The 2005 outburst of GRO J1655−40: spectral evolution of the rise, as observed by Swift

We present Swift observations of the black hole X-ray transient, GRO J1655−40, during the recent outburst. With its multiwavelength capabilities and flexible scheduling, Swift is extremely well suited for monitoring the spectral evolution of such an event. GRO J1655−40 was observed on 20 occasions and data were obtained by all instruments for the majority of epochs. X-ray spectroscopy revealed spectral shapes consistent with the ‘canonical’ low/hard, high/soft and very high states at various epochs. The soft X-ray source (0.3‐10 keV) rose from quiescence and entered the low/hard state, when an iron emission line was detected. The soft X-ray source then softened and decayed, before beginning a slow rebrightening and then spending ∼3 weeks in the very high state. The hard X-rays (14‐150 keV) behaved similarly but their peaks preceded those of the soft X-rays by up to a few days; in addition, the average hard X-ray flux remained approximately constant during the slow soft X-ray rebrightening, increasing suddenly as the source entered the very high state. These observations indicate (and confirm previous suggestions) that the low/hard state is key to improving our understanding of the outburst trigger and mechanism. The optical/ultraviolet light curve behaved very differently from that of the X-rays; this might suggest that the soft X-ray light curve is actually a composite of the two known spectral components, one gradually increasing with the optical/ultraviolet emission (accretion disc) and the other following the behaviour of the hard X-rays (jet and/or corona). Ke yw ords: accretion, accretion discs ‐ stars: individual: GRO J1655−40 ‐ X-rays: binaries.

[1]  C. Brocksopp,et al.  The orbital modulation in the radio emission of Cygnus X-1 , 2002, astro-ph/0206460.

[2]  P. Podsiadlowski,et al.  The quiescent light curve and the evolutionary state of GRO J1655–40 , 2001, astro-ph/0109136.

[3]  Tasso Tzioumis,et al.  XTE J1550–564: a superluminal ejection during the September 1998 outburst , 2000, astro-ph/0011051.

[4]  J. Orosz,et al.  Optical Observations of GRO J1655–40 in Quiescence. I. A Precise Mass for the Black Hole Primary , 1996, astro-ph/9610211.

[5]  Elena Belsole,et al.  Proceedings of the X-ray Universe 2005, Vols 1 and 2 , 2006 .

[6]  S. J. Tingay,et al.  Relativistic motion in a nearby bright X-ray source , 1995, Nature.

[7]  J. Lasota.  The disc instability model of dwarf novae and low-mass X-ray binary transients , 2001, astro-ph/0102072.

[8]  T. Belloni,et al.  GRS 1915+105 and the Disc-Jet Coupling in Accreting Black Hole Systems , 2004 .

[9]  Richard M. Ambrosi,et al.  Readout modes and automated operation of the Swift X-ray Telescope , 2003, SPIE Optics + Photonics.

[10]  M. C. Begam,et al.  The optical counterpart of the superluminal source GRO J1655 – 40 , 1995, Nature.

[11]  Chris Biemesderfer,et al.  Astronomical Data Analysis Software and Systems X , 2001 .

[12]  B. A. Harmon,et al.  The Dual Nature of Hard X-Ray Outbursts from the Superluminal X-Ray Transient Source GRO J1655–40 , 1996, astro-ph/9610212.

[13]  Alan A. Wells,et al.  The Swift Gamma-Ray Burst Mission , 2004, astro-ph/0405233.

[14]  R. Hunstead,et al.  Radio Emission from GRO J1655–40 during the 1994 Jet Ejection Episodes , 2000, astro-ph/0003466.

[15]  Marc Klein-Wolt,et al.  Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate , 2000 .

[16]  I. Mirabel,et al.  A superluminal source in the Galaxy , 1994, Nature.

[17]  W. Lewin,et al.  Compact stellar X-ray sources , 2006 .

[18]  R. P. Fender,et al.  Initial low/hard state, multiple jet ejections and X-ray/radio correlations during the outburst of XTE J1859+226 , 2001, astro-ph/0112137.

[19]  G. J. Fishman,et al.  Correlations between X-ray outbursts and relativistic ejections in the X-ray transient GRO J1655 – 40 , 1995, Nature.

[20]  Roger D. Blandford,et al.  Relativistic jets as compact radio sources , 1979 .

[21]  J. Orosz,et al.  An Optical Precursor to the Recent X-Ray Outburst of the Black Hole Binary GRO J1655–40 , 1997, astro-ph/9701098.

[22]  R. K. Jain,et al.  X-Ray States and Radio Emission in the Black Hole Candidate XTE J1550–564 , 2001, astro-ph/0102114.

[23]  T. Takahashi,et al.  Broadband High-Energy Observations of the Superluminal Jet Source GRO J1655–40 during an Outburst , 1996, astro-ph/9611023.

[24]  Carlos E. C. J. Gabriel,et al.  Astronomical Data Analysis Software and Systems Xv , 2022 .

[25]  N. Gehrels,et al.  Detection of Absorption-Line Features in the X-Ray Spectra of the Galactic Superluminal Source GRO J1655–40 , 1998 .

[26]  R. M. Hjellming,et al.  Episodic ejection of relativistic jets by the X-ray transient GRO J1655 - 40 , 1995, Nature.

[27]  Alan M. Levine,et al.  Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550–564 , 2000 .