INTEGRAL detection of hard X-ray emission from the Very Faint X-ray Transient IGR J17285-2922

ATel #12646; L. Ducci (IAAT, Germany; ISDC, Switzerland), V. Grinberg (IAAT, Germany), J. Wilms (ECAP, Germany), J. Rodriguez (CEA Saclay/lab AIM, France), E. Bozzo, C. Ferrigno, V. Savchenko (ISDC, Switzerland)
on 9 Apr 2019; 19:14 UT
Credential Certification: Lorenzo Ducci (lorenzo.ducci@unige.ch)

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During the INTEGRAL observations of the Galactic Center field performed from 2019 April 8 at 17:30 to April 9 at 8:12 (UTC) the IBIS/ISGRI instrument on board INTEGRAL detected hard X-ray emission from IGR J17285-2922 (aka XTE J1728-295).

The source is detected in the 20-40 keV IBIS/ISGRI mosaic (effective exposure time 28 ks) with a significance of about 10.6 and a flux of 17.3 ± 1.6 mCrab, while it is barely detected in the energy band 40-80 keV (significance 3.4) with a flux of 9.9 ± 2.9 mCrab. IGR J17285-2922 is not detected in the JEM-X mosaics (effective exposure time 6.8 ks). We estimated a 3 σ upper-limit in the 3-10 keV energy band of 6.3 mCrab and a 3 σ upper-limit of about 16 mCrab in the 10-20 keV energy band. Assuming a distance of 8.5 kpc (Barlow et al. 2005, A&A 437, 27), the measured flux corresponds to a luminosity of about 10³⁶ erg/s, consistent with previous observations (Barlow et al. 2005; Sidoli et al. 2011, MNRAS 415, 3, 2373).

The hard X-ray emission detected by INTEGRAL could indicate a possible onset of an outburst. The previous was detected in 2010 August 26-30 (ATels: #2823, #2824, #2825), when IGR J17285-2922 was detected by INTEGRAL in the energy range 20-40 keV with a flux of 10 ± 2 mCrab and classified as a possible Very Faint X-ray Transient (Sidoli et al. 2011). The 3-10 keV 3 σ upper-limit could indicate that IGR J17285-2922 is slightly fainter than in the previous outburst of 2010 August, when its 2-10 keV flux (measured with RXTE/PCA) was about 6.5 mCrab (ATel: #2823).

Further INTEGRAL observations of the region around IGR J17285-2922 are planned for the coming weeks. Multiwavelength observations of the source are encouraged.