Volume 553, May 2013
|Number of page(s)||19|
|Published online||18 April 2013|
Using the criteria in Sect. 5.1, we classified sources Nos. 21, 143, 182, and 174 as foreground stars according to their optical and infrared properties (Figs. 4, 6), and their optical-to-X-ray ratios as a function of the hardness ratios (Fig. 5). Although the hardness ratio criterion HR3 ≲ −0.4 of source No. 174 is not fulfilled, we classified this source as a foreground star because of the large uncertainty of the hardness ratio (see Fig. 5).
has optical and infrared counterparts and log 10(fx/fopt) < −1, but violates the hardness ratio HR2 criterion (see Fig. 5). The optical counterpart is bright (mB,No. 24 = 14.1), and the B − R and J − K colours are consistent with those of foreground stars (Figs. 4 and 6), thus this source most likely belongs to the Milky Way. It has been detected in observations 1 and 2 in all three EPIC cameras. In all cases, source No. 24 shows hard HR2 (Fig. 5, left panel), inconsistent with the expected X-ray spectra of foreground stars. The properties of the optical companion and the hard X-ray spectra may indicate a cataclysmic-variable nature for this source. This class of sources can show short- and long-term time variability, therefore we produced the X-ray lightcurve in the energy range 0.5–4.5 keV to give more evidence for this identification. However, the resulting X-ray lightcurve (with a bin-time of 2000 s) shows neither short- nor long-term variability.
coincide with ROSAT sources H2, H31, H34 and H36. They were classified by Immler et al. (1999) as foreground stars based on positional coincidences with optical sources of the APM Northern Sky Catalogue (Irwin et al. 1994). We found possible optical counterparts in the USNO-B1 catalogue for source No. 164 (USNO−B1 0601−0299090) and source No. 12 (USNO−B1 0602−0301227). However, their X-ray-to-optical flux ratios (Eq. (2)) are log (fx/fopt) ≈ 0.10 and 0.11 respectively (fopt of both sources was calculated using visual magnitude), hence the foreground star classification for these sources is ruled out. The refined positions of sources Nos. 137 and 189 obtained with XMM-Newton, allowed us to exclude their association with the optical counterparts proposed by Immler et al. (1999). Source No. 189 can be associated with a new optical counterpart, USNO−B1 0600−0300832, which is ~3 orders of magnitude fainter than the previous one (USNO−B1 0600−0300831). However, the new X-ray-to-optical flux ratio is log (fx/fopt) ≈ 0.68 (fopt was calculated using visual magnitude), too high for a foreground star (see Sect. A.3). Hardness ratios of sources Nos. 164 and 137 are consistent with a powerlaw or disk-blackbody spectrum. Therefore, the spectra of these sources are too hard to be classified as foreground stars.
We found radio counterparts of the sources Nos. 20, 37, and 189 and classified them as AGN candidates for the first time.
is located outside the D25 ellipse (D25 = 11.5′; Tully 1988) at ~0.41° from the centre of the galaxy. It coincides with the radio source NVSS J133618−301459. We detected this source with XMM-Newton in observations 2 and 3 in the outer disc of M 83. Source No. 20 shows a significant long-term variability (Table 3), and the hardness ratios are roughly consistent with a spectrum described by an APEC model with a temperature of kTapec ~ 0.5 keV (HR2 = −0.2 ± 0.1; HR3 = −0.81 ± 0.11). Therefore, source No. 20 can be classified as an AGN candidate (with a soft spectral component) or an SNR candidate. The distance of this source from the nuclear region of M 83 of ~32 kpc rather indicates that source No. 20 does not belong to the galaxy, therefore it is more likely an AGN than an SNR candidate.
coincide with the radio sources NVSS J133630−301651 and NVSS J133805−295748, respectively. Source No. 189 was previously classified as a foreground star by Immler et al. (1999) (see Sect. A.2). We detected these sources with XMM-Newton in observation 3. Their hardness ratios are consistent with a spectrum described with a powerlaw or disc-blackbody model (No. 37: HR2 = 0.62 ± 0.12; HR3 = −0.37 ± 0.13; No. 189: HR2 = 0.07 ± 0.10; HR3 = −0.25 ± 0.12). Therefore, they can be classified as AGN candidates.
coincides with the Chandra source [SW03] 33, classified as an accreting X-ray pulsar, with a hard spectrum (Γ ≈ 1.7) and a spin period of 174.9 s.
