Shock-powered X-rays from Nova Vulpeculae 2024 (V615 Vul) detected by Swift

ATel #16788; Kirill Sokolovsky (UIUC), Elias Aydi, Laura Chomiuk, Peter Craig, Isabella Molina (MSU), Jennifer Sokoloski (Columbia), Justin Linford (NRAO), Montana Williams (NMT), Koji Mukai (NASA/GSFC)
on 24 Aug 2024; 18:33 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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Swift observed Nova Vulpeculae 2024 (V615 Vul, PNV J19430751+2100204; ATel #16743, #16746, CBET #5423) for a total exposure of 6.9 ks between 2024-07-30 and 2024-08-22 (1 to 24 days after the optical discovery of the eruption on 2024-07-29.8317 UT). After the initial X-ray non-detection on day 1 (ATel #16751), the second Swift/XRT observation on day 11 resulted in detection of an X-ray source with a 0.3-10 keV count rate of 0.033 +/-0.007 cts/s. The XRT count rate peaked on day 16 at 0.068 +/-0.011 cts/s and declined to 0.042 +/- 0.006 by day 24. The hard 2-10 keV count rate was higher than the soft 0.3-2 keV rate at all epochs indicating substantial absorption.

Stacking 5.1 ks observations obtained between day 11 and 24, when the X-ray source is visible, we obtain a spectrum that can be fit by thermal plasma emission with kT = 1.8 +/-0.4 keV behind an absorbing hydrogen column of (5 +/-1)x10^22 cm^-2, which is a few times larger than the total line-of-sight Galactic value derived by Kalberla et al. (2005, A&A, 440, 775). This corresponds to an unabsorbed 0.3-10.0 keV flux of 1x10^-11 erg cm^-2 s^-1.

The observed X-ray emission likely originates from shock-heated plasma within the nova ejecta. As the fast wind from the nuclear-burning white dwarf collides with slower-moving material ejected earlier in the eruption, it forms shocks that heat the gas to X-ray temperatures. The high intrinsic absorption suggests that the shock is deeply embedded within the ejecta. This type of X-ray emission is typical for classical novae days to weeks after the eruption.

Swift/UVOT photometry of the nova in Vega magnitudes:

 
   JD        Band   Mag.  Err.  
 2460522.37  UVW2  15.07  0.02  
 2460532.79    U   14.27  0.02  
 2460537.01    U   14.70  0.02  
 2460540.96    U   15.10  0.02  
 2460545.24    U   15.40  0.02  

We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for their continuing support of nova monitoring observations.

Swift/XRT lightcurve of nova V615 Vul