On the Progenitor and Nature of ASASSN-18gq

ATel #11518; Peter Pessev (GRANTECAN, IAC, ULL), Stefan Geier (GRANTECAN, IAC, ULL)
on 9 Apr 2018; 14:43 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Peter Pessev (peter.pessev@gtc.iac.es)

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ASASSN-18gq was reported as a low-luminosity transient in the direction of the nearby galaxy ESO325-G011 on April 03 2018 (ATel#11500). The transient was detected as a g=16.5 source on 2018 April 1.89 and was not present during the earlier observations of the field carried out on 2018 March 28.27 (detection limit of g~17.6). Blue UV-optical colors reported by ATel#11501 constrained the nature of the transient as broadly consistent with a young core-collapse supernova. Furthermore the transient was observed by the ePESSTO survey and the spectrum revealed Balmer lines in absorption at zero redshift, contradicting the supernova classification (ATel#11509).

We observed the transient with the 2.5m NOT telescope at the ORM observatory during the night of April 03/04 2018 at ~02:00 UT, close to transit of the field at El~19 degrees and AM~3. Our goal is to better constrain the nature of the transient, carrying out a search of the possible progenitor. Astrometry of the field was carried out, utilizing the IRAF imtools.imcoords suite. It was based on 8 individual dithered frames, using between 3 and 7 reference stars in the field, coming from the Sky Mapper Southern Sky Survey DR 1.1. (Wolf et al. 2018, PASA, arXiv:1801.07834). We measure the following mean position:

RA= 13:45:03.495+/-0.003
Dec= -41:52:23.34+/-0.23

The reported uncertainties are the statistical errors of the mean. We also note that in general there is a 0.16 arcsec median offset between the Sky Mapper and Gaia astrometry (Wolf et al. 2018). Our position is coinciding within the uncertainties with the position reported by ATel#11500. A comprehensive search in various databases and archives revealed no counterpart around the reported position even with a generously relaxed search radius of 2 arcsec. In particular the non-detection in the Sky Mapper Southern Sky Survey DR1.1 implies progenitor magnitudes higher than ~18 for the entire filter system of the survey. Based on the Gaia DR1 magnitude histogram at https://www.cosmos.esa.int/web/gaia/dr1 we can conclude that the progenitor should be fainter than G~24. A search in the Hubble Source Catalog (HSCv2), revealed a single counterpart at about 1.5 arcsec from the transient position with a W2_814W=23.5m. The reported limits set up an amplitude of the outburst of the order of 8 magnitudes. Based on the data available, we propose the hypothesis that ASASSN-18gq is a Luminous Red Nova outburst in the direction of ESO325-G011. Further observations are strongly encouraged.

Based on observations made with the Nordic Optical Telescope operated on the island of La Palma by NOTSA in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias under the joint IACNordic Observing Time programme. The national facility capability for SkyMapper has been funded through ARC LIEF grant LE130100104 from the Australian Research Council, awarded to the University of Sydney, the Australian National University, Swinburne University of Technology, the University of Queensland, the University of Western Australia, the University of Melbourne, Curtin University of Technology, Monash University and the Australian Astronomical Observatory. SkyMapper is owned and operated by The Australian National University's Research School of Astronomy and Astrophysics. The survey data were processed and provided by the SkyMapper Team at ANU. The SkyMapper node of the All-Sky Virtual Observatory (ASVO) is hosted at the National Computational Infrastructure (NCI). Development and support the SkyMapper node of the ASVO has been funded in part by Astronomy Australia Limited (AAL) and the Australian Government through the Commonwealth's Education Investment Fund (EIF) and National Collaborative Research Infrastructure Strategy (NCRIS), particularly the National eResearch Collaboration Tools and Resources (NeCTAR) and the Australian National Data Service Projects (ANDS).