Dwarf Nova AT2021afpi: X-ray Observations
ATel #15087; Derek B. Fox (PSU)
on 5 Dec 2021; 22:15 UT
Credential Certification: Derek B. Fox (dfox@astro.psu.edu)
Subjects: X-ray, TeV, Neutrinos, Nova
Referred to by ATel #: 15115
The high-amplitude dwarf nova AT2021afpi / Master OT
J030227.28+191754.5 (ATel #15067, #15074; see also ATels #15069,
#15072, #15073), discovered by the MASTER telescope network during
optical follow-up of the IceCube high-energy neutrino event
IceCube-211125A (GCN #31126), and with possible coincident TeV
gamma-ray emission detected by HAWC (ATel #15079), has been observed
at three epochs by NASA's Neil Gehrels Swift Observatory (Swift) and
at one epoch by NASA's Nuclear Spectroscopic Telescope Array
(NuSTAR). The source is detected in all four datasets; analysis with
HEASOFT v6.29 and UK Swift Science Data Centre tools (Evans
et al. 2009, MNRAS, 397, 1177) yields the following results (uncertainties in
trailing significant figures quoted at 90%-confidence level,
throughout).
The first epoch of Swift observations (ObsID 14935001) began at
19:35 UT on 28 Nov 2021, with 1.96 ks exposure yielding 926 counts in the
source aperture with an estimated 10 counts background; the observed
flux is 2.8e-11 erg cm-2 s-1 or 31 photons cm-2 ks-1 (see also ATel
#15073). Single-component models do not provide a satisfactory fit. A
sum of thermal bremsstrahlung and power-law components provides a
satisfactory fit, with no evidence for soft X-ray absorption. The
thermal bremsstrahlung component has kT=0.116(6) keV and contributes
29(6) photons cm-2 ks-1 (0.3-10 keV), while the power-law component
has photon index Gamma=1.13(13) and contributes 2.6(4) photons cm-2
ks-1 (0.3-10 keV).
The second epoch of Swift observations (ObsID 14935002) began at
22:52 UT on 29 Nov 2021, with 0.99 ks exposure yielding 182 counts in
the source aperture with an estimated 1 count background; the observed
flux is 1.1e-11 erg cm-2 s-1 or 9.4 photons cm-2
ks-1. Single-component models do not provide a satisfactory fit. A sum
of thermal bremsstrahlung and power-law components provides a
satisfactory fit, again with no evidence for soft X-ray
absorption. The thermal bremsstrahlung component has kT=0.10(4) keV
and contributes 7(4) photons cm-2 ks-1 (0.3-10 keV), while the
power-law component has photon index Gamma=1.4(3) and contributes
2.2(6) photons cm-2 ks-1 (0.3-10 keV). The temperature of the thermal
bremsstrahlung component is only weakly constrained from below.
The third epoch of Swift observations (ObsID 14935004) and the single
NuSTAR observation (ObsID 90701341002) were coordinated: Swift
observations began at UT 19:47 on 1 Dec 2021, with 0.95 ks exposure
yielding 80 counts in the source aperture with an estimated 1 count
background. NuSTAR observations began at 23:31 UT on 1 Dec 2021, with
roughly 34 ks exposure yielding 1848 (1761) counts in the source
apertures for modules A (B), respectively, with estimated backgrounds
of 835 (789) counts.
A joint spectral fit with normalizations free across the instruments
yields a satisfactory fit for an unabsorbed power-law spectrum with
photon index Gamma=2.24(5) across the full 0.3-100 keV bandpass (with
caveats as noted below). The observed flux is 3.0(3)e-12 erg cm-2 s-1
over the 0.3-10 keV XRT bandpass, and 3.1(3)e-12 erg cm-2 s-1 over the
3-100 keV NuSTAR bandpass. The Swift model normalization is roughly
53% of the NuSTAR model normalization, which is well in excess of the
indicated statistical uncertainties and may reflect either a flux
decline between the two observations or some unaccounted-for
complexity in the spectrum at this epoch, for example, a break in the
power-law component or the presence of soft X-ray absorption along
the line of sight.