Detection of a 9.4 min periodicity in the XMM-Newton and Chandra X-ray light curves of V407 Lup (Nova Lup 2016)

ATel #10749; Andy Beardmore (U. Leicester), Andrej Dobrotka (Slovak University of Technology in Bratislava), Jan-Uwe Ness (ESA/ESAC, on behalf of a larger XMM-Newton-related collaboration), Marina Orio (University of Wisconsin and Padova Observatory), Julian Osborne and Kim Page (U. Leicester)
on 14 Sep 2017; 21:04 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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We report on the timing analysis of two long, uninterrupted X-ray observations of V407 Lup (also know as ASASSN-16kt and Nova Lup 2016; see ATel #9538, #9539, #9550, #9554, #9587, #9594 and #9644, #10632, #10722) performed with XMM-Newton for 22,000 s on 2017 March 11 and with the Chandra HRC_s and Low Energy Transmission Grating on 2017 August 30 for 34,000 s.

The light curve obtained with XMM-Newton Reflection Grating Spectrometer (RGS) in the 15 - 37 angstrom (i.e. 0.33 - 0.83 keV) range, 168 days after discovery, shows evidence of multi-periodic behaviour. This includes two broad (~2-3 ks wide) dips separated by 12.6 ks, consistent with a 3.6 hour modulation seen in the Swift-UVOT data reported in ATel #10632, which might be related to the orbital period of the binary.

Superimposed on the light curve are shorter timescale variations. A Lomb Scargle periodogram of the light curve reveals multiple peaks, at 524 s, 543 s, 564 s and 590 s. The modulation amplitude appears strongest during the broad dips, where it reaches fractional amplitudes of 10 - 15%. Simulations of a fixed frequency component with a variable modulation amplitude does not match the complicated periodogram patterns around the detected frequencies, but improvement can be achieved with a variable frequency component.

Splitting the wavelength range over which we extract the light curve into the 15 - 23.5 Å (0.53 - 0.83 keV) and the 23.5 - 37 Å (0.33 - 0.53 keV) bands, the Lomb Scargle periodogram of the hardness ratio reveals a dominant 564 s period, stronger in the lowest energy band.

The Lomb Scargle periodogram obtained from the zeroth order light curve of the Chandra data, taken 340 days after discovery, is dominated by a strong modulation at a period of 565 s.

The 565 s X-ray period and longer 3.6 hour Swift-UVOT period are clearly reminiscent of the typical rotation and orbital periods seen in intermediate polar cataclysmic variables and may imply that the white dwarf of this nova is magnetised. Further X-ray observations in quiescence will be necessary to confirm this hypothesis. Similar multiple periods as we measured here with XMM-Newton have been observed in other novae in the supersoft X-ray phase; see measurements and discussions in Drake et al., 2003, ApJ, 584, 448; Ness et al., 2003, ApJ, 594, L127; Leibowitz et al., 2006, MNRAS, 371, 424; Dobrotka & Ness, 2010, ApJ, 405, 2628 and 2017, MNRAS, 467, 4865; Ness et al., 2011, ApJ, 733, 70. Apart from the white dwarf rotation period, non-radial g-mode oscillations have also been discussed.