Palomar Gattini-IR identification of a likely highly reddened infrared transient counterpart to SRG J181414.6-225604 = SRGe J181415.1-225617

ATel #14524; 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 (NAOC), T. Travouillon (ANU) on behalf of the Palomar Gattini-IR team
on 7 Apr 2021; 17:14 UT
Credential Certification: Kishalay De (kde@astro.caltech.edu)

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We report the identification of a likely infrared variable counterpart to the highly reddened X-ray transient SRG J181414.6-225604 = SRGe J181415.1-225617 (ATel #14510). The counterpart was identified in data taken with the Palomar Gattini-IR (PGIR) survey (Moore & Kasliwal 2019) that scans the entire visible sky from Palomar Observatory to a depth of 15.7 AB mag every two nights (De et al. 2020).

Within the eROSITA localization region, we find a bright IR source detected in 2MASS and IRAS (as IRAS 1811-2257). Performing forced photometry on the source coordinates in the PGIR images, we find that the source has exhibited large amplitude flares detected since the start of the survey. The source was detected in a high state at J ~ 10.3 mag for 100 days starting from JD ~ 2458550, followed by a fading to J ~ 11.7 mag by JD ~ 2458750. The source remained in the low flux state at J ~ 11.7 mag throughout 2020, before emerging at a magnitude of J ~ 11.3 mag after solar conjunction in April 2021.

Since the first two SRG sky surveys in 2020 found the source in a low flux X-ray state (ATel #14510), the brightening of the IR source in 2021 further confirms a temporal association. Together, based on the spatial and temporal coincidence of the variability, we suggest IRAS 1811-2257 as the likely counterpart to SRG J181414.6-225604 = SRGe J181415.1-225617.

On UT 2021-04-07, we obtained a very low resolution spectrum of the source using the SED Machine spectrograph (Blagorodnova et al. 2018) on the Palomar 60-inch telescope. The spectrum shows a very red continuum with a very weak emission feature at the wavelength of H alpha. In addition, we detect broad absorption features between 7000 and 9000 A, similar to M-type giants. Using the combined optical and infrared SED of the source, we thus propose that the source is a new Galactic symbiotic X-ray binary.

Spectroscopy and multi-wavelength follow-up of the source are encouraged to confirm the nature of the source. An improved X-ray localization could further confirm the association.