Swift J1845.7-0037: 199s period detection, possible BeXRB?

ATel #13195; J. A. Kennea (PSU), P. A. Evans, A. P. Beardmore (U Leicester), A. Bahramian (ICRAR), H. A. Krimm (NSF/USRA), P. Romano (INAF-OAB), K. Yamaoka (Nagoya U.), M. Serino (RIKEN) and H. Negoro (Nihon U.)
on 16 Oct 2019; 12:18 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

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We report on the observations performed by Swift of the X-ray transient Swift J1845.7-0037 (ATEL#4130, ATEL #13191) which was recently found to be in outburst again by MAXI (ATEL #13189), the first detection since it was first discovered in 2012. On October 14 and 15, 2019, Swift/XRT performed 2 additional 1ks Windowed Timing mode observations of the source. The source was well detected, and the light-curve in both observations shows a clear modulation. A periodicity search finds a period of 199.2s is present, but due to the small number of cycles in these short observations, the detection is relatively low significance. However, analysis of archival data from the 2012 observations by Swift, show a clear detection of a periodicity at 199.4s, suggesting that this period is real.

This period detection would suggest that Swift J1845.7-0037 contains an accreting neutron star. The relatively short outburst time seen in the 2012 observation may indicate that the X-ray flaring is due to Type 1 outbursts from a Be/X-ray Binary. This would naively suggest a ~7.5 year orbital period for the system, if the outbursts occurred at periastron, although it's not possible to determine this without further outburst detections, or observations at other wavelengths.

The spectrum of the source is hard, and well fit with a power-law of photon index 0.63 +/- 0.20. The detected flux from the system is 2 x 10^-10 erg/s/cm² (0.3 - 10 keV), not corrected for absorption, which is high (5 x 10²² cm^-2).

Further observations have been requested in order to monitor the current outburst, and better determine the source type.