We observed source No. 81 in all XMM-Newton observations. The hardness ratios are consistent with an absorbed powerlaw spectrum, and this source shows a significant long-term X-ray variability (Vf = 2.5, S = 3.0, Table 3). We applied a Fourier transform periodicity search and a Z analysis (Sect. 4.1), which did not reveal any significant periodicity. We calculated the upper-limit on the pulsed fraction (defined as the semi-amplitude of the sinusoidal modulation divided by the mean count rate) using the procedure described by Vaughan et al. (1994). The upper limit on the pulsed fraction obtained from the combined PN and MOS events of observation 1 is 16% at the 99% confidence level. This upper limit is marginally compatible with the pulsed fraction of (50 ± 15)% of source [SW03] 33.
corresponds to the X-ray source [SW03] 113. Using the spectral properties and the 201.5 s periodicity detected with Chandra, SW03 classified source [SW03] 113 as an XRB in a soft state.
We observed source No. 120 with XMM-Newton in observations 1 and 3. The hardness ratios of this source are consistent with an absorbed powerlaw spectrum with NH ~ 5 × 1021 cm-2 and Γ ~ 1.5. Similarly to source No. 81, a Fourier transform periodicity search and a analysis did not reveal any significant periodicity. At the 99% confidence level, the upper limit on the pulsed-fraction of source No. 120 derived from the MOS events is 49%. This upper limit is compatible with the (50 ± 19)% pulsed fraction of [SW03] 113.
The position of this source corresponds to the position of the ROSAT source H15 (Immler et al. 1999) and the Chandra source [SW03] 27. The Chandra spectrum shows emission lines, suggesting the possibility of emission from optically thin thermal plasma, and has been fitted by SW03 with an absorbed powerlaw with Γ ~ 1.4 and NH ~ 7 × 1020 cm-2. SW03 classified this source as a young SNR candidate. Another possible explanation for the hard powerlaw spectrum with superposition of emission lines of [SW03] 27 is that the source is an XRB surrounded by a photoionised nebula (SW03). However, XRBs showing these spectral properties usually have a higher absorbing column density than that of [SW03] 27 (see e.g. Sako et al. 1999).
The XMM-Newton hardness ratios of source No. 79 below 2 keV are consistent with an APEC model with temperature kTapec ≳ 1.5 keV, while at higher energies the hardness ratios are consistent with a powerlaw with photon index ~2. The spectral shape of source No. 79 derived from XMM-Newton hardness-ratio diagrams agrees with the X-ray spectrum of [SW03] 27 presented by SW03 (see Fig. 6 in SW03) and can be interpreted as an SNR exhibiting both a thin-thermal emission (below ~2 keV) and an additional hard component, which dominates at energies above ~2 keV. Also, source No. 79 does not show any significant long-term variability (see Table 3).
Hardness-ratio diagram of source No. 91 observed with XMM-Newton. Thick lines are different spectral models as function of the NH, thin lines are different column densities NH (from left to right: 1020, 1021, 1022 cm-2) as a function of the spectral parameters.
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coincides with Einstein source 3 (Trinchieri et al. 1985) and Chandra source [SW03] 55 classified by Di Stefano & Kong (2003) as an SSS candidate (source M 83-50 in Di Stefano & Kong 2003). Di Stefano & Kong (2003) fitted the X-ray spectrum of M 83-50 with an absorbed blackbody with a temperature of eV, a column density of cm-2, and a luminosity of Lx = 2.8 × 1037 erg s-1 (0.3−7 keV, d = 4.5 Mpc).
