INTEGRAL/OMC optical observations of the bright nova ASASSN-18fv
On 20 March a new bright transient optical source, near the Galactic plane,
was found at V<10 mag by the
All Sky Automated Survey for SuperNovae (ASAS-SN; ATel
#11454).
These observations showed an outburst amplitude of more than 7 mag. About a day later
it was already brighter than 6 mag (see, e.g.,
AAVSO Alert Notice 626). Early spectroscopy (ATels
#11456,
#11468)
had not ruled out a Galactic classical nova, but the transient might also be a
large outburst of a young stellar object or other peculiar explosion. Near-IR
spectroscopy obtained about 10 days later (ATel
#11506)
showed that the spectrum is consistent with that observed for normal Fe II and
transition (Fe II + He/N) class novae after peak. Two weeks later, Fermi/LAT and
AGILE detected prolonged gamma-ray emission above ~100 MeV (ATels
#11546,
#11553),
and a few days after that, NuSTAR in X-rays (3.5-78 keV; ATel
#11608).
If the source is indeed a classical nova, one may expect nucleosynthesis line
emission from the decay of 7Be at 478 keV or of 22Na at 1275 keV, depending on the nova type.
Since ASASSN-18fv looks like a CO nova, 7Be is favoured with respect to 22Na. This
makes ASASSN-18fv a very interesting target for INTEGRAL.
On 23 April INTEGRAL started an
out-of-TAC public observation
of ASASSN-18fv. The source continued to be very bright (V~6.8 mag) one month after
discovery, and it was, therefore, observed with the OMC in Fast monitoring mode. This
is one of the few observations using this observing mode. With this mode, integrations
of 3 seconds are performed at intervals of 4.5 seconds, and only the sections of the
CCD containing the target of interest are read from the CCD and transmitted to ground.
On 18 May, the OMC was configured back to Normal monitoring mode when the source
brightness decayed to V~8.5 mag. Observations are currently still ongoing (see the
INTEGRAL scheduling information.
The optical light curve as obtained by the AAVSO/visual estimates (open white
circles) and with the INTEGRAL/OMC V-band (filled red circles) are shown in the main
image. It can be seen that AAVSO data are important due to the large time span covered.
The OMC data are of extremely good quality, although they are affected by time gaps
produced when the source falls outside its FoV, because of the observing 5x5 dithering
pattern. In the zoomed-in OMC light curve (inset figure top right) the excellent time
resolution of the Fast monitoring mode and a photometric accuracy of about 0.02 mag
can be seen (note that systematic effects not included). Short-scale time variability
superposed on the general decline is clearly revealed. In some cases, the amplitude of
the variability reaches 0.3 mag on timescales of several hours to one day. Due
to the long INTEGRAL ToO program on ASASSN-18fv, the OMC is collecting a legacy
optical data set on the source.
Oscillations were also reported in ATel
#11508,
around maximum light on timescales 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, the observed variations by the OMC seem to be essentially random
and start before the peak, while O-class novae are characterized by quasi-periodic
oscillations which generally start around 3 mag below the peak. Future observations
will help in the definitive classification of ASASSN-18fv.
Credits: Albert Domingo (INTEGRAL/OMC team, CAB/CSIC-INTA, Spain)
and Margarita Hernanz (ICE-CSIC and IEEC, Spain).
We acknowledge with thanks the variable star observations from the AAVSO International
Database contributed by observers worldwide and used in this image.
Background image: An artist's rendering of a classical nova;
credit (c) David A. Hardy/www.astroart.org
