The detection of the X-ray emission from V572 Vel (Nova Vel 2025) with Swift XRT.
ATel #17367; G. J. M. Luna (UNAHUR/CONICET), K. L. Page (U. Leicester), N. P. M. Kuin (MSSL),M. Orio (Univ. of Wisconsin and INAF-Padova), I. J. Lima (UNESP)
on 1 Sep 2025; 20:29 UT
Credential Certification: Gerardo Juan Manuel Luna (juan.luna@unahur.edu.ar)
Subjects: Ultra-Violet, X-ray, Nova
Referred to by ATel #: 17425
V572 Vel entered into an outburst on June 25.44 (CBET #5574), reaching U=5.6 magnitudes on June 25.57. The optical light curve available at the AAVSO database reports visual magnitudes of about V=4.8 on June 27 and 5.8 on July 1st. FERMI LAT reports a detection just a few hours after the optical discovery (Jean et al. Atel #17252). Lima et al. (RNAAS, in press) found no evidence of intrinsic optical linear polarization on observations obtained 1 day and 1 month after the outburst.
Swift has observed V572 Vel on average every two days, starting on June 26th for about 1.5 ks each time in WT mode, and no X-rays were detected. By the end of August, the optical emission had faded sufficiently such that the XRT could be switched to the more sensitive Photon Counting mode. The first of these observations, taken on August 31, revealed a clear, but faint, X-ray source, with a count rate of 0.030 +0.005/-0.004 count s^-1 (0.3-10 keV, single pixel events only). The spectrum (comprising only 27 photons) can be approximated by an absorbed single-temperature optically-thin plasma with kT > 2.0 keV and NH = (2.1 +4.8/-1.2) x 1022cm-2. The observed (unabsorbed) flux over 0.3-10 keV is 4.2 (5.8) x 10-12 erg cm-2 s-1.
Comparing the spectrum taken on Aug 31st to that taken Aug 17th we see largely the same spectral features at a reduced brightness. The HI Balmer series is present as well as the HeII lines. The following other lines are deemed present: N III] 1750, Al III ? 1840, C III] 1909, N II]/[N IV] 2147/2151, O II/S iII 2330, O II 2476 blend with He II 2511, Mg II 2800, O III 3134, C III/N III 4630, as well as second order lines of N III] 1750(2) at 2966 and C III] 1909(2) at 3273. All numbers are in angstroms. The line at 2000A is unknown. Some weak zeroth orders are close to the spectrum but no obvious signs have been seen. We took the latest spectrum with the nominal grism setting, unlike the first spectrum, which was done to avoid strong contaminating background sources. The Mg II line is now much narrower and shows a small dip at -40A, corresponding to a velocity of -4200km/s if that is due to an absorbing outflow.
Further Swift observations are planned, and we thank the Swift PI, Brad Cenko, and his deputies for approving the time, as well as the Swift planning and operations teams for their continuing support.
