V572 Vel: Change of 11-Minute Period within two months

ATel #18121; Jan-Uwe Ness (ESA-ESAC), Andrej Dobrotka and Jozef Magdolen (Slovak University of Technology in Bratislava), Marina Orio (University of Wisconsin and INAF-Padova), Gerardo Juan Manuel Luna (CONICET-University of Buenos Aires), Kim L. Page (U. Leicester), Julian Osborne (U. Leicester), Gloria Sala, Yuri Cavecchi (Universitat Politecnica de Catalunya, UPC)
on 28 Oct 2026; 20:36 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jan-Uwe Ness (jan.uwe.ness@esa.int)

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The nova V572 Vel was observed with XMM-Newton starting on UT 2025 Nov 28.3, 155.86 days after discovery (t_0 = 2025 Jun 25.440 UT; ATel#17603). We report here on a second observation obtained 55.6 days later, on UT 2026 Jan 23 (211.5 days after t_0).

We find a significant change from the 661.88+/-0.46 period reported in ATel#17603 for day 155.86, measuring 660.11+/-0.33s on day 211.5 from the pn X-ray light curve using the same instrumental setup as in ATel#17603. The periodicity is also present in all other X-ray instruments. The light curve from day 155.86 exhibits complicated contamination and scatter toward lower count rates, which can affect the period analysis. To verify the change in periodicity between the two observations, we therefore revised our analysis of the day-155.86 light curve. Filtering the light curve for outliers and removing newly discovered dips (see below) yields a revised period of 662.05+/-0.23s for day 155.86, strengthening the evidence for a period change between the two observations.

The modulation amplitude increased substantially, from approximately 10% on day 155.86 to approximately 50% on day 211.5. In addition, a significant first overtone is detected in the later observation but was not present on day 155.86. This is consistent with a change in the pulse profile, which now appears flattened near maximum while remaining approximately sinusoidal around minimum.

Closer inspection of the day-155.86 light curve also reveals that some of the downward scatter consists of short (<1 minute) dips reaching depths of up to 45% of the mean count rate. These dips appear preferentially near, or slightly before, the minimum of the sinusoidal modulation.

The RGS spectrum on day 211.5 is qualitatively similar to that on day 155.86, showing the same absorption-line spectrum, blueshifted by approximately 1600 km/s, and comparable continuum-to-line ratios. However, the flux decreased by a factor of about 2 at wavelengths longer than 32A and by a factor of about 7 below 32A. This differential decline implies a substantial deepening of the C V absorption edge at 31.62A, suggesting an increase in the column density of the absorbing material, possibly reflecting a change in its ionization balance. It is noteworthy that the depths of the C VI and C V absorption lines relative to the continuum remain largely unchanged, indicating that a more detailed analysis is required to understand the origin of the spectral evolution.