Swift upper limit on early X-ray emission of Nova Sagittarii 2021 No. 2 = V6595 Sgr = PNV J17581670-2914490

ATel #14535; Kirill Sokolovsky, Elias Aydi, Laura Chomiuk, Adam Kawash, Jay Strader (MSU), Koji Mukai (NASA/GSFC), Kwan-Lok Li (NCKU)
on 9 Apr 2021; 07:46 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The eruption of V6595 Sgr (Nova Sgr 2021 No. 2, PNV J17581670-2914490) was first noted by A. Pearce on DSLR images obtained on 2021-04-04.825 UT. The object was spectroscopically confirmed as a classical nova (CBET #4952, ATel #14513, #14533). The latest pre-discovery image of the nova field was obtained on 2021-04-02.776 by R. McNaught.

Swift observed V6595 Sgr for 2.0ks split between the two pointings centered on 2021-04-05.7821 and 2021-04-05.8606, 23 and 25 hours post-discovery, respectively. The nova is clearly visible in Swift/UVOT images having the following UVW2 (Vega) magnitudes:

 
2459310.2821 12.49 +/- 0.02 (stat) +/- 0.03 (sys)  
2459310.3606 12.54 +/- 0.02 (stat) +/- 0.03 (sys)  

The Swift/UVOT nova position measured with respect to UCAC3 star in the field is 17:58:16.09 -29:14:56.3 +/-0.2" J2000.

Swift/XRT detected no X-ray source at the position of the nova with a one-sigma upper limit of 0.003 cts/s. Assuming kT= 2 keV thermal plasma emission and HI column density of 3.46x10^21 cm^-2 (Kalberla et al. 2005 A&A, 440, 775) this translates to the unabsorbed 0.3-10.0 keV flux limit of 2x10^-13 erg/cm^2/s.

We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for rapid scheduling these ToO observations.