Swift observations of the classical nova in the symbiotic system V1708 Sco = TCP J17234205-3103072
ATel #14078; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU), Kim Page, Andy Beardmore, Julian Osborne (U. Leicester)
on 9 Oct 2020; 07:21 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)
Subjects: Optical, X-ray, Nova
The eruption of V1708 Sco was first noted by T. Kojima
on 2020-09-08.4213 UT. Optical spectroscopy suggested the object is
a classical nova with an unusual Halpha line profile: slow
~500 km/s emission component was superimposed on a very wide
~9000 km/s emission base (ATel #14015; vsnet-alert 24684;
CBET #4848). OGLE photometry identified the nova progenitor as
a pulsating red giant (ATel #14017). Together with the pre-nova
Halpha emission found in the archival data (ATel #14028), this
suggests the event is a classical nova eruption in a symbiotic
system, somewhat similar to V407 Cyg. SRG/eROSITA observed
V1708 Sco between 2020-09-24.79 and 2020-09-25.79 detecting
a combination of bright super-soft (SSS) and hard power-law X-ray
emission (ATel #14057).
Swift observed V1708 Sco for 5.2ks split between three epochs
2020-09-25.73 (overlapping with the SRG/eROSITA observing window),
2020-09-30.38 and 2020-10-07.79. The source is clearly detected
at the count rates of 0.573 +/-0.017, 0.110 +/-0.010 and
0.157 +/-0.011 cts/s, respectively. The Swift/XRT spectrum can be
fit by a combination of kT ~ 70 eV black body and kT ~ 2 keV
optically thin thermal emission behind an absorbing screen with
nH dropping from 2.7x10^22 cm^-2 at the first two epochs to
1.0x10^22 cm^-2 at the third epoch. The fit requires an
overabundance of Ne in the absorber by a factor of ~4 (by number)
compared to the Solar value (as suggested in ATel #14057).
Alternatively, a good fit can be achieved for a solar abundance
absorber by adding an ionized O absorption edge at ~0.87 keV to
the blackbody model, as expected for a hot white dwarf atmosphere.
The Swift/UVOT position of the nova 17:23:41.94 -31:03:07.7 +/-0.2"
J2000 (measured with respect to UCAC3 stars in the field) is
consistent with the astrometry reported by S. Kiyota and A. Pearce
via the CBAT Transient Object Confirmation Page (TOCP) and supports
the identification of Gaia DR2 4058144727782811648 as the likely
progenitor. Comparing Swift/UVOT photometry (Vega system) to
the measurements reported at the TOCP, the time of decline by two
magnitudes in V band can be constrained as t_2 < 10d:
JD V err. Observer
2459103.878 14.40 0.02 A. Pearce
2459103.949 14.39 .... S. Kiyota
2459118.23 17.16 0.04 Swift/UVOT
2459122.88 17.59 0.07 Swift/UVOT
2459130.29 17.82 0.08 Swift/UVOT
The rapid optical decline and the emergence of SSS 16 days past
eruption put V1708 Sco in the category of fast novae.
The ~9000 km/s emission component in the optical spectrum is the
one likely associated with the bulk of nova ejecta.
The lower velocity emission probably originates in gas streams
within the symbiotic system that may have been present before
the eruption. Fast novae are usually associated with massive white
dwarfs that may be of ONeMg composition (as hinted by
the overabundance of Ne; ATel #14057).
We thank the Neil Gehrels Swift Observatory team and PI,
Brad Cenko, for scheduling these ToO observations.
CBAT Transient Object Followup Reports page for V1708 Sco
