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A 71 second oscillation in the super-soft X-ray flux of V5668 Sgr (Nova Sgr 2015 No. 2)

ATel #8133; K. L. Page, A. P. Beardmore & J. P. Osborne (U. Leicester)
on 6 Oct 2015; 16:21 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

Subjects: X-ray, Nova

On 2015 August 30 (168 days after outburst), the bright, dust-forming (ATEL #7643, #7862), gamma-ray emitting (ATEL #7283, #7315) nova V5668 Sgr was seen to enter a super-soft X-ray phase (ATEL #8054). Since that time, the X-ray flux measured by the Swift XRT has shown a general brightening trend, reaching ~10 c/s on September 12 (day 181) and ~8 c/s between September 23-24 (day 192-193). Observations of ~1-1.5 ks have been obtained approximately daily between the end of August and September.

We have searched the XRT WT light-curves (binned at 0.018s, i.e. 10x the WT time resolution) for variability on short timescales; WT data were regularly collected after September 17 (day 186). An oscillation with a central period of 71 +/- 2 s was first detected on September 19 (day 188). This modulation is not detected in all observations, and shows a varying strength, up to ~15% modulation amplitude, though usually much less. There has not been a significant (>99%) detection of the modulation since September 25 and the most recent observation on October 01 (day 193-200).

This short-period modulation appears similar to the quasi-periodic oscillations (QPOs) which have been detected previously in the super-soft X-ray emission of a number of novae, although we do not have strong constraints on the coherence of the modulation in V5668 Sgr at this stage. Oscillations centred on a period of ~35 s were seen in both RS Oph (Osborne et al. 2011 ApJ 727, 124; Ness et al. 2007 ApJ 665 1334) and KT Eri (ATEL #2423), while a ~54 s period was found in V339 Del (ATEL #5573); see also the XMM detections discussed by Ness et al. (2015, A&A, 578, A39). The origin for such oscillations is not certain; they might be related to WD spin or a nuclear burning oscillation (eg Osborne et al. 2011).

Swift continues to monitor V5668 Sgr , and we thank the Swift PI and mission operations team for their ongoing support.