New X-ray outburst of accreting millisecond pulsar SWIFT J1756.9-2508 detected by INTEGRAL

ATel #11497; I. A. Mereminskiy, S. A. Grebenev, R. A. Krivonos (Space Research Institute, Moscow), R. A. Sunyaev (Max Plank Institute for Astrophysics, Garching)
on 3 Apr 2018; 09:47 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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During recent observations (1-2 Apr 2018, PI: E.Bozzo) of the weak X-ray burster IGR J17379-3747 (#11447,#11487, Chelovekov et al. 2006, AstL, 32, 456) and regular observations of Galactic center region (2-3 Apr 2018, PI: R.A. Sunyaev) we detected a new X-ray transient in 20-60 keV sky maps obtained by IBIS/ISGRI. Its position is consistent with that of known accreting millisecond pulsar SWIFT J1756.9-2508.
In order to identify the transient we performed ToO observation with XRT onboard The Neil Gehrels Swift Observatory. Although the field is heavily polluted by single scattered photons from nearby bright LMXB GX 5-1 we clearly detected the source, with best-estimated position of (J2000) 269.23892, -25.10790 (error is 2.5", 90% confidence), which is 0.65" from catalog position of SWIFT J1756.9-2508.
Fitting the XRT spectrum with absorbed powerlaw model we found strong absorption NH = (7.9±2.7)×10²² and a moderate slope of 1.6+-0.6. Total observed flux in 0.3-10 keV is (3.0±0.5)×10^-10 erg cm^-2s^-1 and quasi-simultaneous observations of INTEGRAL yield an (5.5±0.3)×10^-10 erg cm^-2s^-1 (44±3 mCrab) flux estimate in 20-60 keV.
Given the positional coincidence and strong obscuration, that is consistent with previous observations (e.g. Krimm et al, 2007, ApJ,688,147) we proposing that the observed transient is a new outburst of accreting millisecond pulsar SWIFT J1756.9-2508.
Follow-up observations are strongly encouraged. We are very grateful to The Neil Gehrels Swift Observatory team and its PI Brad Cenko for rapid scheduling of our observations.