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MAXI J1630-276: Swift/XRT non-detection

ATel #11198; J. A. Kennea (PSU), P. A. Evans, A. P. Beardmore (U Leicester), H. A. Krimm (NSF/USRA), P. Romano (INAF-OAB), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 18 Jan 2018; 15:18 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Subjects: X-ray, Neutron Star, Transient

At 19:20UT on 2018 January 16, Swift/XRT performed a 7-point tiling observation (500s per tile) of the error regions of MAXI J1630-276 as provided by Negoro et al. (ATEL #11173). MAXI J1630-276 is a soft X-ray transient first detected on 2017 November 17 by MAXI (Negoro et al., ATEL #10984).

We detect no point sources inside any of the XRT pointings. The XRT observations completely cover the error regions in ATEL #11173, including additional systematic errors. ROSAT source 1RXS J163200.2-273335, is inside the observed region covered by the XRT pointings, but is not detected. 1RXS J163035.5-275500 is outside of the field of view. Neither ROSAT source is consistent with the updated MAXI error regions.

We therefore conclude that MAXI J1630-276 has likely turned off. Based on the low background level, and non-detection of any sources, we place a 90% confidence upper limit on the brightness of MAXI J1630-276 of ~10-2 XRT count/s, assuming the spectral parameters given in ATEL #11173, this equates to a flux of 8 x 10-13 erg/s/cm2 (0.5 - 10 keV). We note that this level is not consistent (~2 orders of magnitude lower) than the expected flux as extrapolated from the MAXI measured decay rate between 2017 December 6 and 2017 December 24 (ATEL #11173). This suggests that the source may have turned off rapidly, for example as a result of the propeller effect.