INTEGRAL/OMC optical photometric observations of nova ASASSN-18fv

ATel #11677; A. Domingo (CAB/CSIC-INTA, Spain), M. Hernanz (ICE, CSIC and IEEC, Spain), E. Kuulkers (ESA/ESTEC, The Netherlands), Paul Kuin (MSSL/UCL, UK)
on 1 Jun 2018; 14:06 UT
Credential Certification: Albert Domingo (albert@cab.inta-csic.es)

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On 23 April 2018 INTEGRAL started an out-of-TAC public observation of nova ASASSN-18fv (ATel #11454, #11456, #11460, #11468, #11506, #11508, #11546, #11553, #11608). The source continued to be very bright in the optical band (V~6.8 mag) one month after discovery (20 March 2018), and it was, therefore, observed with the optical monitoring camera (OMC) in Fast monitoring mode. We report here one of the few observations using this observing mode. With this mode, 3 second-long integrations are performed at intervals of 4.5 seconds, and only the sections of the CCD containing the target of interest are read out from the CCD. On 18 May 2018, the OMC was configured back to Normal monitoring mode when the source brightness decayed to V~8.5 mag. INTEGRAL observations are currently still ongoing.

We report here on the INTEGRAL/OMC optical photometric data accumulated from 23 April 2018 (revolution 1945) to 23 May 2018 (revolution 1956). The OMC photometric data are affected by time gaps produced when the source falls outside its field-of-view, because observing is in a 5x5 dithering pattern by the spacecraft. The light curve shows a general decline from V=6.76 on 23 April 23:10 (UTC) to V=8.06 on 13 May 05:30 (UTC). The next set of photometric data started on 13 May 20:50 (UTC) with a magnitude of V=8.35. Thus, the source brightness dropped ~0.3 magnitudes in 15 hours. After the drop, the light curve continues the general decline trend with the same slope as before the drop. In addition to this general decline at long time-scales, short time-scale variability superposed on the general decline is clearly revealed in the OMC optical light curve, thanks to the excellent time resolution of the Fast monitoring mode and the photometric accuracy of about 0.02 mag (systematic effects not included). In some occasions, the amplitude of the variability reaches 0.3 mag on time-scales of several hours to one day.

Oscillations in the optical photometry of this source were also reported in ATel #11508, around maximum light on time-scales of days. According to the classification by Strope et al. (2010), ASASSN-18fv can be tentatively classified as a J-class nova. Light curves of this class are characterized by substantial jittering above the base level. The variations observed in the light curve could also resemble oscillations like those seen in the O-class. However, a structure-function analysis of the OMC light curve indicates that the variability is essentially random and, as reported in ATel #11508, starts before the peak, while O-class novae are characterized by quasi-periodic oscillations which generally start around 3 mag below the peak. Future observations should help in the definitive classification of ASASSN-18fv.

Due to the long INTEGRAL ToO program on ASASSN-18fv, the OMC is collecting a legacy set of well calibrated optical data on the source. The INTEGRAL/OMC optical light curve and additional information can be found at INTEGRAL Picture of the Month (June 2018).

INTEGRAL will continue monitoring the source over the upcoming revolutions, so new OMC optical photometric observations will be available in the near future.

We thank the INTEGRAL Science and Mission Operations Centres for their effort in making these observations possible.