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 (email@example.com)
Referred to by ATel #: 10756
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
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
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.