NuSTAR detects X-rays from a deeply embedded shock in the Fermi-detected nova ASASSN-18fv

ATel #11608; Thomas Nelson (U. Pittsburgh), Koji Mukai (NASA/GSFC & UMBC), J. L. Sokoloski, Brian Metzger (Columbia), Laura Chomiuk (MSU), Justin Linford (GWU), Indrek Vurm (Tartu Observatory)
on 4 May 2018; 14:43 UT
Credential Certification: Thomas Nelson (tjnelson@pitt.edu)

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In response to the detection of gamma-ray emission with the Fermi LAT instrument in the recent nova ASASSN-18fv (ATel #11546), we carried out a target-of-opportunity observation of the system with the NuSTAR observatory between 2018 April 20 and 2018 April 22. The total exposure time was ~64 ks. A simultaneous soft X-ray observation was carried out using the Swift satellite at the end of the NuSTAR exposure on 2018 April 22, lasting 1000s. The Swift observation utilized the XRT instrument in Windowed Timing mode, due to the high optical brightness of the nova.

ASASSN-18fv was not detected in the Swift XRT observation, with a 3-sigma upper limit on the 0.3-10 keV count rate of 0.016 cts/s. The nova was, however, clearly detected in the NuSTAR image, with ~2350 net counts detected in a 50 arcsec circular region centered on the source in both the FPMA and FPMB instruments. We find that the nova spectrum is well fit by an absorbed thermal plasma model (phabs*apec in XSpec), consistent with a deeply embedded internal shock in the ejecta. The best fit energy of the plasma is kT = 7.8 +/- 1.5 keV. Absorption is seen to be strong enough to eliminate all photons below ~4 keV, requiring an equivalent hydrogen column density (Nh) of a few 10^24 cm^-2 if solar abundances are assumed. The observed flux was 3e-12 erg/s/cm^2 in the 3.5-78 keV range, and the implied unabsorbed flux was 1e-11 erg/s/cm^2. We find no evidence of non-thermal emission at high energies in the spectrum.

Further X-ray observations of this nova are encouraged to monitor the evolution of this internal shock as the ejecta expand.