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 (firstname.lastname@example.org)
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
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
Swift X-ray light curves of RS Oph