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X-Ray Emission from Wolf-Rayet Stars

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X-Ray Emission from Wolf-Rayet Stars X-ray Emission from Wolf-Rayet Stars Steve Skinner1, Svet Zhekov2, Manuel Güdel3 Werner Schmutz4, Kimberly Sokal1 1CASA, Univ. of Colorado (USA) [email protected] 2Space Research Inst. (Bulgaria) and JILA/Univ. of Colorado (USA) 3ETH Zurich (Switzerland) 3PMOD (Switzerland) Abstract We present an overview of recent X-ray observations of Wolf-Rayet (WR) stars with XMM-Newton and Chandra. Observations of several WC-type (carbon-rich) WR stars without known companions have yielded only non-detections, implying they are either very feeble X-ray emitters or perhaps even X-ray quiet. In contrast, several apparently single WN2-6 stars have been detected, but data are sparse for later WN7-9 stars. Putatively single WN stars such as WR 134 have X-ray luminosities and spectra that are strikingly similar to some known WN + OB binaries such as WR 147, suggesting a similar emission mechanism. 1 X-rays from WR Stars: Overview 3 Single Nitrogen-rich WN Stars 4 Wolf-Rayet Binaries WR stars are the evolutionary descendants of massive O • Sensitive X-ray observations have now been obtained • High-resolution X-ray grating spectra have been ob- stars and are losing mass at very high rates. They are in ad- of several putatively single WN2-6 stars with XMM tained for a few binaries such as γ2 Vel (WC8 + vanced nuclear burning stages, approaching the end of their and Chandra. All but one were detected (Fig. 1). O7.5; Fig. 3) and WR 140 (WC7 + O4-5). CCD lives as supernovae. Strong X-rays have been detected from CCD spectra exist (Fig. 2), but no grating spectra spectra have been obtained of WR 147 (WN8 + OB; several WR binary systems, which are thought to originate (at have been obtained of single WN stars due to low Fig. 2). These spectra stringently test shock models. least partially) in a colliding wind shock between the two binary count rates. components. Much less is known about the X-ray emission 2 • Wind Shocks: Spectral features in γ Vel (Skinner from single (non-binary) WR stars, but theoretical models pre- • Observations of later WN7-9 stars are sparse. The et al. 2001; Schild et al. 2004) imply line formation dict that they should emit soft X-rays (kT < 1 keV) via shocks WN8h star WR 40 was undetected by XMM (Gosset far from the star, perhaps in colliding wind shocks. that are set up by instabilities in their supersonic line-driven et al. 2005; see also Fig. 1). Are WNL stars X-ray winds. We have recently detected X-ray emission from two pu- sources? We don't yet know. • tatively single WN stars, and both show hard emission that is Models: Colliding wind models of WR 147 roughly reproduce XMM CCD X-ray spectra, but suggest a not anticipated if their X-rays originate in radiative wind shocks. • Putatively single WN stars such as WR 2 and WR No single WC star has so far been detected in X-rays. revision of adopted mass-loss parameters may be 134 show high-temperature plasma in Chandra ACIS needed (Skinner et al. 2007; Zhekov 2007). spectra (Fig. 2). This is not expected if their emission arises solely in radiative wind shocks. WN (detected) WN (detected) WN (undetected) WN (undetected) WC (undetected) −6 WC (undetected) 20b 24 33 WR 147 (WN8+B0.5V) 20b h m s h m s h m s Chandra HETG (MEG 1st order) 147 110 08 09 33 08 09 32 08 09 31 110 • −47˚20´00˝ 134 134 X-ray luminosities of several single WN stars are as Gamma Vel 2 24 0.15 2 −7 ) Si XIII Si XIV S XV bol 32 Mg XII /L large as some WR binaries (Fig. 1). If these stars x (ergs/s) x −47˚20´05˝ 40 5 OB Stars Mg XII 136 | log (L | log L −8 40 are indeed single, the belief that single WR stars are Mg XI 57 136 − companion | 114 31 5 57 0.1 ||| 90 less-luminous than WR binaries is probably incorrect −47˚20´10˝ Ar XVII 114 −9 135 | 90 (but subject to distance uncertainties). − Gamma Vel S XV S XVI 30 S XVI 135 Counts/s/Angstrom | −47˚20´15˝ | 38 38.5 39 39.5 40 2 3 4 5 6 7 8 9 0.05 Si XIII | Ne X log L (ergs/s) Spectral Subtype (WN, WC) bol | Ne X Fe XX | Ne IX | Ne IX −47˚20´20˝ Fe XXV | | || | 0 2 4 6 8 10 12 14 WAVELENGTH (Angstroms) FIGURE 1: Left: Lx versus Lbol for single WR stars. The WN binary sys- WR2 (WN2) N VII ? Mg XI Si XIII tem WR 147 is also shown for comparison. The WR star number is shown | | | next to each data point. The solid line is the regression fit for OB stars de- 0.04 tected in the ROSAT All-Sky Survey (Berghöfer et al. 1997). L from 2 bol FIGURE 3: Left: Chandra zero order ACIS image of γ Vel. The WC8+O7.5 system is Hamann et al. 2006 (AA, 457, 1015). Right: L /L versus spectral sub- 0.02 00 x bol unresolved. A faint companion lies 4.8 to its north. Right: Chandra HETG grating spectrum type. There are no existing X-ray detections of single WC stars or of single Counts/s/keV 2 (MEG, 1st order) of γ Vel (Skinner et al. 2001). WN stars later than WN6. 0 WR 134 (WN6) | | 0.06 0.04 S XV 2 Single Carbon-rich WC Stars | 5 Open Questions • Sensitive X-ray observations have now been obtained 0.02 • Are all single WC stars X-ray quiet? Are dense metal- of five putatively single WC stars with XMM and 0 rich winds responsible for the absence of X-ray detec- | | Chandra. All are of spectral type WC5 or later. Sur- WR 147 (WN8+OB) tions of WC stars? 0.4 prisingly, none is detected (Fig. 1). S XV 0.3 | • Are late-type single WN7 - WN9 stars X-ray emitters? • The most stringent upper limit is from a 19 ksec 0.2 Fe XXV | Chandra observation of the WC8 star WR 135 (Skin- 0.1 • Why do the X-ray spectra of putatively single stars like ∼ ner et al. 2006). Upper limits on Lx/Lbol are 100 0 WR 134 (WN6) so closely resemble known binaries times below that of detected O stars and other WR 0.1 1 10 Energy (keV) such as WR 147? Does WR 134 have an optically stars (Fig. 1). faint companion? • The observed sample is small, but results so far sug- FIGURE 2: Chandra ACIS CCD X-ray spectra of putatively single stars WR • Do other processes besides colliding winds con- gest that WC stars are X-ray quiet. 2 (WN2; AV = 1.52 mag) and WR 134 (WN 6; AV = 1.41 mag). X-ray tribute to the X-ray emission of luminous WR+OB bi- absorption below 1 keV is stronger in WR 134 than in WR 2, despite similar naries? • Where are the X-rays? If they are produced in shock AV . Both stars show high-temperature plasma (kT ≥ 3 keV) and high-T emission lines, at odds with a purely radiative wind shock origin. A strong regions close to the star in the wind acceleration N VII line may be present in WR 2. Bottom panel shows the XMM PN spec- References & Acknowledgment zone, as appears to be the case for some O stars trum of the binary WR 147 (AV = 11.2 mag) for comparison (Skinner et al. (e.g. σ Ori AB, Skinner et al. 2008) then they may be 2007). Berghöfer, T. et al. 1997, A&A, 322, 167 Gosset, E. et al. 2005, A&A, 429, 685 totally absorbed by the metal-rich WC wind. Schild, H. et al., 2004, A&A, 422, 177 Skinner, S. et al., 2001, ApJ, 558, L113 Skinner, S. et al., 2006, Ast. Space Sci., 304, 97 Skinner, S. et al., 2007, MNRAS, 378, 1491 Skinner, S. et al., 2008, ApJ, 683, 796 Zhekov, S., 2007, MNRAS, 382, 886 This work was supported by NASA grants NNG05GA10G & GO8-9008..
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