X-ray Spectroscopy of H1743-322 in Outburst

ATel #7652; J. Neilsen (MIT), S. Motta (Oxford), M. Coriat (IRAP), R. Fender (Oxford), G. Ponti (MPE), S. Corbel (CEA Saclay), A. Sanna (University of Cagliari)
on 17 Jun 2015; 15:59 UT
Credential Certification: Joey Neilsen (jneilsen@space.mit.edu)

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Following the report of the black hole H1743-322 in outburst (ATel #7607), we triggered a Chandra campaign to observe the rising portion of the outburst. We are extremely grateful to Harvey Tananbaum, the Chandra Director's Office, and Chandra Mission Planning for executing our observations on short notice.

H1743-322 was observed for 9.4 ks (good exposure 9.2 ks) three times at high spectral resolution with the Chandra High Energy Transmission Grating Spectrometer (starting on 2015-06-11.857, 2015-06-12.535, and 2015-06-13.393, respectively). We extract first-order grating spectra from each observation and model them with an absorbed power law, which provides a satisfactory fit from 1-10 keV. No strong line features are apparent. All three spectra are consistent with an interstellar absorbing column density NH = (2.60 +/- 0.09) x 10^22 cm^-2. The mean photon index is Gamma = 1.26 +/- 0.04, and there are ~1-sigma indications of steepening with time. H1743-322 increased in brightness between each observation. In chronological order, we find absorbed fluxes of 1.04 +/- 0.02, 1.08 +/- 0.02, and 1.13 +/- 0.02 (in units of 10^-9 erg/s/cm^2). For a distance of 8.5 kpc (Steiner et al. 2012), the implied unabsorbed luminosity is ~10^37 erg/s.

We also requested several Swift observations, which give similar spectral results but also provide constraints on the timing properties of the source. The 0.1-64 Hz fractional rms variability is about 30% over the full XRT energy band. The power density spectrum can be described as flat-topped noise with a QPO between 0.2-0.4 Hz; first and second harmonics are also visible.

The spectral and timing parameters reported here for H1743-322, along with the MAXI and Swift/BAT light curves, are consistent with a black hole in the early phases of a hard state. Our results are fairly similar to those reported by McClintock et al. (2009) for the initial ~10 days of the long, bright outburst in 2003. The BAT flux increase may be slowing now, so it remains to be seen how the present outburst will compare to the recent decade of activity from H1743-322.

We are grateful to Neil Gehrels and the Swift team for promptly scheduling and executing the XRT observations. A JVLA observation coincident with the third Chandra observation will be reported on separately, but we are indebted to the NRAO staff for their assistance facilitating our campaign as well.

We have a number of follow-up observations planned with Chandra, JVLA, and NuSTAR, but additional multiwavelength observations of the source are strongly encouraged.

References:

McClintock, J., et al. 2009, ApJ, 698, 1398
Steiner, J., et al. 2012, ApJ, 745, L7
Zhang, H., et al. 2015, ATel, 7607, 1