Swift/XRT Observations of IGR J17091-3624 and IGR J17098-3628

ATel #1140; J. A. Kennea (PSU) and F. Capitanio (IASF-INAF)
on 11 Jul 2007; 15:02 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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At 14:47UT, July 9th 2007, Swift/XRT took a 2.5ks exposure of the field containing IGR J17098-3628. The observation, which was taken in Window Timing mode, reveal detections of two sources in the XRT field of view, IGR J17098-3628 (e.g. Grebenev et al., ATEL #444), and IGR J17091-3624 (Kuulkers et al., ATEL #149). Both sources were in relatively bright states as of the time of the observation.

The spectrum of IGR J17091-3624 is well fit (reduced chi² = 1.4 by an absorbed (N_H = 0.78 +/- 0.05 x10²²) power-law model with photon index = 1.60 +/- 0.05, similar to that seen when the source was previously in a hard state (Capitanio et al., 2006). An absorbed disk blackbody model provides a higher quality fit (reduced chi² = 1.1), and the following parameters: N_H = (0.51 +/- 0.03) x 10²² cm², kT = 2.1 +/- 0.1 keV. The 0.2-10 keV Flux from the source is ~7x10^-11 erg/s/cm2, or ~10^-10 erg/s/cm² corrected for absorption.

The spectral fitting of IGR J17098-3628, is poorly fit by a power-law model (reduced chi² = 4.9). However the source can be well fit by an absorbed disk blackbody model, which N_H=(0.89+/-0.02)x10²² cm^-2 and kT = 0.89 +/- 0.01 keV. The 0.2-10 keV flux is ~4 x 10^-10 erg/s/cm², or ~9 x10-10 erg/s/cm2 corrected for absorption.

A follow-up observation of the field was taken in Photon Counting mode starting 16:36UT, July 10th 2007 for 500s, in order to determine positions for both sources. This observation reveals that the IGR J17091-3628 is at the following location:

RA(J2000) = 17h 09m 07.6s,
Dec(J2000) = -36d 24' 24.9'',

with an estimated uncertainty of 3.6 arcseconds radius (90% confidence). This location lies 20 arcseconds from the INTEGRAL position (Kuulkers et al., ATEL #149) and as such is consistent with being the X-ray counterpart of the INTEGRAL source. However this X-ray position appears to rule out the tentative radio counterpart of of IGR J17091-3624 (Rupen et al, ATEL #152, Pandey et al., 2006), which lies outside of the XRT error circle, approximately 86 arcseconds away. This also rules out the optical counterpart suggested by Negueruela and Schurch (2006) on the basis of the radio counterpart.

The refined XRT position for IGR J17098-3628 is:

RA(J2000) = 17h 09m 45.9s,
Dec(J2000) = -36d 27' 58.8',

with an estimated uncertainty of 3.5 arcseconds (90% confidence). This position is 1.8 arcseconds from the XRT position reported by Kennea et al. (ATEL #476) and 1.5 arcseconds from the probable radio counterpart reported by Rupen et al. (ATEL #490).