Supersoft sources in M 31: Comparing the XMM-Newton Deep Survey, ROSAT and Chandra catalogues

📝 Abstract

To investigate the transient nature of supersoft sources (SSSs) in M 31, we compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton observations. While 12 of the XMM-Newton sources were brighter than the limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10 yr earlier. Five correlate with recent optical novae which explains why they were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below the ROSAT detection threshold. Nevertheless we found one correlation with a ROSAT source, which had significantly larger fluxes than during the XMM-Newton observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr between the observations. Five were also classified as SSSs by Chandra. Of the 30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other classifications are suggested. Of the remaining 16 sources one correlates with an optical nova. Of the 42 Chandra very-soft sources five are classified as XMM-Newton SSSs, while for 22 we suggest other classifications. Of the remaining 15 sources, nine are classified as transient by Chandra, one of them correlates with an optical nova. These findings underlined the high variability of the sources of this class and the connection between SSSs and optical novae. Only three sources, were detected by all three missions as SSSs. Thus they are visible for more than a decade, despite their variability.

💡 Analysis

To investigate the transient nature of supersoft sources (SSSs) in M 31, we compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton observations. While 12 of the XMM-Newton sources were brighter than the limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ~10 yr earlier. Five correlate with recent optical novae which explains why they were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below the ROSAT detection threshold. Nevertheless we found one correlation with a ROSAT source, which had significantly larger fluxes than during the XMM-Newton observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1-~6 yr between the observations. Five were also classified as SSSs by Chandra. Of the 30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other classifications are suggested. Of the remaining 16 sources one correlates with an optical nova. Of the 42 Chandra very-soft sources five are classified as XMM-Newton SSSs, while for 22 we suggest other classifications. Of the remaining 15 sources, nine are classified as transient by Chandra, one of them correlates with an optical nova. These findings underlined the high variability of the sources of this class and the connection between SSSs and optical novae. Only three sources, were detected by all three missions as SSSs. Thus they are visible for more than a decade, despite their variability.

📄 Content

arXiv:0912.0624v1 [astro-ph.HE] 3 Dec 2009 Astron. Nachr. / AN 999, No. 88, 789 – 794 (2006) / DOI please set DOI! Supersoft sources in M 31: Comparing the XMM-Newton Deep Survey, ROSAT and Chandra catalogues ⋆ H. Stiele1,⋆⋆, W. Pietsch1, F. Haberl1, V. Burwitz1, D. Hatzidimitriou2,3, and J. Greiner1 1 Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany 2 Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis, 15784 Zografos, Athens, Greece 3 IESL, Foundation for Research and Technology, 71110 Heraklion, Greece Received ??, accepted ?? Published online later Key words galaxies: individual (M 31) – X-rays: galaxies – X-rays: individuals (supersoft sources) – novae, cataclysmic variables To investigate the transient nature of supersoft sources (SSSs) in M 31, we compared SSS candidates of the XMM-Newton Deep Survey, ROSAT PSPC surveys and the Chandra catalogues in the same field. We found 40 SSSs in the XMM-Newton observations. While 12 of the XMM-Newton sources were brighter than the limiting flux of the ROSAT PSPC survey, only two were detected with ROSAT ∼10 yr earlier. Five correlate with recent optical novae which explains why they were not detected by ROSAT. The remaining 28 XMM-Newton SSSs have fluxes below the ROSAT detection threshold. Nevertheless we found one correlation with a ROSAT source, which had significantly larger fluxes than during the XMM- Newton observations. Ten of the XMM-Newton SSSs were detected by Chandra with <1–∼6 yr between the observations. Five were also classified as SSSs by Chandra. Of the 30 ROSAT SSSs three were confirmed with XMM-Newton, while for 11 sources other classifications are suggested. Of the remaining 16 sources one correlates with an optical nova. Of the 42 Chandra very-soft sources five are classified as XMM-Newton SSSs, while for 22 we suggest other classifications. Of the remaining 15 sources, nine are classified as transient by Chandra, one of them correlates with an optical nova. These findings underlined the high variability of the sources of this class and the connection between SSSs and optical novae. Only three sources, were detected by all three missions as SSSs. Thus they are visible for more than a decade, despite their variability. c ⃝2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The class of supersoft sources (SSSs), was established by ROSAT and is based on observable characteristics. SSSs show extremely soft spectra with equivalent blackbody tem- peratures of ∼15–80eV. The bolometric luminosities lie in the range of 1036–1038 erg s−1. The favoured model for these sources is that they are close binary systems with a white dwarf (WD) primary, burning hydrogen-rich matter at the surface. Symbiotic systems, which contain a white dwarf in a wide binary system, were also observed as SSSs (Kahabka & van den Heuvel 1997). SSSs are often observed as transient X-ray sources (Greiner 2000, and references therein). Pietsch et al. (2005a, 2007, hereafter PFF 2005 and PHS2007) showed that many SSSs in M 31 corre- late with classical novae. The observations of the ‘Deep XMM-Newton Survey of M 31’ (Stiele et al. 2008a) provide, together with archival observations, a full coverage of the D25 ellipse of M 31 with high spatial and spectral resolution. A description of ⋆Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Mem- ber States and NASA. ⋆⋆Corresponding author: e-mail: hstiele@mpe.mpg.de the observations, the source detection procedure and the full source catalogue will be published in a separate paper (Stie- le et al., in preparation). Here, we discuss the 40 SSSs con- tained in the Deep XMM-Newton Survey catalogue (here- after XMM LP-total). The SSSs were selected on the ba- sis of their hardness ratios (HR1<0, HR2−EHR2 <−0.96 or HR2 not defined; energy bands: B1:0.2–0.5keV, B2:0.5– 1.0 keV, B3:1.0–2.0keV). The hardness ratios and errors are defined as: HRi = Bi+1 −Bi Bi+1 + Bi and (1) EHRi = 2 p (Bi+1EBi)2 + (BiEBi+1)2 (Bi+1 + Bi)2 , (2) where Bi and EBi denote count rates and corresponding errors in energy band i. By cross-correlating with our nova catalogue1 we found that 14 out of the 40 sources can be classified as X-ray counterparts of optical novae. The main properties of the four sources that are reported for the first time are given in Table 1, while the remaining ten were re- ported in PFF2005, PHS2007 or Smirnova et al. (2006). To study the long term variability of the SSS population of M 31 we performed cross-correlations with the ROSAT 1 http://www.mpe.mpg.de/ ˜m31novae/opt/m31/M31_table.html c ⃝2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 790 H. Stiele et al.: Supersoft sources in M 31 Table 1 Properties of four newly detected SSSs, which correlate with optical novae Name t∗ S t+ E Dist& Perr† T ‡ bb L# X M31N (d) (d) (′′) (′′) (eV) (1037 erg s−1) 1997-10c 982 116

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