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)

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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.