Detection of X-rays from the symbiotic star V1329 Cyg

arXiv:1102.1976v1 [astro-ph.HE] 9 Feb 2011 Received 2010 September 27; accepted 2011 February 3 Preprint typeset using LATEX style emulateapj v. 11/10/09 DETECTION OF X-RAYS FROM THE SYMBIOTIC STAR V1329 CYG Matthias Stute1,2, Gerardo J. M. Luna3, and Jennifer L. Sokoloski4 Received 2010 September 27; accepted 2011 February 3 ABSTRACT We report the detection of X-ray emission from the symbiotic star V1329 Cyg with XMM-Newton. The spectrum from the EPIC pn, MOS1 and MOS2 instruments consists of a two-temperature plasma with k T1 = 0.11+0.02 −0.02 keV and k T2 = 0.93+0.12 −0.14 keV. Unlike the vast majority of symbiotic stars detected in X-rays, the soft component of the spectrum seems to be absorbed only by interstellar material. The shock velocities corresponding to the observed temperatures are about 300 km s−1 and about 900 km s−1. We did not find either periodic or aperiodic X-ray variability, with upper limits on the amplitudes of such variations being 46% and 16% (rms), respectively. We also did not find any ultraviolet variability with an rms amplitude of more than approximately 1%. The derived velocities and the unabsorbed nature of the soft component of the X-ray spectrum suggest that some portion of the high energy emission could originate in shocks within a jet and beyond the symbiotic nebula. The lower velocity is consistent with the expansion velocity of the extended structure present in HST observations. The higher velocity could be associated with an internal shock at the base of the jet or with shocks in the accretion region. Subject headings: binaries: symbiotic – stars: individual (V1329 Cyg=HBV 475) – stars: white dwarfs – X-rays: stars – ISM: jets and outflows 1. INTRODUCTION V1329 Cygni (=HBV 475) is one of a small number of symbiotic novae (M¨urset & Nussbaumer 1994) that have outbursts of 3 to 7 mag and return to their pre- vious brightness only slowly over decades. The canon- ical model (e.g. Baratta & Viotti 1990) for symbiotic novae involves a hot white dwarf star that has a sur- face thermonuclear flash, with the fuel supplied by a red giant companion. The only recorded major out- burst of V1329 Cyg began in about 1965 and reached B magnitude < 11.5 mag in October 1966. Some time elapsed between discovery as an eruptive object (Kohoutek 1969; Kohoutek & Bossen 1970) and recog- nition that it is a binary, with the object first being called a proto-planetary nebula (Crampton et al. 1970). The photographic brightness dropped to mV < 16 mag over the past century, with several abrupt drops of as much as another 2.5 mag between 1925 and 1962. These repeated with a period of 950–959 days and were ex- plained as the eclipses of the hot component by the red giant (Stienon et al. 1974; Grygar et al. 1979). Schild & Schmid (1997) improved the determination of the orbital period to 956.5 days and found from polarime- try an inclination of 86±2 degrees. From optical and UV emission lines, orbital parameters of the hot component have been determined yielding minimum masses of about 0.71 and 2 M⊙for the white dwarf and red giant, respec- matthias.stute@tat.physik.uni-tuebingen.de 1 Institute for Astronomy and Astrophysics, Section Compu- tational Physics, Eberhard Karls Universit¨at T¨ubingen, Auf der Morgenstelle 10, 72076 T¨ubingen, Germany 2 Dipartimento di Fisica Generale ”A. Avogadro”, Universit`a degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy 3 Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS 15, Cambridge, MA, 02138, USA 4 Columbia Astrophysics Laboratory, 550 W. 220th Street, 1027 Pupin Hall, Columbia University, New York, NY 10027, USA tively (Schild & Schmid 1997). These authors derived an EB−V value of 0.6, corresponding to nH = 2.3×1021 cm2 (using the conversion factor of Groenewegen & Lamers 1989). About 200 symbiotic stars are known (e.g. Belczynski et al. 2000), but jets have been de- tected at different wavelengths only in 10 of them (Brocksopp et al. 2004). V1329 Cygni is a mem- ber of this list, since Brocksopp et al. (2003) found two peaks and extended emission in HST WFPC2 snapshot images taken in October 1999 with F502N and F656N filters. These were separated by about 950 AU, assuming a distance of 3.4 kpc as given by M¨urset & Nussbaumer (1994). After comparison with HST images of Schild & Schmid (1997) taken in July 1991, they derived an expansion velocity of 260 ± 50 km s−1, suggesting that this mass ejection was not associated with the nova outburst in 1965, but with an event in 1982. As an additional riddle, the position angle of the orbital plane of 11 ± 2◦determined by Schild & Schmid (1997) furthermore suggests that the mass ejection occurred along the orbital plane, instead of perpendicular to it as expected for a jet. Using the ephemeris of Schild & Schmid (1997) JDmin = 2444890.0 + 956.5 × E , (1) the system was at phase 0.733 during the observations of Schild & Schmid (1997) and at phase 0.880 during that of Brocksopp et al. (2003). Whethe

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