Point Like Emission Components

The bright X-ray sources in galaxies discussed in this section include X-ray binaries (XRBs), supersoft sources (SSS), supernovae (SN) and supernova remnants (SNRs), nuclear sources, and a number of very luminous sources, so-called ultra-luminous X-ray sources (ULXs). ULXs are radiating at luminosities above the Eddington limit for a 1M0 object and their nature is not fully understood. But at least part of them could belong to the classes of objects mentioned above.

Often, the X-ray source population of a target galaxy is confused by foreground stars in the Milky Way and background objects (galaxies, galaxy clusters, and AGN) observed in the same field. For fainter X-ray sources, the fraction of these spurious objects rises in number and in Local Group galaxies it can dominate the detected sources. One therefore has to find ways to separate the different galactic and extra-galactic source classes. This can be achieved by identifying detected X-ray sources with the help of their position and/or time variability with counterparts that were categorized in surveys in other wavelength regimes (e.g., as SNR, globular clusters, SN). Because of the high source density in the fields, good X-ray positions are of utmost importance for this approach. Classification can also be based on X-ray properties like time variability, extent, hardness ratios, or energy spectra. Classification schemes including X-ray hardness and extent were successfully applied to ROSAT PSPC sources in the LMC (758 sources cataloged), SMC (517) (see Fig. 20.2) and

ROSATPSPC SURVEY ■ color image

? degree

ROSAT PSPC

1 degree

Fig. 20.2 ROSAT PSPC images of the LMC observed in the ROSAT all sky survey (left) and of the SMC (right [31]) in the energy bands 0.1-0.4, 0.5-0.9, and 0.9-2keV combined in red-green-blue color coded images

Fig. 20.3 Three color XMM-Newton EPIC images of the Andromeda galaxy M 31 (left: together with a zoom-in on the M 31 bulge area, [52]) and M 33 (upper right [55]). Red, green, and blue show, respectively, the 0.2-1.0, 1-2, and 2-12keV bands. The ellipses indicate the optical extent of the galaxies. The X-ray color/color plot (lower right) [55] using energy bands 0.2-0.5, 0.5-1, and 1-2 keV can be used - together with informations from other wavelength regimes - to classify the detected X-ray sources as SSS, SNRs, foreground stars, or hard sources (XRBs, plerions, background sources)

Fig. 20.3 Three color XMM-Newton EPIC images of the Andromeda galaxy M 31 (left: together with a zoom-in on the M 31 bulge area, [52]) and M 33 (upper right [55]). Red, green, and blue show, respectively, the 0.2-1.0, 1-2, and 2-12keV bands. The ellipses indicate the optical extent of the galaxies. The X-ray color/color plot (lower right) [55] using energy bands 0.2-0.5, 0.5-1, and 1-2 keV can be used - together with informations from other wavelength regimes - to classify the detected X-ray sources as SSS, SNRs, foreground stars, or hard sources (XRBs, plerions, background sources)

M 33 (184) fields [31-33]. Figure 20.3 shows X-ray color images of deep XMM-Newton EPIC surveys of the bright Local Group spirals M 31 and M 33 together with a hardness ratio diagram for M 33. More than 850 respectively 400 pointlike sources were detected in these surveys [52,55]. The X-ray sources have been classified using similar schemes as developed for the ROSAT observations. Many foreground stars, SSS, SNRs, and candidates can be identified by these procedures. However, even with the broader energy band of XMM-Newton and the better energy resolution, Crab-like SNRs, XRBs, and AGN do not separate in the hardness ratio diagrams. XRBs may be separated if they show strong time variability, or as sources correlating with an optically identified globular cluster in the galaxy. A source may be classified as AGN or Crab-like SNR if additional optical or radio indicators are available. The results of the identifications will be discussed later and compared to results for source populations in other galaxies.

