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New Galactic transient IGR J17451-3022 still soft

ATel #6501; A. Bahramian, C. O. Heinke (Alberta), A. P. Beardmore (U. Leicester), D. Altamirano (Southampton), R. Wijnands (Amsterdam)
on 26 Sep 2014; 20:32 UT
Credential Certification: Arash Bahramian (bahramia@ualberta.ca)

Subjects: X-ray, Black Hole, Neutron Star, Transient

Referred to by ATel #: 6533, 7028

We report the analysis of new Swift/XRT observations of the transient IGR J17451-3022. This is a new transient discovered by INTEGRAL JEM-X (ATel #6451) which has been monitored by Swift/XRT multiple times per week (ATels #6459, #6469, #6486).

IGR J17451-3022 was observed by Swift/XRT in WT mode on September 25th. The source shows an absorbed flux (0.5-10 keV) of 1.1e-10 erg/s/cm2, similar to previous observations. We performed spectral analysis in the 0.8-10 keV band, comparing a blackbody, a disk blackbody and a power-law model. We found that a blackbody model with a temperature of 1.0±0.1 keV, or a disk blackbody model with inner disk temperature of 1.4±0.2 keV, better fit the spectrum (reduced chi-squared of 1.2 and 1.3 for blackbody and disk blackbody respectively), compared to a power-law with photon index 2.9±0.5 (reduced chi-squared of 1.5 for 12 degrees of freedom). These temperatures are consistent with the previous observations reported in ATel #6459.

In ATel #6486, we reported an apparent spectral hardening in the Swift/XRT observation performed on Sep. 21st, where there was a 40 arcsec offset between the source and the active part of the detector. Performing a refined spectral analysis with a smaller extraction region (15 pixels radius) and careful extraction of the background from a same-size region, we find that the spectrum in the Sep. 21st observation is consistent with the previous observations, indicating that the observed hardening was spurious.

As our new analysis shows a consistent soft spectrum for this transient, its nature remains unclear; a low-mass X-ray binary or a magnetar remain viable possibilities.

We thank the Swift team for arranging our observations.