INTEGRAL hard X-ray constrains on the new X-ray transient Swift J174038.1-273712

ATel #14552; V. Sguera (INAF-OAS Bologna, Italy), L. Sidoli (INAF-IASF MIlano, Italy), E. Kuulkers (ESA/ESTEC, Noordwijk, Netherlands) on behalf of the Galactic bulge monitoring team
on 15 Apr 2021; 13:04 UT
Credential Certification: Vito Sguera (sguera@iasfbo.inaf.it)

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Swift J174038.1-273712 is a new X-ray transient recently discovered during the Swift Bulge Survey (ATel #14536) on 9 April 2021, when the source was detected at a flux level equal to about 3 x 10^-11 erg cm^-2 s^-1 (0.5-10 keV). Follow-up observations were performed with NICER approximately 19 hours later (ATel #14545) and with Neil Gehrels Swift 5 days later (ATel #14550). In particular, the latter Neil Gehrels Swift observation measured a flux smaller by less than a factor of 2 over 5 days, suggesting that the transient activity had already reached its peak but that it was not rapidly decaying (ATel #14550).

The sky region of Swift J174038.1-273712 was covered by INTEGRAL observations of the Galactic Center region carried out from 8 April 2021 at 23:04 UTC to 12 April 2021 at 12:59 UTC, for a total on source exposure time of about 8.5 ks. Swift J174038.1-273712 was not detected in the energy bands 25-60 keV and 60-100 keV; the inferred 3 sigma upper limits are of the order of 6 mCrab or 5.4 x 10^-11 erg cm^-2 s^-1 (25-60 keV) and 16 mCrab or 7.8 x 10^-11 erg cm^-2 s^-1 (60-100 keV). Assuming a source distance of 8 kpc, the 25-60 keV upper limit implies a relatively low luminosity (i.e. smaller than 4 x 10³⁵ erg s^-1). Clearly, INTEGRAL did not detect any bright hard X-ray activity from the transient source; our measured flux upper limits and the fluxes reported in other ATels (#14536, #14545, #14550) are not compatible with standard bright outburst X-ray activity (i.e. Lx about 10³⁶ erg s^-1 or more) from both Low Mass X-ray binaries and High Mass X-ray binaries. Conversely, they are more typical of the so-called Very Faint X-ray Transients.

In order to constrain possible transient activity from the source that may have occurred before its discovery with Swift, we considered preceding INTEGRAL observations of the sky region. Swift J174038.1-273712 was in the INTEGRAL field of view during Galactic Center region observations performed from 2 April 2021 at 06:32 UTC to 3 April 2021 at 05:08 UTC (3.3 ks on source exposure). No detection was achieved, with inferred 3 sigma upper limits of the order of 10 mCrab or 8.7 x 10^-11 erg cm^-2 s^-1 (25-60 keV) and 24 mCrab or 1.2 x 10^-10 erg cm^-2 s^-1 (60-100 keV).