We detected source No. 91 in observation 1, where the hardness ratios are consistent with a blackbody spectrum (with column density in the range ≈1020−1021 cm-2) and marginally compatible with an APEC spectrum with temperature in the range ≈0.2–0.5 keV (Fig. A.1). Source No. 91 has a 0.2–4.5 keV luminosity of Lx = (2.2 ± 0.2) × 1037 erg s-1 and does not show any significant variability compared to the Chandra observation.
EPIC counts spectra, together with residuals in units of standard deviations for source No. 133 detected in the observation 2. Left panel shows the fit with an absorbed cool disc-blackbody plus hard powerlaw, while the right panel shows the fit with an absorbed cool blackbody plus a warm disc-blackbody (see Table A.1).
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Best-fitting parameters of the X-ray spectra of source No. 133 (errors at 90% confidence level).
Two ULXs have been discovered in M 83: H2 (Trinchieri et al. 1985), and a transient ULX discovered with Chandra on 23 December 2010 with a luminosity of Lx ~ 4 × 1039 erg s-1 (0.3–10 keV) by Soria et al. (2010), and classified as an accretion-powered black hole with mass MBH ≈ 40−100 M⊙ (Soria et al. 2012). This ULX has not been detected in the XMM-Newton data. Soria et al. (2012) measured an upper limit to the X-ray luminosity of ~1037 erg s-1 (0.3–10 keV) from the three XMM-Newton observations.
We observed the ULX as source No. 133 in all XMM-Newton observations. Ehle et al. (1998) and Immler et al. (1999) found a faint extended optical source within the error circle of the ROSAT source position. Roberts et al. (2008) used HST images in three Advanced Camera for Survey (ACS) filters to find the counterparts to six ULXs in different galaxies. For the ULX in M 83, they compared the optical position with the X-ray position from a Chandra High Resolution Camera for Imaging (HRC-I) observation. They detected a counterpart to the ULX with magnitudes B = 25.66 ± 0.13, V = 25.36 ± 0.17. They also noticed that the ULX is located at ~5′′ from the centre of a background galaxy, and although the latter is outside the error circle, Roberts et al. (2008) did not completely rule out a possible association between the ULX and the background galaxy.
Stobbart et al. (2006) reported the XMM-Newton spectral analysis of source No. 133 during observation 1. They found that the X-ray spectrum is well fitted with a cool disc-blackbody (kTin ~ 0.2 keV) plus a powerlaw (Γ ~ 2.5), or with a cool blackbody (kTbb ~ 0.2 keV) plus a warm disc-blackbody (kTin ~ 1.1 keV). The first spectral model is the standard IMBH model, where the low disc temperature is due to a black hole with mass of ~1000 M⊙, while the origin of the powerlaw component is still not clear (see Roberts et al. 2005). Instead, the spectral parameters obtained with the second spectral model suggest that No. 133 is a stellar-mass black hole accreting close to the Eddington limit. In this model, the cool blackbody component represents the optically thick wind from the stellar-mass black-hole accreting at or above the Eddington limit, while the high temperature of the disc follows the standard trend Lx ∝ T4 shown by the Galactic stellar-mass black-hole binaries.
We analysed all XMM-Newton observations of the ULX No. 133 and fitted the PN, MOS1 and MOS2 spectra simultaneously with a model assuming an IMBH (phabs*[diskbb + powerlaw] in XSPEC), and a model assuming a stellar-mass BH (phabs*[bbody + diskbb]). We used two absorption components: the Galactic absorption column density (NH = 3.69 × 1020 cm-2) and the absorption within M 83 plus the intrinsic column density of the ULX. In all fits we obtained a good fit with both spectral models with the resulting spectral parameters in agreement with those obtained by Stobbart et al. (2006) from observation 1. However, the spectral parameters in observation 3 are only poorly constrained due to the poor statistics (only MOS1 and MOS2 data were available for this observation). Therefore, we fitted the spectrum of observation 3 with a single component model and found that an absorbed powerlaw can adequately fit the data (Fig. A.2, Table A.1).
Best-fitting parameters of sources No. 16, 61, 103, 126, 153.
© ESO, 2013
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