Chandra High Energy Transmission Gratings Observations of the Accreting millisecond Pulsar IGR J17591-2342

ATel #11988; Michael Nowak (Dept. of Physics, Washington University, St. Louis), Ada Paizis (Istituto Nazionale di Astrofisica), Jérôme Chenevez (DTU Space), Sylvain Chaty (Université Paris Diderot, Institut Universitaire de France et CEA), Jörn Wilms (Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics (ECAP), Universität Erlangen-Nürnberg), Jérôme Rodriguez (CEA/IRFU-CNRS/INSU-Universit ?e Paris Didero)
on 25 Aug 2018; 20:39 UT
Credential Certification: Michael Nowak (mnowak@space.mit.edu)

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Starting on 2018 Aug 23 at 17:53 UTC, Chandra performed a 20 ksec follow up observation of IGR J15791-2342 (see ATELs 11941, 11942, 11946 11954, 11957, 11970, 11981, and 11985) using the High Energy Transmission Grating Spectrometer (HETGS). The Chandra position, determined from intersecting the dispersed spectra of the High Energy Gratings (HEG) and Medium Energy Gratings (MEG) is (J2000) RA = 17:59:02.83, Dec = -23:43:0.8.0. This position is within 0.6 arcsec of that reported from VLT observations of an IR counterpart (ATel 11970) and within 0.5 arcsec of that reported from ATCA observations of a radio counterpart (ATel 11954). Without any other sources within the Chandra field of view to further refine the astrometry, these small differences are consistent with typical Chandra astrometric uncertainties. The source remains bright with a 1-9 keV flux of (1.56+/-0.03) x 10^-10 erg/cm^2/s (90% confidence level, not corrected for absorption), with minimal long time scale variability over the course of this observation. Preliminary fits indicate that the X-ray spectra of IGR J15791-2342 are well-described by an absorbed power law with photon index of 1.80+/-0.06 (90% confidence level) and NH column of (4.9 +/-0.2) x 10^22 cm^-2, where we have modeled the absorption with the prescription (including abundances) of Wilms, Allen, & McCray (2000, ApJ, 542, 914). The measurement of the absorption column density is partly driven by detection of a Si edge; however, further work is required to assess what fraction of the absorption is local to the system and what fraction is attributable to the interstellar medium. There is some evidence for further high resolution structure within the HETGS spectra. As the source remains bright, additional multi-wavelength observations are encouraged.