Swift Bulge Survey: Follow-up observations of the transient source Swift J174038.1-273712

ATel #14550; Rivera Sandoval, L. E., Heinke, C. O. (U. Alberta), Maccarone, T. (TTU), Shaw, A.(U. Nevada, Reno), Sivakoff, G. (U. Alberta) on behalf of the SBS team
on 15 Apr 2021; 03:51 UT
Credential Certification: Liliana Rivera Sandoval (lriveras@ualberta.ca)

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We report Neil Gehrels Swift Observatory follow-up observations of the recently discovered transient source Swift J174038.1-273712 (Atel #14536, #14545), towards the Galactic bulge. The transient has decayed by less than a factor of 2 over 5 days, suggesting that it has already peaked in Lx, but that it is not rapidly decaying.

Observations were obtained on 2021-04-14 with a total exposure time of 635s. Fitting the X-ray spectrum of the source with an absorbed power-law where N_H is fixed to the Galactic value of 5.5E21 cm^-2 and using the Cash statistics, we have obtained a power-law photon index of 1.5(+/-0.3) and a X-ray flux of 1.9(+0.4/-0.3)*1E-11 erg/s/cm^2 in the 0.5-10 keV band. There are too few counts in the Swift spectrum to fit the absorbed (blackbody+thermally comptonized continuum) model indicated by the NICER data. This flux is slightly fainter than the 0.5-10 keV flux measured by Swift [3.2(+2.3/-1.1)*1E-11 erg/s/cm^2, Atel #14536] at about 2021-04-08 20:06 (the 2021-04-09 date reported by Atel #14536 was the last time of the Bulge survey fields). Approximately 19 hours later, the flux measured by NICER [1.5(+/-0.2)*1E-11 erg/s/cm^2 0.5-10 keV, Atel #14545] is consistent with the Swift flux from 2021-04-14. It is possible that NICER's absolute flux could be slightly incorrect, as it is not an imaging instrument and this is a faint source in a crowded field. It is unclear whether this transient may have had a relatively bright outburst that peaked before our initial observations, but INTEGRAL data of this region may constrain this scenario.

Neither 2021-04-14 UVOT observations of the field in the UVM2 filter, nor archival UVOT images in the same filter, reveal any source at the position of Swift J174038.1-273712. Further Swift observations are planned.

We thank the Swift team for scheduling our ToO request.