Palomar Gattini-IR discovery and SpeX classification of the reddened Galactic transient PGIR21fyy as a microlensing event

ATel #14742; K. De (Caltech), V. Karambelkar (Caltech), M. M. Kasliwal (Caltech), M. Hankins (Caltech), J. Jencson (Arizona), J. Sokoloski (Columbia), M. Ashley (UNSW), A. Babul (Columbia), R. M. Lau (ISAS/JAXA), A. Moore (ANU), E. O. Ofek (Weizmann), M. Sharma (Columbia), J. Soon (ANU), R. Soria (UCAS/Sydney), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 28 Jun 2021; 07:08 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Kishalay De (kde@astro.caltech.edu)

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We report the discovery and spectroscopic classification of a reddened Galactic transient PGIR21fyy/AT2021qvq. The transient was discovered in regular survey operations of Palomar Gattini-IR (Moore & Kasliwal 2019; De et al. 2020), and saved as part of a filter to identify large amplitude Galactic transients (De et al. 2021).

The source PGIR21fyy was first detected on UT 2021-06-21 at J = 12.7 +/- 0.3 mag, at J2000 coordinates
RA 18:37:00.67
Dec -18:18:47.9
The source was not detected on UT 2021-06-20 to a 5 sigma depth of J = 12.9 mag. The source subsequently brightened to J = 11.76 +/- 0.2 mag on UT 2021-06-22.

On UT 2021-06-23, we obtained a NIR spectrum of the source using SpeX (Rayner et al. 2003, PASP 115, 362) on the NASA Infrared Telescope Facility, as part of program 2021A083 (PI: De). The data were obtained in SXD mode using a 0.3" slit. The reduced spectrum shows a relatively featureless continuum with narrow absorption features of Mg I, Si I and Ca II. The overall spectrum is consistent with a G-type star (Rayner et al. 2009).

The transient is also detected in the ZTF alert stream (Bellm et al. 2019), at g = 16.2 mag, suggesting a large g - J color of ~ 3.5 mag. The transient is coincident with a faint PS1 source at g ~ 21.3 mag. The NIR transient has since faded to J > 13.0 mag on UT 2021-06-27. Overall, the spectrum, quiescent history and large amplitude of the brightening are consistent with a microlensing event in the direction of the Galactic bulge.