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RS Oph: Swift X-ray observations find short period modulation and highly variable low energy flux

ATel #770; J Osborne, K Page, A Beardmore M Goad (Leicester), M Bode (Liverpool John Moores), T O'Brien (Manchester), G Schwarz (West Chester), S Starrfield, J-U Ness (Arizona State), J Krautter (Heidelberg), J Drake (SAO), A Evans (Keele), S P S Eyres (Central Lancashire)
on 22 Mar 2006; 17:49 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

Subjects: X-ray, Nova

Referred to by ATel #: 801, 838, 1301, 2392, 2423, 6147

Swift X-ray telescope observations following the recent explosion of the recurrent nova RS Oph over 2006 Mar 17.65 - Mar 19.02 UT have shown a much increased count rate compared to the days before, and have revealed two new forms of modulation.

The Swift XRT 0.3-10 keV light curve can be characterised as a high level of 100-150 c/s which appears to be cut by two deep dips (centred on Mar 18.25 & 18.79 UT) during which the count rate fell below ~20 c/s. These high count rates compare with an initial secular decline to ~6 c/s on Mar 10.82. The increased count rate is due to a higher flux from the new low energy component (E<0.7 keV) reported in ATEL #764. The Swift data do not strongly reject a periodic recurrence of the dips, which are not of uniform width. It is also possible that their apparent separation is due to the ~96 min sampling by Swift; the rest of the light curve is suggestive of aliasing, and a period near 1.7 hours may be present. The high-low flux episodes are visible both above and below 0.7 keV.

The XRT data also show faster modulation at more than one period. Strong modulation (amplitude up to 9%) at periods of 35.0 and 34.5 sec is seen. Other nearby periods may also be present, and the period often changes from one Swift observation to the next. The modulation is quasi-sinusoidal, and has similar fractional depth above and below 0.7 keV. Occasionally a first harmonic is also seen.

The X-ray flux during Mar 20.75 - 22.16 was at a similar level and displayed similar behaviours, including: 3 dips or rapid declines again separated by ~0.54 days; possibly related to this, unresolved variations again suggestive of an underlying ~1.7 hr modulation; and multiple periods around 35 sec.

Throughout the entire interval the harder X-ray flux (E>1.5 keV) shows a declining trend, broadly consistent with the earlier behaviour, and with a slight imprint of the low energy variations in the Mar 22 observation.

The 0.3-10 keV Swift X-ray light curve for the two intervals can be viewed at http://www.star.le.ac.uk/~julo/RSOph/

The substantially increased flux at low energies appears similar to the emergence of the super-soft phase in other novae, caused by the unveiling of the still-burning white dwarf. The origin of the intensity variations at timescales longer than one hour is unclear. The modulation timescales of ~13 hrs and/or 1.7 hrs are both much shorter than those expected for the accretion disk, and RS Oph is thought to be at low inclination (Dobrzycka & Kenyon 1994 AJ 108, 2259).

The poor stability of the ~35 sec periods appears to disfavour an explanation based on the rotation period of the white dwarf. The nova V1494 Aql showed multiple soft X-ray periods, albeit mostly above 1000 sec, attributed by Drake et al 2003 (ApJ 584, 448) to pulsations. Similarly, Ness et al 2003 (ApJ 594, 127) found a soft X-ray period in the nova V4743 Sgr at 1324 sec. Even so, spinning magnetized white dwarfs are known to have short periods (eg AE Aqr at 33 sec), and a number of novae contain rapidly spinning magnetic white dwarfs (eg DQ Her, V533 Her, GK Per).

Swift observations continue, thanks to the efforts of the Swift MOC at Penn State.

Swift X-ray light curves of RS Oph