VLBA observations of the nuclear transient AT 2017gbl in IRAS 23436+5257

ATel #10712; Miguel Perez-Torres (Instituto de Astrofisica de Andalucia - CSIC, Granada), Erik Kool (Australian Astronomical Observatory, AAO), Stuart Ryder (Macquarie Unviersity/AAO), Seppo Mattila (University of Turku), Morgan Fraser (University College, Dublin), Erkki Kankare (Queen's University, Belfast)
on 6 Sep 2017; 13:17 UT
Credential Certification: Miguel A. Perez-Torres (torres@iaa.es)

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We report on simultaneous radio interferometric observations of the luminous nuclear transient AT 2017gbl, carried out at 4.4 and 7.6 GHz with the Very Long Baseline Array (VLBA) on 2017 August 15.38 UT. AT 2017gbl was discovered in the near-infrared on MJD 57942.56 (ATel #10651) superimposed on the nucleus of the galaxy IRAS 23436+5257 (D= 146 Mpc). We imaged a field of (1x1) arcsec^2 around the position of AT 2017gbl in search for its radio counterpart. We detected one single, unresolved source above the noise (rms figures of merit of 38 and 39 microJy/beam at 4.4 and 7.6 GHz, respectively), with clean beam major and minor axes of (4.5x1.3) milliarcsec^2 and (2.8x0.9) milliarcsec^2 at 4.4 and 7.6 GHz, respectively. The coordinates of this radio source are (RA, Dec.) = (23:46:05.517275, +53:14:01.26050), and agree within 0.20 arcseconds with the near-IR position reported by Kool et al. (ATel #10651). The source has peak flux densities of (908 +/- 59) and (1653 +/- 91) microJy/beam at 4.4 and 7.6 GHz, respectively, implying a rather inverted spectral index (alpha=1.1 +/- 0.2; S_nu ~ nu^(alpha)), and the corresponding 7.6 GHz monochromatic luminosity is L_nu = 4.2e28 erg/s/Hz. The compactness of the source, as well as its radio emission level and inverted spectrum agree well with expectations from a low-luminosity AGN, which would then correspond to the hitherto undetected AGN in IRAS 23436+5257. However, the radio source can also correspond to the transient object, now being past the radio peak, while still being optically thick at low (< 5 GHz) frequencies. Further Very Long Baseline Interferometry (VLBI) observations with the VLBA and the European VLBI Network (EVN) are scheduled to shed further light on the nature of this intriguing new source in the nucleus of IRAS 23436+5257. We thank the staff at the National Radio Astronomy Observatory (NRAO) for scheduling these observations. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.