS1758–258 obser- vation, i.e. containing the source as well as the Galactic ridge contribution (blue (black)), and spectrum for the Galactic ridge emission alone (red (grey)) servation 1.5◦ offset from GRS1758–258 was per- formed by RXTE in 1999. Figure 4 contains the spectrum modeled with two bremsstrahlung compo- nents (F1,3−8keV =0.015keVs −1cm−2, kT1 =8keV, F2,3−8 = 0.0027keVs −1cm−2, kT2 = 1.2keV). The iron line complex wasmodeled according to Galactic ridge observations performedwith Suzaku [7]: Three lines at6.4keV,6.67keVand7keV, respectively,have equivalent widths that scale as 85:458:129. 2.2 Spectral parameters The RXTE-PCA spectra in the 3–20keV band were fittedwithanabsorbedpowerlaw,aweakneutral iron Kα line, and a black body disk component where re- quired, always including the Galactic ridge emission (see Sect.2.1). The column density due to interstel- lar absorption in the direction of GRS1758–258 is fixed at NH = 1.5 × 1022cm−2 according to earlier results [8]. An example is shown in Figure 5. The Fig. 4: Spectrum of the Galactic Ridge emission as seen byRXTE. The data were fittedwith two bremsstrahlung components (1: orange dashed line, 2: red dash-dotted line) and an iron line complex as described in [7] Fig. 5: Example spectrum taken byRXTE on 2009 April 08, containing the absorbedpowerlaw component (orange dash-dotted line), the disk (red dashed line) and the iron line (green (grey) solid line) 50 Acta Polytechnica Vol. 51 No. 6/2011 Fig. 6: Spectral parameters from RXTE monitoring observations of GRS1758–258: The photon index, temperature and normalization of the disk component, and the reduced χ2 Fig. 7: Hardness intensity diagram (HID) from RXTE monitoring observations of GRS1758–258 from 1997 to 2007. The seven dim soft states are highlighted as in Figures 6 and 2 disk becomes visible in the dim soft states, the low source flux increasing the error bars (Figure 6). 3 Hardness intensity diagram For energies < 20keVthe hardness intensity diagram (HID) of GRS1758–258 shows a clear hysteresis for hard and soft state fluxes (absorbed fluxes, see Fi- gure 7). This behavior is similar to that shown by blackhole transientsover their outbursts [9,q-shaped HID]. Different fromtransients, there is no rise in the hard state fromquiescence. During themost extreme soft state the 3–20keV flux is clearly below the low- est hard state flux, with no full return to the hard branch observed down to near-quiescence. A com- parison at these energies with our long-term RXTE monitoring observations of the persistent black hole X-ray binary Cyg X-1 is in preparation. Results from spectral fits to 2003–2009 INTE- GRAL monitoring data of GRS 1758–258 [1] allow us to extend the HID studies to higher energies (Fi- gure 8). As expected, neither source shows hystere- sis for energies > 20keV, i.e., in an energy range where only one, namely the hard, spectral compo- 51 Acta Polytechnica Vol. 51 No. 6/2011 Fig. 8: HID from INTEGRAL monitoring of GRS1758–258 from 2003 to 2009 (red (dark grey): monthly binning), compared to the HID from RXTE monitoring of CygX-1 from 1998 to 2010 (black: indi- vidual pointings, green (light grey): monthly binning) nent dominates. Overall, the tracks ofGRS1758–258 in both HIDs are consistent with a persistent hard state source with occasional softening due to a tem- porary decrease in the mass accretion rate as sug- gested by [5]. The hard state HIDs of both sources show a remarkably similar range of hardnesses. As- suming a distance of 1.9kpc for Cyg X-1 as recently determined from radio parallax [10] and dust scatter- ing halo measurements [11], the > 20keV luminosi- ties of both sourceswouldbe similar ifGRS1758–258 had a distance of 6.5kpc. Note, however, that while the decay towards softer, lower luminosity states is qualitatively similar in both sources as well, the lu- minosity ofGRS1758–258hasdroppedmore severely at a given hardness level than that of Cyg X-1. 4 Summary and outlook The analysis of the 1996–2007 RXTE data of GRS1758–258 reveals many details about the long- termbehaviorof the source. Typicalhardstatefluxes after taking into account the Galactic ridge emission are 0.2–0.4keVs−1cm−2. The spectra can be well describedwith an absorbed power lawwith a photon index of 1.5–3. There are indications for a possible detection of an additional weak Fe line. During themonitoring, there are 7 occurrences of soft states with photon indices softer than 2. These states are most likely due to a decrease in the mass accretion rate. During the soft states, a marginal disk detection with kTin ∼ 500–800eV can be seen. The < 20keV HID shows hysteresis with no full return to the hard branch observed down to near- quiescence. The > 20keV HID shows the same hard state luminosity and hardness as CygX-1 but a dif- ferent decay. Acknowledgement This research has been partly funded by the Euro- peanCommission under contract ITN215212 “Black HoleUniverse”andbyDLRcontractDLR50OR1007. K. Pottschmidt andA. M. Lohfink acknowledge sup- portbyNASAINTEGRALGOgrantNNX08AY17G and in additionA. M. Lohfink acknowledges support by the DAAD. References [1] Lohfink, A. M., Pottschmidt, K., Wilms, J., Lubiński, P.: ApJ, 2011, in prep. [2] Muñoz-Arjonilla, A. J., Mart́ı, J., Luque- Escamilla, P. L., et al.: A & A, 519, A15, 2010. [3] Pottschmidt, K., Chernyakova, M., Zdziar- ski, A. A., et al.: A & A, 452, 285, 2006. [4] Smith, D. M., Heindl, W. A., Markwardt,C. B., Swank, J. H.: ApJ, 554, L41, 2001. [5] Smith,D.M.,Heindl,W.A., Swank, J.H.: ApJ, 569, 362, 2002. [6] Soria, R., Broderick, J. W., Hao, J., Han- nikainen, D. C., et al.: MNRAS, 2011, in press (arXiv:1103.3009). [7] Ebisawa, K., Yamauchi, S., Tanaka, Y., et al.: Progress of Theoretical Physics Supplement, 169, 121, 2007. [8] Pottschmidt,K.,Chernyakova,M., Lubiński, P., et al.: InProc. 7th INTEGRALWorkshop. PoS, 2008, p. 98. [9] Fender, R. P., Belloni, T. M., Gallo, E.: MN- RAS, 355, 1105, 2004. [10] Reid, M. J., McClintock, J. E., Narayan, R., et al.: 2011, arXiv:1106.3688. [11] Xiang, J., Lee, J. C., Nowak, M. A., Wilms, J.: ApJ, 2011, in press (arXiv:1106.3378). M. Obst Remeis Observatory/ECAP Bamberg, Germany 52 Acta Polytechnica Vol. 51 No. 6/2011 K. Pottschmidt CRESST/NASA-GSFC Greenbelt, MD, USA UMBC Baltimore, MD, USA A. Lohfink UMCP College Park, MD, USA J. Wilms Remeis Observatory/ECAP Bamberg, Germany M. Böck Remeis Observatory/ECAP Bamberg, Germany D. M. Smith SCIPP/UCSC Santa Cruz, CA,USA J. A. Tomsick SSL/UCB Berkeley, CA, USA I. Kreykenbohm Remeis Observatory/ECAP Bamberg, Germany 53