Soft X-rays from the direction of the Type Ia SN AT2019daj are not associated with the supernova but from foreground diffuse emission within our own Galaxy

ATel #12686; Dheeraj Pasham (MIT), Teruaki Enoto (Kyoto Univ.), Michael Loewenstein (NASA/GSFC/UMBC), Jon M. Miller (U. of Michigan), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Diego Altamirano (Southampton), Michael Corcoran (NASA/GSFC), Paul S. Ray (NRL), Ron Remillard (MIT), Kenji Hamaguchi (NASA/GSFC), Tod E. Strohmayer (NASA/GSFC), Andrew Fabian (U. of Cambridge), and Deepto Chakrabarty (MIT) on behalf of the NICER team.
on 25 Apr 2019; 02:10 UT
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The Type 1a SN AT2019daj (z=0.038) was detected in late March and early April by the ATLAS and the MASTER sky surveys, respectively (GCN #24076; ATel #12644; GCN #24069). NICER promptly followed up the target, and detected clear X-ray emission in that direction which we modeled with various diffuse gas emission models (ATel #12672). NICER continued to monitor this field of view as the detection of X-ray emission from a young Type 1a SN is of high scientific merit.

However, analysis of archival ROSAT and new Swift/XRT data indicate that the X-ray emission NICER detected in the direction of the Type 1a AT2019daj is NOT associated with the supernova but instead originates from a diffuse (extended) foreground region.

We have revisited archival data going back to the 1990s and found that the NICER pointing is spatially coincident with the diffuse X-ray foreground detected by the ROSAT all-sky survey in the 1990s (Snowden et al. 1997). Also, the flux we estimate from NICER spectral modelling is consistent with earlier measurements from ROSAT.

We also obtained a 1.5 ks Swift/XRT exposure and did not find any point sources within the 3 arcmin field of view of NICER. We detect 2 counts in 0.3-8 keV bandpass within a 47'' radial extraction region (count rate of roughly 0.001 cps), which is consistent with the background. Nevertheless, by associating these events to a point source with an X-ray spectrum similar to that of the Type 1a SN 2012a with X-rays (Bochenek et al. 2018; plasma with temperature of 2 keV with an intrinsic nH of 6x10^22 /cm^2 at z=0.038), we set an upper limit on the unabsorbed 0.3-8 keV X-ray flux of a point source to be 4x10^-13 erg/s/cm^2. However, the XRT X-ray spectrum derived from a 3 arcmin extraction region is consistent with our best-fit NICER plasma model (ATel #12672) again asserting a foreground origin for this detection.

NICER has thus far accumulated over 38 ks of exposure time from this field of view centered on the Galactic coordinates RA=211.3261, and Dec.=-39.88604 (J2000.0). These datasets will soon be public and would provide a detailed spectral view of the diffuse X-ray emission from a region previously not observed with any of the large X-ray telescopes including XMM-Newton, Suzaku and Chandra. Given the high sensitivity of NICER and the large 3 arcmin field of view these datasets also demonstrate NICER's ability to carry out highly sensitive studies of extended X-ray emitting regions.

NICER can carry out prompt follow-up observations of transients and is planning to systematically follow up alerts from LIGO and other X-ray bright extra-galactic transients in the future.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.

We thank the Swift team for executing a TOO request.