A highly variable XMM-Newton X-ray light curve of the recurrent nova V3890 Sgr.

ATel #13124; J.-U. Ness (ESA/ESAC), M. Orio (INAF Padova and U. of Wisconsin), S. Starrfield (Arizona State U.), B. Schaefer (U Louisiana), K. L. Page, A. P. Beardmore, and J. P. Osborne (U. Leicester), S. Shore (U Pisa), J. J. Drake (CfA), E. Kuulkers (ESTEC/ESA), R. Saxton (ESA/ESAC), O. Koenig (ECAP)
on 23 Sep 2019; 20:34 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jan-Uwe Ness (juness@sciops.esa.int)

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The third recorded outburst of the recurrent Nova V3890 Sgr (1962, 1990, 2019) was observed with XMM-Newton for 25,000 seconds during the time period 2019-09-14 23:30:25 to 2019-09-15 06:27:05 UT, starting 18 days after outburst.

An illustration of RGS 1+2 X-ray light curve, high-resolution spectra, and spectral evolution can be viewed under
https://www.cosmos.esa.int/web/personal-profiles/jan-uwe-ness#v3890sgr

The X-ray light curve is consistent with the highly variable Super Soft Source phase as reported in Atel #13104 which is here observed with a continuous 8-hour observation. The MOS light curve starts at a count rate of 15 counts per second (cps) and rises within only 460 seconds to above 30 cps. The same behaviour is seen in the RGS light curve while the pn cameras opened later. The EPIC MOS1 count rate then oscillates between ~10 and ~45 cps. For 60% of the time, the count rate was above 30 cps and 12% of the time below 12 cps. About 10ks after the start of the observation, there was a pronounced dip with a steep decline from a level of 30-40 cps to the 8-12 cps range which took only 440 seconds. The decline and increase appear grey, thus at the same rate at all wavelengths. The dip lasted for one hour, and the count rate then increased equally rapidly. The same behaviour is seen in the pn and RGS light curves.

We searched for the 83s period found in NICER data (ATel #13086) but found no evidence for a quasi-periodic oscillation between 10 and 100 seconds.

The high-resolution RGS spectrum contains emission lines from the shock-heated ejecta and circumstellar medium, similar to the Chandra high-resolution spectrum taken on day 6 after outburst which was described in ATel #13083. The Si and Mg lines have faded while the NeX line has the same flux. Additional lines of FeXVII and OVIII at 15, 16, and 17A are detected which were not seen on day 6.

The overall continuum roughly resembles a blackbody of effective temperature of 600,000K which, of course, is nowhere near a secure determination of a photospheric temperature. The spectrum is highly structured indicating strong effects of atmospheric deviations from a blackbody. Absorption and emission features are difficult to disentangle, placing this SSS spectrum in between the SSa and SSe subclassification introduced by Ness et al. (2013, A&A 599, A50). During the dip, all SSS emission disappeared leaving behind the soft part of the shock spectrum with emission lines of OVIII (19A), OVII (21.6A, 22.1A), NVII (24.8A), and NVI (28.7A). These lines may be produced by a combination of photoionization by the shock precursor, collisional excitation within the giant's wind from the ejecta shock, and photoexcitation from the SSS continuum.

Steep edges from interstellar OI and local NVII can be seen at 23.5A and 19A, respectively. The OI edge can be fitted with the cold absorption model TBnew, with a hydrogen column density of 6.3e21 cm^-2 and an O abundance 90% of solar, whereas the NVII edge requires an additional absorption component corresponding to an NVII column density of 2e17 cm^-2.

The Optical Monitor (OM) took five exposures with the UVM2 filter (2310+/-48A) in Fast Mode. The UVM2 light curve is constant and the deep dip in the X-ray light curve is not seen in the UV. If the dip is caused by an obscuration event, this either means the UV emission originates further away or that the X-ray occulter is transparent to UV emission.

A pre-outburst deep, 57ks XMM-Newton observation of V3890 Sgr was obtained on 2010-04-08, yielding an EPIC upper limit of 0.003 cps (6e-15 ergs cm^-2 s^1 over 0.2-10keV), determined with the multi-mission upper limit server:
http://xmmuls.esac.esa.int/upperlimitserver/

Spectral Time Map