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Swift detects a large flux increase and a probable period of 1.77 days in the X-ray and UV emission of the nova CSS081007:030559+054715

ATel #1873; A. P. Beardmore, J. P. Osborne, K. Page (University of Leicester), G. J. Schwarz (West Chester Unversity), S. Starrfield (Arizona State University), J.-U. Ness (ESAC, ESA)
on 11 Dec 2008; 16:02 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

Subjects: Ultra-Violet, X-ray, Request for Observations, Nova

Referred to by ATel #: 1910, 1938, 1940, 1942

Swift has continued to monitor the nova CSS081007:030559+054715 (ATEL #1825, ATEL #1835, ATEL #1847) in the X-ray and UV wavebands. The Swift-XRT 0.3-10 keV light curve showed a relatively constant count rate of 0.18-0.21 count/s from 2008-Nov-10 to 2008-Nov-21. Over the same time interval the Swift-UVOT uvw2 light curve (central wavelength 1928 A) had a mean magnitude of 14.9, with +/-0.2 magnitude deviations from this value.

Following an 8 day hiatus, daily Swift observations were performed between 2008-Nov-30 and 2008-Dec-09. The X-ray light curve from this interval shows a large flux increase and also large amplitude variations, with the X-ray count rate varying from 0.2 to 2.2 count/s. A Lomb-Scargle periodogram reveals a strong signal at a frequency of 6.53e-06 Hz, in addition to the odd harmonics of this frequency. These suggest that a non-sinusoidal modulation with a period of P=1.77 days is present in the data. An analysis of test periods using the observed window function gives us further confidence in the period determination.

When the X-ray light curve is folded at this 1.77 day period we see a rapid decline (lasting 0.02P) to the minimum count rate level (0.2 count/s) which lasts for approximately 0.15P. Due to limited phase coverage the duration of the maximum is more uncertain, but could extend between 0.4 to 0.8 cycles. The modulation is not apparent in a 0.5-1.5 keV / 0.3-0.5 keV hardness ratio.

The X-ray spectrum remains essentially as reported in ATEL #1847, ie simply a very soft blackbody.

Plots of the Swift 0.3-10 keV X-ray and uvw2 UV light curve, the periodiogram of the X-ray count rate, and the X-ray count rate folded at 1.77 days are available at http://www.swift.ac.uk/CSS081007.shtml

Swift-UVOT uvw2 photometry continued from 2008-Nov-30. It also shows that the source has brightened, and it is also suggestive of a 1.77 day periodicity with a modulation ranging from a magnitude of 14.6 to 14.0 in phase with the X-ray modulation.

We aim to continue Swift observations at a higher cadence over the next week to constrain the periodicity further. Ground based photometry is encouraged.

We thank the Swift PI, Neil Gehrels, the Swift science team and the Swift mission operations team for their support of these observations.