Palomar Gattini-IR discovery of a peculiar, rapidly rising and highly reddened Galactic transient PGIR21fjn/AT2021kgk
ATel #14567; R. Soria (UCAS/Sydney), K. De (Caltech), M. M. Kasliwal (Caltech), K. S. Tinyanont (Santa Cruz), R. M. Lau (ISAS/JAXA), M. Hankins (Caltech), J. Sokoloski (Columbia), M. Ashley (UNSW), A. Babul (Columbia), V. Karambelkar (Caltech), J. Jencson (Arizona), A. Moore (ANU), E. O. Ofek (Weizmann), M. Sharma (Columbia), J. Soon (ANU), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 22 Apr 2021; 03:15 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Roberto Soria (rsoria@physics.usyd.edu.au)
Subjects: Infra-Red, Request for Observations, Nova, Transient
We report the discovery of a highly reddened Galactic transient by the Palomar Gattini-IR survey (Moore & Kasliwal 2019). Palomar Gattini-IR is a wide-field NIR transient survey scanning the entire visible sky every two nights to a depth of J ~ 15.7 AB mag (De et al. 2020). The source was identified as part of a dedicated filter to select large amplitude Galactic plane transients, as described in De et al. (2021).
The source PGIR21fjn / AT2021kgk was first detected on UT 2021-04-19 at a magnitude of J = 12.28 +/- 0.01 Vega mag, at J2000 coordinates RA 19:07:58.625, Dec +08:43:45.14. It was not detected on UT 2021-04-18 to a depth of J ~ 14 Vega mag, thus constraining the transient to be ~ 2 days old. Subsequent data shows that it brightened to J ~ 9 Vega mag on UT 2021-04-21, confirming a rapidly rising transient.
There is a quiescent counterpart in the UKIDSS DR6 Galactic Plane Survey (Lucas et al. 2012), with J = 17.50 +/- 0.02 Vega mag, H = 16.32 +/- 0.02 Vega mag, K = 15.61 +/- 0.02 Vega mag. There is also a quiescent detection in the Pan-STARRS PS1 catalog (Chambers et al. 2016), with z = 20.6 +/- 0.3 Vega mag, y = 19.8 +/- 0.1 Vega mag. A faint source is also seen in 2004 Spitzer images at 3.6 micron and 4.5 micron. Instead, there is no source at this position in archival 2MASS images. Based on the average between the UKIDSS and PS1 positions, the location of the quiescent counterpart is RA 19:07:58.65, Dec +08:43:45.3. The integrated extinction along this line of sight is ~ 23 mag in the g band and ~ 5 mag in the J band (Schlafly et al. 2011).
On UT 2021-04-21, we obtained a low-resolution optical spectrum using the SED Machine Spectrograph on the Palomar 60-inch telescope (Blagorodnova et al. 2018). From the huge increase in J-band brightness (already almost 9 mag) we expected to see the spectrum of a nova. However, the spectrum shows a featureless and steep reddened continuum. We are unable to identify strong emission lines of the Balmer series or O I, typically seen in classical nova eruptions. Photometry from the SEDM guider camera suggests that the transient is at r ~ 17.5 mag, indicating a r - J color of ~ 8.5 mag.
The nature of the source thus remains unclear, and we encourage rapid
spectroscopic follow-up to identify its type.