High-resolution X-ray Spectra Of The Symbiotic Star SS73 17

📝 Abstract

SS73 17 was an innocuous Mira-type symbiotic star until Integral and Swift discovered its bright hard X-ray emission, adding it to the small class of “hard X-ray emitting symbiotics.” Suzaku observations in 2006 then showed it emits three bright iron lines as well, with little to no emission in the 0.3-2 keV bandpass. We present here followup observations with the Chandra HETG and Suzaku that confirm the earlier detection of strong emission lines of Fe Kalpha fluorescence, Fe XXV and Fe XXVI but also show significantly more soft X-ray emission. The high resolution spectrum also shows emission lines of other highly ionized ions as Si XIV and possibly S XVI. In addition, a reanalysis of the 2006 Suzaku data using the latest calibration shows that the hard (15-50 keV) X-ray emission is brighter than previously thought and remains constant in both the 2006 and 2008 data. The G ratio calculated from the Fe XXV lines shows that these lines are thermal, not photoionized, in origin. With the exception of the hard X-ray emission, the spectra from both epochs can be fit using thermal radiation assuming a differential emission measure based on a cooling flow model combined with a full and partial absorber. We show that acceptable fits can be obtained for all the data in the 1-10 keV band varying only the partial absorber. Based on the temperature and accretion rate, the thermal emission appears to be arising from the boundary layer between the accreting white dwarf and the accretion disk.

💡 Analysis

SS73 17 was an innocuous Mira-type symbiotic star until Integral and Swift discovered its bright hard X-ray emission, adding it to the small class of “hard X-ray emitting symbiotics.” Suzaku observations in 2006 then showed it emits three bright iron lines as well, with little to no emission in the 0.3-2 keV bandpass. We present here followup observations with the Chandra HETG and Suzaku that confirm the earlier detection of strong emission lines of Fe Kalpha fluorescence, Fe XXV and Fe XXVI but also show significantly more soft X-ray emission. The high resolution spectrum also shows emission lines of other highly ionized ions as Si XIV and possibly S XVI. In addition, a reanalysis of the 2006 Suzaku data using the latest calibration shows that the hard (15-50 keV) X-ray emission is brighter than previously thought and remains constant in both the 2006 and 2008 data. The G ratio calculated from the Fe XXV lines shows that these lines are thermal, not photoionized, in origin. With the exception of the hard X-ray emission, the spectra from both epochs can be fit using thermal radiation assuming a differential emission measure based on a cooling flow model combined with a full and partial absorber. We show that acceptable fits can be obtained for all the data in the 1-10 keV band varying only the partial absorber. Based on the temperature and accretion rate, the thermal emission appears to be arising from the boundary layer between the accreting white dwarf and the accretion disk.

📄 Content

arXiv:0912.2766v1 [astro-ph.HE] 14 Dec 2009 Draft version November 9, 2018 Preprint typeset using LATEX style emulateapj v. 08/22/09 HIGH-RESOLUTION X-RAY SPECTRA OF THE SYMBIOTIC STAR SS73 17 R.N.C. Eze1,2, G.J.M. Luna1, and R.K. Smith1 Draft version November 9, 2018 ABSTRACT SS73 17 was an innocuous Mira-type symbiotic star until INTEGRAL and Swift discovered its bright hard X-ray emission, adding it to the small class of ”hard X-ray emitting symbiotics.” Suzaku observations in 2006 then showed it emits three bright iron lines as well, with little to no emission in the 0.3–2.0 keV bandpass. We present here followup observations with the Chandra HETG and Suzaku that confirm the earlier detection of strong emission lines of Fe Kα fluorescence, Fe XXV and Fe XXVI but also show significantly more soft X-ray emission. The high resolution spectrum also shows emission lines of other highly ionized ions as Si XIV and possibly S XVI. In addition, a reanalysis of the 2006 Suzaku data using the latest calibration shows that the hard (15-50 keV) X-ray emission is brighter than previously thought and remains constant in both the 2006 and 2008 data. The G ratio calculated from the Fe XXV lines shows that these lines are thermal, not photoionized, in origin. With the exception of the hard X-ray emission, the spectra from both epochs can be fit using thermal radiation assuming a differential emission measure based on a cooling flow model combined with a full and partial absorber. We show that acceptable fits can be obtained for all the data in the 1-10 keV band varying only the partial absorber. Based on the temperature and accretion rate, the thermal emission appears to be arising from the boundary layer between the accreting white dwarf and the accretion disk. Subject headings: symbiotic stars: white dwarf: iron lines: accretion

