We presented the results of an analysis of four XMM-Newton observations of the starburst galaxy IC342 taken over a four-year span from 2001 to 2005, with an emphasis on investigating the long-term flux and spectral variability of the X-ray point sources. We detected a total of 61 X-ray sources within 35' $\times$ 30' of the galaxy down to a luminosity of (1-2)$\times$1037 erg s-1 depending on the local background. We found that 39 of the 61 detected sources showed long-term variability, in which 26 of them were classified as X-ray transients. We also found 19 sources exhibiting variations in hardness ratios or undergoing spectral transitions among observations, and were identified as spectral variables. In particular, 8 of the identified X-ray transients showed spectral variability in addition to flux variability. The diverse patterns of variability observed is indicative of a population of X-ray binaries. We used X-ray colors, flux and spectral variability, and in some cases the optical or radio counterparts to classify the detected X-ray sources into several stellar populations. We identified a total of 11 foreground stars, 1 supersoft sources (SSS), 3 quasisoft sources (QSS), and 2 supernova remnants (SNR). The identified SSS/QSS are located near or on the spiral arms, associate with young stellar populations; the 2 SNR are very close to the starburst nucleus where current star formation activities are dominated. We also discovered a spectral change in the nuclear source of IC342 for the first time by a series of X-ray spectrum analysis.
IC342 is a nearby (1.8Mpc; see Buta & McCall 1999 for a review) late-type Sc/Scd galaxy in the Maffei Group which is one of the closest groups to our Galaxy. Its spiral arms are well developed and are almost face-on (i = 25 • ± 3 • ; Newton 1980). Its active star formation activities in the nuclear region has made it a popular target for infrared and submillimeter observations (e.g. Böker, van der Marel, & Vacca 1999, Meier & Turner 2005). These observations revealed that the physical properties of molecular clouds, the infrared luminosity, and the presence of a nuclear stellar cluster in IC342 are similar to the Milky Way in many ways (Schinnerer et al. 2003;Schulz et al. 2001). Its proximity and its orientation provide a unique possibility to study the Xray sources in IC342 from a very favorable viewing angle. One major drawback of X-ray study is that this galaxy is located near the galactic plane (b = 10.6 • ) and has a large absorption (N H = 3 × 10 21 cm -2 ) towards the Galactic center (Stark et al. 1992). This limits us to constrain local absorption and determine X-ray emission below 1 keV. On the other hand, IC342 has been studied in the X-ray with many missions since Einstein. With the advent of high angular resolution and high sensitivity instruments such as Chandra and XMM-Newton it is possible to study the X-ray source population in depth.
The first X-ray observation of IC342 from Einstein (Fabbiano & Trinchieri 1987) showed that the X-ray emission was dominated by three ultraluminous X-ray sources (ULXs) which had luminosities above 10 39 erg s -1 (named X-1, X-2, and X-3 based on designations of Okada et al. 1998). Subsequent ROSAT HRI observation in 1991 revealed seven additional point sources in the disk with a detection limit of L 0.1-2.5keV ≈ 2 × 10 37 erg s -1 (Bregman et al. 1993). Two ASCA observations taken in 1993 and 2000 on X-1 and X-2 (Okada et al. 1998;Kubota et al. 2002) showed that both X-1 and X-2 exhibited spectral and intensity transitions which resembled the spectral/intensity states of X-ray transients in our Galaxy. Kong (2003) and Bauer et al. (2003) presented the result from a 2001 XMM-Newton 10 ks observation and both reported detections of about 35 sources. The slope of the X-ray luminosity function was found to be ∼ 0.5 in both studies, suggesting the X-ray population of IC342 was consistent with other starburst galaxies and Galactic HMXBs. A detailed analysis on the Chandra HRC-I image observed in 2006 was presented in Mak et al. (2008) in which the nuclear X-ray source (X-3) was found to be spatially coincident with a nuclear stellar cluster which had been identified in earlier optical and infrared observations (e.g. Böker et al. 1999). Thus they proposed that the source was not an ULX and was instead associated with starburst activities, together with the possibility of an embedded low background light curves in the > 10 keV band. High background level with count rate over 0.2 cps for MOS and 1.0 cps for EPIC-PN were excluded and the good time intervals (GTI) were obtained for each observation which ranged from 5.6 to 23.6 ks. The resulting GTI of the two observations in 2004, with exposure time over ≥ 20 ks for each MOS camera, roughly double those of the 2001 and 2005 data. Therefore the sensitivity of the 2004 observations were expected to be higher and fainter sources could be detected. We selected only good event patterns for imaging: ≤ 12 for MOS and ≤ 4 for PN, and restricted our analysis in the energy range 0.2-12 keV. These filtered event files were then used for data analysis using HEAsoft v6.4 and XSPEC v12.4.
Source detection using maximum likelihood fitting was done simultaneously on each of the EPIC-PN, MOS1, and MOS2 image in the three energy bands: soft (S; 0.2-1.0 keV), medium (M; 1.0-2.0 keV), and hard (H; 2.0-12.0 keV), using the SAS task EDETECT_CHAIN. A likelihood value of 10 was used, corresponding to a significance level of 3.6σ. The outputs from EDE-TECT_CHAIN were inspected for spurious sources at the edges and boundaries between chips. To generate the final source list, we imposed two additional selection criteria for sources detected by EDETECT_CHAIN for a source to be confirmed as a true detection if it satisfied either one of the followings: First, the source had to be detected in at least one additional observation within a searching radius of 6 ′′ ; second, if the source was detected in only one observation, the S/N of the source had to be greater than 5 in that detection. These criteria were applied to all observations, except for the 2001 February observation in which the sources found by EDETECT_CHAIN were consistent with the source list in Kong (2003) and Bauer et al. (2003) and thus we just adopted the source list in Kong (2003) for this observation. About one third of the sources detected in the four observations were eliminated by these criteria.
Using this algorithm, we found 37, 43, 51, and 30 sources in the observation ta
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