A new 33-s period in the SSS X-ray light curve of the nova LMC 2009a and confirmation of the 35 s modulation in KT Eri

ATel #6147; J.-U. Ness, E. Kuulkers, M. Henze (ESAC/ESA), G. Schwarz (American Astronomical Society), J. P. Osborne, A. P. Beardmore (U. Leicester), A. Dobrotka (Slovak University of Technology), A. L. Piro (Caltech), S. Starrfield (Arizona State U.)
on 20 May 2014; 07:27 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jan-Uwe Ness (juness@sciops.esa.int)

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In addition to four known systems with super soft source (SSS) X-ray emission (<1keV) with short transient periods in the range 35-67s, we report the discovery that another such system, nova LMC 2009a, hosts a transient 33-s period. So far such periods have been found in XMM-Newton data of the persistent SSS Cal 83 (67s: Odendaal et al. 2014, MNRAS, 437, 2948), in Swift/XRT and XMM-Newton data of the nova RS Oph (35s: ATel#770, Osborne et al. 2011, ApJ, 727, 124, Ness et al. 2007, ApJ 665, 1334), in Swift/XRT data of the nova KT Eri (35s: Beardmore et al. ATel#2423), and in Swift/XRT and XMM-Newton data of the nova V339 Del (54s: Beardmore et al. ATel#5573, Ness et al. ATel#5626).

We have searched for similar oscillations in all XMM-Newton and Chandra observations of all objects containing SSS emission, that is persistent SSS and novae during their SSS phase. Each observation was split into overlapping 1000-sec segments whose light curves were searched for periodic signals with the Lomb-Scargle method. We constructed a time series of the period power which relates to the detection probability of a tested period in terms of the false alarm probability defined by Scargle (1982, ApJ 263, 835). This allows us to detect transient periods that are only present for a short time during an observation that might not be detectable in the entire observation.

The nova LMC 2009a was observed with XMM-Newton four times during 2009: May 6.4, July 20, Aug. 20.6 and Sep. 23, i.e., 90, 165, 197 and 230 days after the start of the outburst. We found a clear transient 33-s period on days 165, 197, and marginally at the beginning of the day-230 observation. In the observations taken on days 165 and 197, the period is detected at 90% confidence for 7% and 20% of the total time, respectively.

Our time-resolved method also revealed the 35-s period of KT Eri in a Chandra observation taken on day 159 after outburst (2010 April 21.4) that was detected for 69% of the time above 90% confidence. This period is also detected above 87% confidence for 10% of the time on day 84.6 (2010 Feb 6.3).

A comprehensive analysis of the XMM-Newton and Chandra observations with the timing properties of all SSS emitting objects is in preparation.