Swift observations of the super-soft nova ASASSN-21fh
ATel #14955; K. L. Page and J. P. Osborne (U. Leicester)
on 5 Oct 2021; 15:54 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)
Subjects: Ultra-Violet, X-ray, Nova
The classical nova ASASSN-21fh (= AT2021jiu), discovered on 2021
Apr 12 by Stanek, Kochanek for the ASAS-SN team and classified by
Joshi et al. (ATel #14544), following the eROSITA X-ray detection
of Sep 17 (ATel #14927) was observed by Swift on 2021 Sep 30
(for 0.75 ks) and Oct 4 (for 1.09 ks).
There is no evidence for variability within or between these two Swift
observations, either in the XRT or UVOT data.
The XRT detected a soft X-ray source at 0.62 +/- 0.03 count s-1.
Its spectrum was well fit with an absorbed blackbody and
optically thin plasma emission model. We find NH
= (9.4 +2.0/-1.8) x 1021 cm-2 (consistent with the total Galactic column of 9.6
x 1021 cm-2; Willingale et al. 2013
MNRAS 431 394) and kTBB = 37 +/- 4 eV,
kTAPEC = 0.99 +0.21/-0.22 keV. The blackbody and APEC
emission models have unabsorbed 0.3-2 keV fluxes of 7.9 x
10-8 and 3.2 x 10-12 erg cm-2
s-1 respectively. The observed XRT flux is 30% lower
than that quoted for eROSITA (ATel #14927) over the same energy
band, while our spectral fit indicates higher absorption.
The mean UVOT magnitudes were uvw1 = 18.62 +/- 0.14, uvm2 >
19.68 and uvw2 > 19.76 (3 sigma upper limits).
These observations, taken 171-175 days after discovery, show the absorbed
nova to be in a steady burning phase. With no evidence of cooling, the
low blackbody temperature at this time suggests a white dwarf mass below
solar (e.g. Sala & Hernanz 2005 A&A 439 1061; Wolf et al. 2014 ApJ 782
117).
