On the very fast fading of the classical EXor V1118 Ori

ATel #13298; Alessio Giunta (AST, Agenzia Spaziale Italiana, Roma, Italy), Teresa Giannini, Fabrizio Vitali, Simone Antoniucci, Dario Lorenzetti (Osservatorio Astronomico di Roma, Monte Porzio, Italy), Domenico Licchelli, Francesco Strafella (Universita’ del Salento, Lecce, Italy)
on 20 Nov 2019; 14:05 UT
Credential Certification: Teresa Giannini (teresa.giannini@inaf.it)

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We recently announced (ATel #12774) that the classical EXor source V1118 Ori started of a new eruptive outburst. Unfortunately, after 2019 May, a ground-based monitoring of that event was hampered by seasonal reasons, but in 2019 August the source was observable again from La Silla (Chile). From there, we started a periodical photometry with REM telescope (Vitali et al. 2002 Proc. SPIE, 4841, 627) in g, r, i, and z bands by using ROSS2. The obtained results are given in the following Table. The values (in magnitudes) have been calibrated using the SDSS catalog. Errors are of the order of ±0.03 mag in all bands.

Date	g	r	i	z
2019.05.09	14.56	13.83	13.31	12.76
2019.08.12	14.00	13.59	12.99	12.60
2019.08.15	14.56	13.81	13.45	12.87
2019.08.18	14.49	13.74	13.34	12.74
2019.08.21	14.19	13.57	13.05	12.64
2019.08.24	14.77	13.99	13.48	12.96
2019.08.27	14.77	14.01	13.47	12.97
2019.09.01	14.56	13.73	13.32	12.83
2019.09.04	14.60	13.85	13.42	12.91
2019.09.07	15.00	14.32	13.86	13.20
2019.09.10	14.71	14.02	13.48	13.01
2019.09.13	15.52	14.82	14.24	13.52
2019.09.18	15.60	14.79	14.27	13.52
2019.09.21	15.59	14.74	14.41	13.72
2019.09.24	15.49	14.33	14.41	13.71
2019.09.27	15.69	14.99	14.49	13.85
2019.09.30	15.75	15.06	14.62	13.88

The peak of the maximum brightness of the V1118 burst evidently occurred in the period June-August 2019, and has been sadly missed. However, a rare case of a very fast luminosity declining has been recorded. Until August 12, the source faded by about 1.5 mag with a rough declining velocity of about 0.033 mag/day. Similarly, we can estimate a rising velocity of 0.018 mag/day. By comparing these values with those calculated for the 2015-2016 outburst (Giannini et al. 2017 ApJ 839, 112), we remark that while the burst amplitude remained substantially the same (about 4 mag), in that occasion the rising and declining velocity were both of about 0.015 mag/day. Of course, a more accurate analysis is in order, by comparing the current values also with those of all the previously documented outbursts, but current data are certainly signalling important differences between repetitive accretion events, likely in the efficiency of the shock heating and subsequent cooling.