NICER detects X-rays from the direction of the Type Ia supernova AT2019daj

ATel #12672; Dheeraj Pasham (MIT), Michael Loewenstein (NASA/GSFC/UMBC), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Jon M. Miller (U. of Michigan), Michael Corcoran (NASA/GSFC), Paul S. Ray (NRL), Ron Remillard (MIT), Teruaki Enoto (Kyoto Univ.), Kenji Hamaguchi (NASA/GSFC), Tod E. Strohmayer (NASA/GSFC), Andrew Fabian (U. of Cambridge), Deepto Chakrabarty (MIT)
on 19 Apr 2019; 17:28 UT
Credential Certification: Dheeraj Pasham (drreddy@mit.edu)

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The transient AT2019daj (MASTER OT J140518.22-395309.9 or ATLAS2019eti) was first detected by the ATLAS sky survey on 2019-03-27 UT, and was later independently detected by the MASTER Global Robotic Net survey on 2019-04-09.07809 UT (GCN #24076; ATel #12644). As this transient was within the LIGO/VIRGO error box for event S190408an (GCN #24069) NICER made several observations between 2019-04-09 and 2019-04-17 with a total exposure of approximately 23 ks. However, in the last week it has been shown that the optical transient is not related to the LIGO/VIRGO event (GCN #24078), and that its optical spectrum is very similar to Type Ia supernovae two to three weeks after maximum (https://wis-tns.weizmann.ac.il/object/2019daj).

Our NICER observations centered on the target clearly reveal the presence of a soft X-ray source, with background-subtracted countrate ~ 0.5 cts/sec. We modeled the average X-ray spectrum with a continuum-only emission spectrum from collisionally ionized diffuse gas (nlapec in XSPEC with best fit kT=0.2 keV) plus five Gaussians, absorbed by the intervening Galactic column density of NH, and this yielded a good fit with a reduced chi-square of 62 for 89 degrees of freedom. The best-fit (local-frame) energies of the Gaussians are 0.350, 0.766, 0.894, 0.673, and 0.574 keV, corresponding to the energies of C VI L-alpha, O VIII L-beta, Ne IX He-alpha, O VIII L-alpha+O VII He-beta, and O VII He-alpha, respectively. Assuming a redshift of 0.038 derived from the optical spectrum implies an unabsorbed X-ray (0.3-2.0 keV) luminosity of ~2x10e42 erg/sec. This is a factor >100 higher than the X-ray luminosity of the only Type 1a SN detected in X-rays thus far (AT2012ca; Bochenek et al. 2018). Good fits may also be obtained with equilibrium or non-equilibrium thermal plasma models; these require large blueshifts with respect to the optical spectrum (z<0.015 for the variable abundance non-equilibrium model).

It is currently unclear whether the X-rays are from the Type 1a SN or from another, foreground transient unrelated to it. We have requested a 1 ks ToO observation with Swift/XRT to obtain a better X-ray localization. NICER is planning several observations in the coming week (see schedule here: https://heasarc.gsfc.nasa.gov/docs/nicer/schedule/), and we encourage follow-up observations at other wavelengths, especially optical spectra to track the evolution of this unusual transient.

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.