Swift X-ray detection during the optical peak of the recurrent nova V3890 Sgr

ATel #13050; K. V. Sokolovsky (MSU), M. Orio (INAF Padova and University of Wisconsin-Madison), K. L. Page, A. Beardmore, J. P. Osborne (U. Leicester), P. Kuin (MSSL/UCL), J. Leahy-McGregor, E. Aydi, L. Chomiuk, A. Kawash, J. Strader (MSU), J. D. Linford (NRAO), M. Rupen (NRC HAA)
on 29 Aug 2019; 18:12 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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The 2019 eruption of the symbiotic recurrent nova V3890 Sgr was discovered by A. Pereira on 2019-08-27.87 UT and confirmed by many observers (ATel #13047; previous eruptions: 1962-06-02 and 1990-04-27, Schaefer 2010, ApJS, 187, 275). The latest pre-outburst observation was performed by R. Stubbings on 2019-08-27.521, who visually estimated the object brightness as 15.2mag (vsnet-alert 23507, AAVSO Alert Notice 677).

Swift observed V3890 Sgr for 2 ks on 2019-08-28.438. Swift/XRT detected an X-ray source with the net count rate of 1.85 +/-0.03 cts/s (grade 0 events) at the position of the nova. The XRT was operating in Windowed Timing mode to avoid optical loading. The nova spectrum can be fitted with a heavily absorbed thermal plasma with kT = 7.9 +2.3/-1.6 keV and n_HI = (3.4 +/-0.3) x10^22 cm^-2. The Fe emission feature with EW ~ 1 keV is clearly visible at 6.67 +0.07/-0.06 keV. The feature can be reproduced by the APEC plasma emission model if the Fe abundance (by number) is increased by a factor of 3 with respect to the solar value. The unabsorbed 0.3-10 keV flux is 1.8x10^-10 ergs/cm^2/s. The ultraviolet count rate is too high for Swift/UVOT allowing us to place only a lower limit on the nova brightness: UVW1 < 8.6.

Optical photometry of the nova was performed with the 0.6m telescope of the MSU Campus Observatory (MPC code 766).

 
   JD(UT)    Band mag  err  
2458723.5492  V   7.36 0.01  
2458723.5514  B   7.99 0.01  
2458723.5528  I   6.68 0.02  
2458723.5536  R   6.76 0.01  
2458723.6188  V   7.17 0.01  
2458723.6204  B   7.78 0.01  
2458723.6216  R   6.58 0.01  
2458723.6222  I   6.44 0.01  
2458723.6516  V   7.28 0.01  
2458723.6529  B   7.91 0.01  
2458723.6540  R   6.65 0.02  
2458723.6546  I   6.51 0.02  
2458723.6734  V   7.41 0.01  
2458723.6746  B   8.01 0.01  
2458723.6758  R   6.76 0.03  
2458723.6764  I   6.60 0.02  

The lightcurve suggests that the nova had peaked at V=7.17 on 2019-08-28.1188. We adopted the following magnitudes for the comparison star HD 170434 B=8.566, V=8.255, R=8.070, I=8.312 (taken or color-transformed from APASS).

V3890 Sgr is one of 4 known symbiotic (red giant donor) recurrent novae, along with T CrB, RS Oph, and V745 Sco. The 2014 eruption of V745 Sco (Page et al. 2015, MNRAS, 454, 3108) and the 2006 eruption of RS Oph (Bode et al. 2006, ApJ, 652, 629) were followed with Swift providing a direct comparison. In both novae, the hard thermal X-ray emission was detected in the first days of the outburst. The emission was attributed to the shock-heated plasma produced by the nova blast wave propagating in the red giant wind.

We encourage a continued high cadence monitoring of V3890 Sgr at all wavelengths.

We thank the Neil Gehrels Swift Observatory team and PI, Brad Cenko, for rapid scheduling of this ToO observation.