X‐Ray Point Sources in the Central Region of M31 as Seen byChandra

Abstract

We report on Chandra observations of the central region of M31. By combining eight Chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central ∼ 17 ′ × 17 ′ region of M31, with a detection limit of ∼ 2 × 10 35 erg s −1 . Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at 1.7 × 10 37 erg s −1 ). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.Subject headings: galaxies: individual (M31) -X-rays: galaxies -X-rays: stars supersoft source populations (Di Stefano et al., in preparation) In this paper, we adopt a distance of 780 kpc (Stanek & Garnavich 1998;Macri et al. 2001) and assume a hydrogen column density equal to the Galactic value of ∼ 7 × 10 20 cm −2 (Dickey & Lockman 1990) unless otherwise specified. The quoted errors throughout this paper are at 1-σ confidence, unless otherwise specified. Observations and Data ReductionM31 was observed with Chandra regularly as part of the AO-1 and AO-2 GTO program during 1999-2001. The program was designed to search for transients. The observations consist of a series of HRC snapshots (∼ 1 ks) that cover the entire galaxy. If a transient is discovered in the HRC mosaic, then a follow-up ACIS image (∼ 5 ks) of the transient is obtained; otherwise an ACIS image of the nucleus is obtained. In this paper we focus only on the ACIS-I (I0, I1, I2 and I3) data obtained for the central 16.9 ′ × 16.9 ′ region of M31. These data consist of 8 separate observations, with exposure times ranging from 4 to 8.8 ks. The details of the observations are given in Table 1. The nuclear region of M31 was placed near the aim-point of the ACIS-I array. The focalplane temperature was −110 • C during the first four observations, and −120 • C for the others. The observations were made at various spacecraft roll angles; consequently, the total region covered by the observations is slightly larger than 16.9 ′ × 16.9 ′ , and sources near the outer edge of ACIS are not observable in all eight exposures.All data were telemetered in Faint mode and were collected with a frame transfer time of 3.2 s. In order to reduce the instrumental background, only data with ASCA grades of 0, 2, 3, 4, and 6 were included. We selected data free from bad columns, hot...

The very short supersoft X-ray state of the classical nova M31N 2007-11a

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The Luminous X‐Ray Source Population in M51 Observed withChandra

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Classifying X‐Ray Sources in External Galaxies from X‐Ray Colors

“…However, once the remnant enters the adiabatic phase, the luminosity should decline gradually (Jones, Smith, & Straka 1981;Hamilton, Chevalier, & Sarazin 1983). In contrast, accretion-powered supersoft sources are known to be variable (Kahabka, Pietsch, & Hasinger 1994;Kong et al 2002), and detection of variability in a large fraction of the soft sources would support the hypothesis that they are accretion-powered. Both M101 and M83 have one deep exposure in the Chandra archive, with a second shorter observation (see Table 1).…”

“…The LMXB region is best defined in Figure 3. By crosscorrelating our M31 inner bulge source list with the list given by Kong et al (2002), we have rejected known foreground and background sources, supersoft sources, and sources for which there may be a contribution from hot gas (e.g., those associated with planetary nebula). The resulting plot is illustrative of the intrinsic dispersion in the colorcolor diagram of LMXBs.…”