Palomar Gattini-IR discovery of a bright Galactic microlensing event PGIR20emh/AT 2020rjf

ATel #13969; V. Karambelkar (Caltech), P. Mroz (Caltech), K. De (Caltech), J. van Roestel (Caltech), M. M. Kasliwal (Caltech), M. Hankins (Arkansas Tech), M. Ashley (UNSW), A. Babul (Columbia), J. Jencson (U. Arizona), R. M. Lau (ISAS/JAXA), A. Moore (ANU), E. O. Ofek (Weizmann), M. Sharma (Columbia), J. Soon (ANU), R. Soria (NAOC), J. Sokoloski (Columbia), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 26 Aug 2020; 02:36 UT
Credential Certification: Viraj Karambelkar (viraj@astro.caltech.edu)

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We report the detection of a bright Galactic microlensing event PGIR20emh/ AT2020rjf during regular survey operations of Palomar Gattini-IR(De et al. 2020, Moore & Kasliwal 2019). Palomar Gattini-IR (PGIR) is a wide-field NIR transient survey scanning the entire Northern sky in J band to a median depth of 15.7 AB mag every two nights.
PGIR20emh was first detected in the Gattini data processing pipeline on UT 2020-07-18 at a J magnitude of 11.56 +/- 0.3 AB mag, at J2000 coordinates of RA 17:30:40.02 Dec -27:48:47.5 corresponding to a Galactic latitude of 3.7 degrees. This location is coincident with a quiescent 2MASS source with a J mag of 13.6 (AB). The integrated extinction along this line of sight is ~ 4 mags in g band, ~ 3 mags in r band and ~ 1 mag in J band (Schlafly et al. 2011). The source subsequently rose to a J-band magnitude of 9.04 +/- 0.01 on UT 2020-08-19, which is the last PGIR observation of this source.
We examined the g and r-band lightcurves of this source using data from the Zwicky Transient Facility (ZTF, Bellm et al. 2018). The lightcurves indicate that the source was at its quiescent brightness of g ~ 19 mags and r ~ 16 mags on UT 2020-05-18. Over the next ~90 days, it brightened substantially to g = 14.83 +/- 0.05 on UT 2020-08-19 and r = 12.64 +/- 0.05 on UT 2020-08-20. Since then, it has faded to g = 15.40 +/- 0.05 on UT 2020-08-23 and r = 13.49 +/- 0.05 on UT 2020-08-24.
We fit the lightcurve with a point-source point-lens model and obtained an excellent fit with the current best-fitting parameters t0 = 2459080.998 +/- 0.095, tE = 79.8 +/- 5.7 d, and u0 = 0.0276 +/- 0.0025.
We obtained an optical spectrum of the source on UT 2020-08-19 using the Low Resolution Imaging Spectrometer (Oke et al. 1995) on the Keck 10-m telescope. The spectrum shows a red continuum together with strong Ca II (8498, 8542, 8662 A), Na I (5890 A), weak H-alpha and broad, weak TiO absorption (6200 A). The spectrum suggests that the lensed source is a cool star. Further follow-up observations are underway and we encourage multi-wavelength follow-up observations of the source.