1. INTRODUCTION Symbiotic stars are interacting binaries whose compo- nents are a red giant and hot companion which accretes mass from the stellar wind of the red giant, produc- ing a blue continuum that ionizes the surrounding gas. In most symbiotic stars the accretor is a white dwarf (WD), although some symbiotic stars (e.g. GX 1+4) have a neutron star companion (Chakrabarty & Roche 1997). In X-ray wavelengths, symbiotic stars were de- tected as moderately bright sources in the ROSAT/All Sky Survey. M¨urset et al. (1997) examined 16 symbi- otic stars seen with ROSAT and categorized them into three classes: (1) super soft emission from the photo- sphere of the white dwarf (α-type), (2) emission from an optically-thin thermal (kT ∼0.2 keV) plasma possibly due to colliding winds from the two stars or to accretion (β-type), and (3) an ill-defined category of relatively hard X-ray sources (γ-type). The origin of this hard emission remains uncertain to this day, and although not in the M¨urset et al. (1997) survey, the ROSAT observation of SS73 17 suggests it would likely have been placed in cat- egory (3), albeit with some uncertainty due to the large column density (NH = 1.8 × 1022 cm2) required to fit ROSAT data (Smith et al. 2008). Earlier in 2005, both INTEGRAL (IGRJ10109- 5746, Revnivtsev et al. 2006) and Swift (SwiftJ101103.3- 574814, Tueller et al. 2005) independently discovered a hard X-ray source which Masetti et al. (2006) quickly identified with CD-57 3057 (SS73 17). Combined with a small group of objects – CH Cyg (Ezuka et al. 1998), RT Cru (Masetti et al. 2005; Luna & Sokoloski 2007), and Electronic address: reze@head.cfa.harvard.edu, romanus.eze@unn.edu.ng 1 Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138 2 University of Nigeria, Nsukka, Enugu State, Nigeria T CrB (Tueller et al. 2005; Luna et al.

2008. – these sources form the “hard X-ray emitting symbiotics”. Af- terwards, a dedicated Suzaku observation of SS73 17 re- vealed the presence of strong iron lines in the 6 −7 keV region (Smith et al. 2008) which pointed to a thermal ori- gin for the X-ray emission. Another remarkable feature in the X-ray spectrum of SS73 17 is its highly absorbed soft X-ray emission (with NH > 1023 cm−2 ; Smith et al. (2008)). The origin of the hard and weak soft X-ray emission have remained a mystery which we hoped to address with a combination of Chandra HETG and Suzaku observa- tions. In §2, we summarize our observations and data processing, §3 contains our spectral analysis, while §4 presents the timing analysis. A discussion of our results is found in §5.

2. OBSERVATION AND DATA ANALYSIS We observed SS73 17 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) and the Suzaku X-ray Imaging Spectrometer (XIS) and Hard X-ray Detector (HXD). The HETG data have a spec- tral resolution of 0.012 ˚A & 0.023 ˚A FWHM for the High and Medium Energy Gratings (HEG, MEG), re- spectively. The Suzaku XIS data cover the 0.3 −12 keV range with ∼150 eV resolution, while the HXD extends the energy coverage from 10 to 600 keV, although the source could not be detected above 50 keV.

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