20.3.1 X-Ray Binaries

In the absence of an AGN or a large amount of hot gas, XRBs contribute the major fraction to the host galaxy's X-ray luminosity, as is the case for the Milky Way or the Andromeda galaxy (see, e.g., a recent review [22]). XRBs are subdivided into low-mass and high-mass systems depending on the mass of the donor star (LMXB, Mopt & 1M0; HMXB, Mopt > 8M0, respectively) and have very different evolutionary time-scales. The lifetime of HMXBs is limited by the nuclear time-scale of the massive donor star to less than 106-107 yrs, i.e., it is comparable to the duration of a star formation event. The onset of the X-ray active phase of a LMXB after the formation of the compact object is determined by the nuclear evolution time-scale of the donor star and/or the binary orbit decay time-scale to about 109-1010 yrs. The active phase then may last for a similar time. Therefore, HMXBs radiate during or shortly after a star formation event while LMXB lifetimes are comparable to that of the host galaxy. The population of LMXBs may be proportional to the total stellar mass of a galaxy (see e.g. [29]).

Pointed archival ROSAT PSPC and HRI observations covered large parts of the LMC. The surveys [32,62] revealed 17 XRBs and candidates. Besides the LMXB LMC X-2 and one LMXB candidate in the LMC bar the other sources are HMXBs or candidates. Ten have been identified as Be-type X-ray binaries, one is a wind fed HMXB, one is fed by Roche lobe overflow, and two are black hole HMXBs [47]. The observed ratio of LMXBs and HMXBs of about 1/7 is as expected from the low LMC mass and moderate star formation rate [65].

X-ray surveys of the SMC with ASCA, BeppoSAX, ROSAT, RXTE, and XMM-Newton revealed a large number of XRBs (67). Most of them are transient and were discovered through the detection of X-ray pulsations, indicating the spin period of a neutron star. Optical follow-up observations revealed in all cases Be stars as optical companions in HMXBs (with the only secure exception of the supergiant system SMC X-1), making the SMC very different to the Milky Way. These Betype XRB systems have a quiescent luminosity of &1034 erg s^1 and are brighter by about a factor of 100 during outbursts, which generally occur close to the time of periastron passage of the neutron star. They sometimes in addition show giant outbursts (Lx > 1037 erg s^1) lasting for several weeks or even months, which are connected to Be star activity. Only during such outbursts pulsations from Be-type XRBs in the MCs were detectable with satellites before XMM-Newton [9,34-36].

M 33 is an Sc spiral in the Local Group at a distance of 795 kpc, about 15 times the distance of the MCs that makes the detection and identification of XRBs much more difficult. In M 33 only two candidates for XRBs could be identified up to now. One is the brightest source in the Local Group, M 33 X-8 with an X-ray luminosity of 1.5 x 1039 erg s^1 in the 0.5-10 keV band, which - based on a Chandra analysis of its X-ray spectrum, time variability, and correlation with a radio source - most likely is an ultraluminous black hole XRB system at the M 33 nucleus similar to the galactic microquasar GRS 1915+105 [15]. Even with Chandra positioning it is not clear yet if the system correlates with an optical star bright enough for an high mass optical counterpart. The second system is the HMXB M 33 X-7 with 3.45 d orbital period, the first eclipsing XRB outside the Milky Way and the MCs. In 1989 it was suggested as an eclipsing XRB with 1.7 d orbital period based on Einstein observations. Adding ROSAT observations revealed that the orbital period is twice as long and allowed to refine the shape of the eclipsing light curve and binary ephemeris. The position of X-7 was covered in several observations of the XMM-Newton survey of M 33. The collecting power of the instruments not only led to a much clearer orbital light curve (resolution of 1000 s) but also allowed detailed modeling of the source spectrum with a disk blackbody or bremsstrahlung spectrum. With the improved X-ray position of the source (including Chandra observations) a B0I to O7I star could be identified as optical counterpart that showed an optical heating light curve of a HMXB with the M 33 X-7 period. The X-ray spectrum together with the lack of an X-ray pulsation period proposed M 33 X-7 as the first eclipsing black hole XRB (see Fig. 20.4 and detailed references in [56]). Further XRB candidates are expected from a more detailed analysis of the time variability of sources in the XMM-Newton raster observations of M 33.

M33 X7 X-ray light curve

•30 XMM-Newton »102640101

>

•33:00

• i*" r T ¿J v -

30

■ v>

• 32:00

■30

.

•30:31:00

1:33:40 36

32

2?

