1 Introduction

Figure 1: The 0.3-10 keV EPIC PN image of the central region of M 31. The data are from the 31-ks exposure taken on June 25, 2000. The intensity of the color is proportional to the logarithm of the number of counts collected (with a maximum of 5000 counts). PN chip boundaries are clearly visible. The positions of ten variable sources are encircled and labelled with the numbers listed in Table 2.

At a distance of 760 kpc (van den Bergh 2000; adopted throughout this paper), M 31 is close enough to allow detailed study of individual sources within the galaxy using modern X-ray telescopes. Over 100 discrete X-ray sources in M 31 were detected with the Einstein  observatory (Trinchieri & Fabbiano 1991, hereafter TF; van Speybroeck et al. 1979). Primini et al. (1993, hereafter PFJ) reported on the detection of 86 X-ray sources in the central 34$^\prime$ of M 31 with the ROSAT  HRI. Supper et al. (2001, hereafter Su01) recently published the complete catalog of 560 sources detected in a $\sim$10.7 deg² survey of M 31 with the ROSAT /PSPC. From the extrapolation of the luminosity distribution, PFJ concluded that the detected population of X-ray sources could account for only $\sim$15-26% of the unresolved X-ray emission in M 31, suggesting that the remaining emission is truly diffuse or due to a new class of X-ray sources. Spectral analysis of ROSAT  data performed by Borozdin & Priedhorsky (2000) and observations with XMM  (Shirey et al. 2001, hereafter Paper I) and Chandra  (Garcia et al. 2001) showed that the unresolved X-ray emission in the bulge of M 31 is significantly softer than most of the point sources and can be approximated with an optically-thin thermal plasma model ( $kT\sim 0.3$ keV) as expected from truly diffuse emission.

 

 

Table 1: XMM-Newton  PV observations of the core of M 31.

Date & Time (UTC)	Rev	Obs. ID	Exp. (ks)
25/6/2000 (10:44-20:25)	100	0112570401	34.8a/30.7b
28/12/2000 (0:10-3:34)	193	0112570601	12.2a/9.8b

^a

For EPIC-MOS.

^b

For EPIC-PN.

In the first Chandra  observation of M 31, the nuclear source seen with Einstein  and ROSAT  was resolved into five sources (Garcia et al. 2000, hereafter G2000). One of these sources is located within 1 $^{\prime\prime}$ of the radio nucleus of M 31 and exhibits an unusually soft X-ray spectrum, suggesting that it may be associated with the central super-massive black hole. A few more pairs of previously unresolved sources and a new transient were also detected within 30 $^{\prime\prime}$ of the nucleus.

We report on observations of the Andromeda Galaxy (M 31) carried out with XMM-Newton  (Jansen et al. 2001) during its Performance Verification (PV) phase. M 31 was selected as an XMM  PV target in order to demonstrate the capabilities of the mission in performing spectral and timing studies in a field of point sources and extended emission. In Paper I we focused on the group properties of the X-ray point sources and on the diffuse emission. The spectral properties of discrete X-ray sources in the XMM-Newton  exposures will be discussed in Trudolyubov et al. (2001, Paper III). In this present paper we discuss the variability of individual X-ray sources in M 31. In Sect. 2, we summarize the XMM-Newton  observations and our data-reduction process. In Sect. 3, we discuss our detections of transient and periodic variability in individual sources. Finally, we present our conclusions.

 

 

Table 2: Bright variable X-ray sources detected in M 31 with XMM-Newton  in 2000.

#	Source namea	$\alpha$(2000)	$\delta$(2000)	$F_{\rm Jun}^b$	$F_{\rm Dec}^b$	Commentsc
1	XR J004205.9+411329	00:42:05.9	41:13:29	<2.7	12.6 $\pm$ 2.6	PFJ#3
2	XR J004212.3+411800	00:42:12.3	41:18:00	1.8 $\pm$ 1.3d	17.5 $\pm$ 4.1	TF#14, PFJ#8
3	XMMU J004234.1+411808	00:42:34.1	41:18:08	27.9 $\pm$ 1.7	<6.4	XMM  X-ray nova (Sect. 3.1)
4	CXO J004242.0+411608	00:42:42.2	41:16:09	58.1 $\pm$ 2.1	45.6 $\pm$ 3.0	Chandra  transient (G2000 and Sect. 3.2)
5	RX J0042.7+4116	00:42:47.2	41:16:28	9.5 $\pm$ 1.3	94.0 $\pm$ 3.6	TF#59, PFJ#50, Su97#198, Su01#195
6	XMMU J004247.5+411158	00:42:47.5	41:11:58	4.0 $\pm$ 1.3	<3.0
7	CXOU J004257.1+411843	00:42:57.1	41:18:43	<2.0	6.3 $\pm$ 2.2	seen on Oct. 13, 1999 with Chandra
8	RX J0043.3+4120	00:43:18.9	41:20:19	23.4 $\pm$ 1.3e	2.5 $\pm$ 1.4e	SSSf, TF#87, Su97#235, Su01#235
9	XMMU J004319.4+411759	00:43:19.4	41:17:59	43.6 $\pm$ 1.7e,g	<5.1e,h	SSSf, 865-s pulsations (Sect. 3.3)
10	RX J0043.4+4118	00:43:27.9	41:18:35	8.6 $\pm$ 0.8e	6.5 $\pm$ 1.5e	SSSf, TF#89, PFJ#80, Su97#240, Su01#241, SNR

^a

The names for ROSAT and Einstein sources were adopted from Su01. If the source had not been found in Su01, we used the acronym XR followed with truncated XMM  coordinates. The sources seen first with Chandra  or XMM  are named according to the naming conventions for these missions.

^b

Source count rates for the XMM-Newton  observations in June and December of 2000 are given in units of 10^-3 counts/s (MOS1) and are corrected for vignetting. Quoted count rates are measured in the 0.3-10 keV energy range, except for supersoft sources (SSS), which are measured in the 0.3-1.5 keV band. In cases when sources are not detected, 2$\sigma$ upper limits are listed.

^c

References to the earlier observations: TF = Trinchieri & Fabbiano (1991); PFJ = Primini et al. (1993); Su97 = Supper et al. (1997); Su01 = Supper et al. (2001); G2000 = Garcia et al. (2000).

^d

Source near the chip edge; the detected count rate might be an underestimation of real count rate.

^e

Source count rates for 0.3-1.5 keV band.

^f

Supersoft source.

^g

With contribution from a nearby faint source.

^h

Upper limit is defined by the contribution of a nearby source.