The first X-ray detection of the dust-forming C-rich nova V1280 Sco

ATel #2063; J.-U. Ness (ESAC/ESA), G. Schwarz (West Chester), C. E. Woodward, L. A. Helton (U. Minnesota), M. Sitko (U. Cincinnati and Space Science Institute), R. W. Russell, D. K. Lynch, R. J. Rudy (The Aerospace Corp), S. Starrfield (Arizona State U.), J. P. Osborne, K. Page (U. Leicester), M. F. Bode (U. Liverpool)
on 28 May 2009; 20:10 UT
Distributed as an Instant Email Notice Novae
Credential Certification: Jan-Uwe Ness (Jan-Uwe.Ness@asu.edu)

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We report the first detection in X-rays of the Classical Nova V1280 Sco (IAUC # 8803 ). After V2362 Cyg, this is only the second carbon-rich dust-forming nova detected in X-rays.

We have obtained X-ray, optical, and Infrared observations on a number of dates. Optical spectra were obtained at the Steward Observatory Bok 2.29-m telescope on 2008-Jun-25.2 and 2009-May-7.4. Spitzer IR spectra were obtained on 2007-Apr-17.6, 2007-May-2.8, and 2008-May-6.2. IRTF/SpeX observations were made on 2009-May-20. Swift observed for 1010s, 1910s, and 4790s on 2007-Feb-21.5 (ATel #1011), 2007-Apr-9.9, and on 2009-May-18.7, respectively.

The first two Swift observations only gave X-ray telescope (XRT: 0.3-10 keV range) 95% upper limits of 0.018 c/s and 0.0014 c/s respectively. However, the latest observation yields a clear detection with a count rate of 0.021+/-0.002 c/s. With only 82 counts, the XRT spectrum is not sufficiently exposed for detailed spectral modelling. However, comparing a black body fit with an APEC (optically thin plasma) model suggests that the emission originates from an optically thin, thermal plasma. Although both models reproduce the data, the best-fit black body temperature is 195+/-45 eV (NH < 3E21 cm^-2, Lbol=2.8E32 erg/s assuming d=1.6 kpc; Chesneau et al. 2008, A&A 487, 223), which is too high for photospheric emission from a white dwarf. The APEC fit yields values of kT of 227+98-53 eV (for NH=(7.2+/-2.5) E21 cm^-2). The XRT spectrum does not resemble that of a Super Soft Source (SSS), even this late in the outburst, and so is probably not being produced by nuclear burning on the white dwarf.

The first detection of formation of dust was reported 2007-Mar-5 (CBET #866), and recent Spitzer and IRTF/SpeX observations indicate that significant amounts of dust are still present (CBET #1809, IAUC # 9046 ). The detection of UIR features suggests the ejection of a dense carbon-rich dust shell. The reddening deduced from OI lines (IAUC # 9046 ) is lower than on 2007-May-31 (IAUC # 8845 ), but still higher than the Schlegel et al. (1998, ApJ, 500, 525) galactic extinction maps, yielding only E(B-V) = 0.36 at V1280 Sco's position. We conclude that significant circumstellar extinction must still be present. Here, we report the continued presence of P-Cyg absorption on 2009-May-7.4 on the HeI 667.8-nm line and on the Balmer lines with a mean velocity of ~1000 km/s. The expansion velocity has thus increased from only 500 km/s on 2007-Feb-5.9-8.7 (IAUC # 8803 and # 8807 ).

Since some of the emission in the Swift XRT spectrum occurs where strong lines are expected (NVII, OVIII, FeXVII, and MgXII), we interpret the X-ray emission as originating from overlapping emission lines, unresolved by the XRT. Compared to other novae, the detection of shock-related hard emission is quite late in the outburst. The detection of P-Cygni profiles at such a late time, on the other hand, may indicate that the conditions for shock-induced X-ray emission are still present. We note that in the novae V1974 Cyg (Krautter et al. 1996, ApJ, 456, 789) and V458 Vul (Ness et al. 2008, AJ, 137, 4160), the earlier "hard" X-ray emission phase was followed by bright SSS X-ray emission, and we may yet detect SSS emission from the white dwarf. More observations at all wavelengths are encouraged.

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