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Detection of Highly-Absorbed X-rays from Nova Mus 2018 with Swift

ATel #11377; Thomas Nelson (U. Pittsburgh), Paul Kuin (MSSL), Koji Mukai (NASA/GSFC and UMBC), Kim Page (U. Leicester), Laura Chomiuk, Adam Kawash (MSU), J. L. Sokoloski (Columbia), Justin Linford (GWU), Michael P. Rupen (Herzberg Astronomy and Astrophysics) and Amy Mioduszewski (NRAO)
on 2 Mar 2018; 14:07 UT
Credential Certification: Thomas Nelson (tjnelson@pitt.edu)

Subjects: Ultra-Violet, X-ray, Nova

We report the detection of X-rays from Nova Mus 2018 with the Swift XRT instrument. We have been carrying out weekly monitoring of the nova with Swift since its discovery on 2018 Jan 15 (see ATel #11220), and observations up to 2018 Feb 24 yielded X-ray non-detections. In the most recent observation carried out on 2018 Feb 27 (net exposure time of 2460 seconds), 57 photons are detected within a 20 pixel radius circle centered on the position of the nova. After background subtraction and correcting for PSF losses, the resulting net count-rate in the 0.3-10 keV energy range is 0.032 +/- 0.004 cts/s.

We extracted a spectrum of the nova and attempted to model it in Xspec as an absorbed thermal plasma (tbabs*apec), but no stable fit to the data was obtained. This is probably due to the low number of source counts. The majority of the detected photons have energies in the range 4-7 keV, indicating that the X-ray emitting region is highly absorbed (at least 5 x 10^22 cm^-2).

This first X-ray detection coincides with a likely dust formation event, as indicated by the precipitous drop in optical brightness since 2018 Feb 20 (see the website linked in ATel #11298). Prompted by this recent reduction in optical brightness, we began obtaining UV grism spectroscopy of Nova Mus 2018 with the Swift UVOT instrument. Our first UV spectrum was taken on 2018 Feb 17 and our most recent on 2018 Feb 27. The UV spectrum shows a strong Mg II emission line, a weak C III 1909 line, and weak lines of He II. There are significant absorption features in the range 190-300 nm, most prominently at 210, 230, and 270 nm which become less deep between the first and second spectra. Similar features were seen during the dust minimum of V5668 Sgr.

UVOT photometry was initially obtained from the readout streak (formed when sources are bright enough to saturate the detector) and most recently using aperture photometry. The uvw2 and uvm2 images show a steady decline until 2018 Feb 24 when the magnitudes (Vega) were uvw2 = 11.49 +/- 0.04, uvm2 = 11.97 +/- 0.04. On 2018 Feb 27 the nova had suddenly decreased to uvw2 = 13.37 +/- 0.04 and uvm2 = 13.53 +/- 0.04, similar to the large drop observed in the optical and also due to the formation of dust.

Nova Mus has now been detected at every wavelength, from radio to gamma-rays (see ATels #11183, #11201, #11221, and #11296). We will continue to monitor the nova with Swift over the coming weeks; observations at other wavelengths are highly encouraged.