Emergence of a Bright and Highly Variable Super-soft Source Phase in Nova KT Eri (2009)

ATel #2392; M F Bode (Liverpool JMU), J P Osborne, K L Page, A P Beardmore (Leicester), F M Walter (Stony Brook), J-U Ness (ESA), G Schwarz (AAS), S Starrfield (ASU), E Kuulkers (ESA), T J O'Brien (Manchester), S Balman (METU), M J Darnley (Liverpool JMU), A Evans (Keele), P Evans (Leicester), S P S Eyres (Central Lancashire), J Krautter (Heidelberg)
on 20 Jan 2010; 19:17 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Michael Bode (mfb@astro.livjm.ac.uk)

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We report Swift satellite X-ray (XRT) and SMARTS optical photometry and spectroscopy of KT Eri. The nova was discovered on 2009 November 25.536 UT but was present in outburst on sky patrol images taken on 2009 November 14.572 (IAUC # 9098 ) which we take as t = 0 days here.

Swift observations of KT Eri commenced on 2009 November 27 UT (day 13.1). No X-ray emission was detected with the XRT until day 39.9, at which time a relatively hard X-ray source was seen (all but one count above 1 keV), as was the case on day 47.5. By the time of the next observation on day 55.5, excess soft emission (below ~0.5 keV) was apparent, with a count rate of 0.019 +/- 0.003 cps. This could be modelled with a BB of kT ~ 25 eV. The X-ray source remained around 0.03-0.05 cps and consistently soft (comparing the 0.3-1 and 1-10 keV bands) between days 60.3 and 62.5 (BB kT ~ 45 eV). On day 64.1 the source was at a similar count rate, the soft emission could be fit by a BB of kT ~ 33 eV at this time. Then, on 2010 January 19 (day 65.7), a large increase in count rate was observed, with the X-ray emission reaching 13 cps and softening dramatically, with the soft emission being modelled with a BB of kT ~ 18 eV. We note that the timescale for emergence of the SSS after t = 0 was very similar to that in the recurrent nova LMC 2009a (ATEL #2025). Observations from day 65.61 to 65.76 showed dramatic changes in count rate by a factor of ~20 in ~3 hours. This fast and rapid soft X-ray variability appears similar to that seen in Nova RS Oph 2006 (ATEL #770). Fitting these spectra simultaneously to determine the absorbing column provides an estimate of (2.8 +/- 0.1) x 10²¹ cm^-2 which is significantly higher than that estimated from the proposed interstellar extinction alone, i.e. NH~5x 10²⁰ cm^-2 (ATel #2327). However caution should be exercised over the applicability of BB fits to what is now a hot stellar atmosphere.

We have been monitoring KT Eri photometrically and spectroscopically with the SMARTS facilities at Cerro Tololo Chile since 2009 November 27 (day 13). The optical spectrum has similarities to those of the recurrent novae YY Dor and N LMC 2009a (ATel #1930), with a distinctive H-alpha emission profile consisting of a broad pedestal (FWZI ~ 5800 km/s) underneath a narrower double-peaked line. The V/R ratio of the central component reverses on timescales of a few days. The entire complex is blue-shifted by about 200 km/s. The He I lines are double-peaked, and mirror the central H-alpha component. He II 4686 emission, presumably pumped by the SSS, is marginally detectable above the wing of the 4060A emission complex from days 32 to 44, at which time the equivalent width of the emission increased by a factor of 6 in 28 hours. It increased another factor of 2 on day 55 to 20A equivalent width, and may have peaked at 30A on day 61.

Approximately nightly BVRI photometry from day 17 to day 65 shows a steady fading of about 2 mag in BVRI and about 0.5 mag at JHK since day 53. A fairly smooth decay in BVRI up to about day 51 transitioned into a more variable light curve with day-to-day variations of at least 0.5 mag. There is a suggestion of a 2.7d periodicity (5 cycles), but we are insensitive to periods shorter than 2 days and the AAVSO light curve suggests more complex variability.

Swift and SMARTS observations are continuing and further ground-based monitoring (both photometry and spectroscopy) is strongly encouraged. We thank the Swift PI, Neil Gehrels, the Swift science team and the Swift mission operations team for their support of these observations.