Introduction - Aims

1. The XMM-Newton Large Project for M 31: The search for High-Mass X-ray Binaries Hatzidimitriou D.1,2, Williams B.F.3, PietschW.N.4, Stiele H.5,Green G.M.3, Haberl F.4, Bonfini, P.6,2 1University of Athens, Department of Physics – Section of Astrophysics, Astronomy & Mechanics, Greece, 2Foundation of Research and Technology – Hellas, Heraklion, Greece, 3Department of Astronomy, University of Washington, Seattle, USA, 4Max-Planck-Institut für extraterrestrische Physik, Garching, Germany, 5Osservatorio Astronomico di Brera, Italy, 6Department of Physics, University of Crete, Heraklion, Crete, Greece Introduction - Aims Introduction - Aims • M31 is the nearest spiral galaxy to our own (780 kpc), and thanks to its moderate galactic foreground absorption and favorable inclination (78o), it is an ideal target for a detailed X-ray inventory of an archetypal low-star formation rate galaxy • Identification and spectral classification of X-ray sources in nearby galaxies is necessary for correctly interpreting the properties of more distant ones, particularly since we can classify nearby systems with X-ray and optical data, while more distant ones via X-rays alone • The HMXRB population in M31 remains elusive, and no HMXRB has been confirmed optically yet. The occurrence of HMXRBs, of the Be type in particular, are a sensitive star-formation indicator, and probably also sensitive to the metal abundance of the parent galaxy • Our aim is to identify HMXRBs among the 1948 X-ray sources discovered by XMM-Newton in the direction of M31, by combining X ray and optical properties The XMM-Newton M31 Large Project Fig.1 • XMM-Newton EPIC observations of the largest Local Group spiral galaxy M 31, taken between June 2006 and February 2008, together with archival observations (Pietsch et al.2005) from June 2000 to July 2004 yielded a total of 1948 X-ray sources (0.2-12.0 keV) covering (for the first time) the entire D25 ellipse of • M 31, down to a limiting luminosity of ~1035 erg/s in the 0.2-4.5 keV band (Stiele et al. 2010). These sources include • sources within M31, i.e. X-ray binaries, supernova remnants and supersoft sources • foreground (galactic) stars (Hatzidimitriou et al. 2006, Bonfini et al. 2009) • background objects, i.e. mostly active galactic nuclei, some normal galaxies and a few clusters of galaxies Fig.1 XMM-Newton M31 – LP : RGB image using bands 0.2-1.0, 1.0-2.0, 2.0-12.0 keV Classification criteria of X-ray Sources • Hardness ratios • X-ray variability • Correlations with catalogues in other wavelengths (optical, radio) – e.g. Local Group Survey (LGS: Massey et al. 2006) • Locus of candidate counterparts on optical Colour Magnitude Diagrams and X-ray Hardness Ratio Diagrams • High Mass X-ray binaries are expected to be classified as hard sources • A high percentage (65%) of the sources can only be classified as “hard” sources, i. e. they can be X-ray binaries (LMXB and HMXB) or Crab-like supernova remnants in M 31, or X-ray sources (mainly AGNs) in the background. • Be-XRBs are expected to have early B spectral types (blue stars) and H-alpha emission Fig.2 Optical Spectroscopy Candidate Be XRB? (+ Hα diffuse emission) • Sample: Hard sources with blue counterparts, brighter than V~21 • We have obtained optical spectra for a sample of 33 objects from the Apache Point Observatory in New Mexico using the • 3.5 meter telescope, equipped with the medium dispersion Dual Imaging Spectrograph. For the blue branch, a 400 lines/mm • grating was used, while the red branch was equipped with a 300 lines/mm grating.The spectra cover the wavelength ranges • ~3750-5400 Å, and ~5200-8500 Å, with a nominal resolution of 1.83 Å/pixel and 2.31 Å/pixel, respectively. • In Figure 2, the positions of the objects observed are overlayed on an optical image of M31 (Calar Alto, using HDAP) • Examples of spectra obtained are shown in Figures 3-4 [PFH 2005] 407 Single counterpart Colour magnitude Diagram of all candidate counterparts Q-parameter distribution for different classes of object Fig. 3 Hardness-Ratio Diagram Fig. 6 Fig. 7 Fig. 5 O + B stars in sample AGNs in sample All counterparts in LGS O & B stars Candidate HMXRB WN+OB (Embedded in HII region) Be stars (MW) [PFH 2005] 146 HII region Brightest counterpart Results Fig. 4 • Four optical counterparts (of sources 407, 688, 696, 757) observed have early B spectral types, visual magnitudes, reddening free parameter “Q” values, and hardness ratiosconsistent with Be XRBs Figures 5-7 • Τwo of these [407 (Fig. 3) and 688] show Hα emission (but there is also diffuse Hα emissionin both cases) These are possible Be-XRBs in M31 • One Of supergiant which can becandidate Supergiant-HMXB. • Two composite spectra • HII region + WR star of WN type:candidate HMXB Wolf Rayet WN + O star(Fig. 4)– colliding wind shock emission • supernova remnant + embedded early type (OB) star • Two old globular cluster spectra, and therefore the X-ray emission may be coming from a LMXB (one of the sources also shows transient behaviour) • Background sources:Nine Seyfert 1 galaxieswith redshifts z~0.7-1.6 + peculiar flaring quasar (Meusinger et al. 2010). • Foreground sources: One cataclysmic variableandone U-Gem (dwarf nova)variable in the Milky Way • New Classification Criteria for identification of X-ray sources, e.g. using Figures 6-7 (Hatzidimitriou et al. 2011) References • Bonfini et al. 2009, A&A, 507, 705 • Hatzidimitriou et al. 2006, A&A, 451, 835 • Hatzidimitriou et al. 2011, in preparation • Massey et al. 2006, AJ, 131, 2478 • Meusinger et al. 2010, A&A, 512, 1 • Pietsch et al.2005, A&A, 434, 483 • Stiele et al. 2010, A&A, under revision