Detection of Halpha emission and optical/IR variability in ZTF18abjpmzf (=SRGA J204318.2+443815=SRGE J204319.0+443820)

ATel #14232; Yuhan Yao (Caltech), S. R. Kulkarni (Caltech), Lynne Hillenbrand (Caltech)
on 28 Nov 2020; 19:47 UT
Credential Certification: Yuhan Yao (yyao@astro.caltech.edu)

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SRGE J204319.0+443820 was discovered by the SRG observatory on 20 Nov 2020 (ATel #14206) and noted to be positionally consistent with ZTF18abjpmzf. We performed photometry of images of this region obtained from the Zwicky Transient Facility (ZTF, Bellm et al. 2019, Graham et al. 2019) during the Dec 2017 to Sep 2020. The median AB magnitude in g-, r-, and i-band are 20.0, 17.1 and 16.1 mag, respectively. In i-band, the light curve shows moderate variability with an amplitude of 0.3 mag on the timescale of a few days, and the source seems to have brightened from 16.3 mag in Sep 2019 to 16.0 mag in Sep 2020.
Inspection of the AllWISE catalog (circa 2010) shows a bright mid-infrared counterpart with the following WISE magnitudes: 10.9 (W1 band), 10.7 (W2), 10.3 (W3) and 8.9 (W4). We extracted the light curve (2014-2019) in W1 and W2 from the NEOWISE catalog (Mainzer et al. 2014). The source has brightened since the cryogenic phase of WISE by 0.8 mag. Both W1 and W2 light curves exhibit secular brightening and undulations on timescales of year as well as transient fluctuations at the level of 0.1 mag on timescales of days.
On UT 26 Nov 2020, we obtained a low-resolution optical spectrum of ZTF18abjpmzf using the IFU spectrograph on the Spectral Energy Distribution Machine (SEDM, Blagorodnova et al. 2018, Rigault et al. 2019) mounted on the robotic Palomar 60-inch telescope. The spectrum shows Halpha in emission at redshift consistent with zero and a red continuum. We propose that SRG[AE] J204319.0+443820 is an accreting neutron star binary.
Multiwavelength observations are encouraged to further elucidate the nature of this source.
ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. SED Machine is based upon work supported by the National Science Foundation under Grant No. 1106171