Luminosity radiated by the symbiotic star V919 Sgr in outburst, as derived from ground-based and Swift observations
Light Curve
Following the announcement of a new outburst of the symbiotic star V919 Sgr (Munari et al., ATel #15528), on 2022 July 31 we observed V919 Sgr with the Swift satellite. The pointing started at 13:57 UT. The source was well detected in all UVOT filters, yielding the Vega-system magnitudes reported in the Table below (errors +/-0.07 mag). It was instead not detected in X-rays: a simultaneous XRT observation lasting about 1700 s did not reveal any emission in the 0.3-10 keV band down to a 3-sigma limit of 3.3x10-3 counts s-1. For the distance and reddening listed below, and assuming a kT=0.1 keV bremsstrahlung, this corresponds to <4x10-13 erg/cm2/s in flux and <1x1033 erg/s in LX luminosity.
To extend the photometric coverage to the optical range, we also observed V919 Sgr with the Asiago 67/92cm Schmidt telescope. The resulting UBVRI magnitudes are listed in the table below (errors <0.005 mag). We have also obtained on Aug 5.959 UT a near-UV, medium-resolution, high S/N spectrum of V919 Sgr with the Asiago 1.22m+B&C telescope, covering the 3190-4400 Ang at 0.58 Ang/pix. The only feature visible in emission is a feeble Hgamma, with Hdelta exactly filling the absorption from the underlying continuum spectrum which is best described as a forest of very sharp absorption atomic lines, so tightly packed that the "true" continuum emerges only at a few pixels sparsely distributed over the recorded spectral interval.
2022 UT UVW2 UVM2 UVW1 U B V R I Jul 31.581 12.63 12.78 11.89 Aug 02.904 11.004 11.127 10.573 10.056 9.313 Aug 03.905 10.988 11.125 10.563 10.051 9.311 The distance to V919 Sgr is determined by Gaia DR3 in 4.6 kpc (4.0-5.5 kpc limits). The high Galactic latitude (-10deg) favors a low reddening, determined in E(B-V)=0.19 by averaging the results of Marshall et al. (2006, A&A 453, 635) and Green et al. (2019, ApJ 887, 93). After subtracting the contribution of the non-variable M3III giant and dereddening by E(B-V)=0.19, the luminosity radiated by the hot component in the UVOT-UBVRI range is 4.27 x 1036 erg/s (3.20 x 1036 and 5.96 x 1036 at lower and upper limits to Gaia distance). V919 Sgr belongs to the group of symbiotic stars characterized by stable nuclear burning on the surface of the WD. The large luminosity radiated at optical and near-UV wavelengths during the present outburst (1100 Lsun) and the drastic drop in the ionization conditions described by Munari et al. (ATel #15528), support the idea that the outburst was caused by a shift toward optical wavelengths of the emission from the WD, driven by an expansion in radius of its burning shell and fueled by a possible surge in mass-transfer from the M3III companion or the accretion disk.
Flickering is ubiquitous in accreting-only symbiotic stars (Luna et al. 2013, A&A 559, A6) and absent for the burning variety (Sokoloski 2003, ASPC 303, 202). During the UBVRI observations of V919 Sgr on Aug 2 and 3, we have collected sparse U-band data distributed over 1.5 hours intervals on both nights, and found the U-band brightness stable to better than 0.01 mag. The absence of flickering agrees with the inferred nuclear burning on the WD.
We thank the Swift Team for the quick approval, scheduling and acquisition of the observation presented in this communication.