Fig. 20.4 Light curve of the XRB M 33 X-7 in the 0.5-3.0keV band and in optical V and B-V folded over the 3.45 d orbital period (left). XMM-Newton EPIC images during the on-state (above) and eclipse (below) demonstrate the strong intensity change of the XRB compared to a constant nearby source (right) [56]

The Andromeda galaxy M 31, a massive SA(s)b galaxy in the Local Group similar to the Milky Way, is located at about the same distance as M 33. As in the Milky Way, a significant part of the luminous X-ray sources are found in globular clusters. Most of the globular cluster sources in the Galaxy show bursts and therefore are low mass neutron star XRBs (see 3.4). There have been extensive surveys for globular clusters in M 31 in the optical and infrared bands. The XMM-Newton catalog of M 31 sources lists 37 globular cluster sources and candidates covering an absorbed luminosity range from 4.5 x 1035 to 2.4 x 1038 erg s^1 in the 0.2-4.5 keV band well in the range allowed for neutron star LMXBs. One of the sources shows intensity dips every 2.78 h that indicate the orbital period of a neutron star LMXB [74]. A search for X-ray bursts in the archival XMM-Newton observations at globular cluster X-ray source positions detected bursts from two sources that can be interpreted as type I radius expansion bursts from sources in M 31 radiating at maximum with a 1 keV black body spectrum with 3.8 x 1038 erg s^1. The bursts identify the sources as neutron star LMXBs in M 31. These type I X-ray bursts are the first detected outside the Milky Way and show that with the help of XMM-Newton X-ray bursts can be used to classify neutron star LMXBs in Local Group galaxies (Fig. 20.5 [53]). In addition to these LMXB sources, detailed work on individual M 31 sources using XMM-Newton and/or Chandra Observatory data, has identified four black hole XRBs, three neutron star LMXBs, an XRB pulsar, and several transients (for references see [52]). While in general transient behavior of bright X-ray sources indicates an XRB nature, with such a selection one may pick up also some highly variable background objects. The XRBs in M 31 selected in this way cover an absorbed luminosity range from 8.4 x 1035 to 2.8 x 1038 erg s^1 in the 0.2-4.5 keV band. Up to now no HMXBs have been identified in M 31. However, several emission line objects correlating with XMM-Newton catalog sources may be good candidates for Be-type HMXBs.

M31 burst source [PFH2005] 253 XMM-Newton, EPIC 0.2 - 7.0 keV

M31 burst source [PFH2005] 253 XMM-Newton, EPIC 0.2 - 7.0 keV

4.36x10* 4.38x10 4.4x10*

Fig. 20.5 XMM-Newton EPIC light curves of source [PFH2005] 253 on January 6/7, 2002 for the individual cameras integrated over 120 s (left) and for the time of the burst integrated over 10 s for all cameras added (right) [53]

4.36x10* 4.38x10 4.4x10*

Fig. 20.5 XMM-Newton EPIC light curves of source [PFH2005] 253 on January 6/7, 2002 for the individual cameras integrated over 120 s (left) and for the time of the burst integrated over 10 s for all cameras added (right) [53]

Several late type galaxies outside the Local Group have been searched for X-ray point-like sources with ROSAT, XMM-Newton, and Chandra. This resulted in source catalogues of up to 100 sources in galaxies with distances of 10Mpc and more (e.g. M 81, M 83, and M 101). However, at these larger distances identifying XRBs gets more and more difficult. [54] report the detection of an eclipsing XRB in the starburst galaxy NGC 253 (distance 2.58 Mpc) based on two changes from low to high state in Chandra Observatory and XMM-Newton observations separated by about a year. They use additional XMM-Newton, Chandra, ROSAT, and Einstein observations to further constrain the orbital parameters and determine an X-ray luminosity during the high state, which is close to the Eddington limit for a 1.4 M© neutron star. In the other galaxies XRB candidates may be identified from globular cluster correlations or their location in the spiral arms together with spectral and time variability arguments. Often, X-ray luminosity functions (XLF) are the only tools to get a hint on the XRB population.

In galaxies with more intense star formation, observations show flatter XLF slopes, which indicate the presence of very luminous sources. The best example is the galaxy merger system NGC 4038/39 (The Antennae, distance 19 Mpc), where nine ULXs were discovered with Chandra Observatory [23]. [30] suggest that the XLFs of star forming galaxies scale with the star formation rate (SFR) and propose HMXBs as SFR indicators in galaxies. They demonstrate that for starburst galaxies the total X-ray luminosity of a galaxy is linearly correlated to the SFR and suggest a universal luminosity function for HMXBs, a proposal that seems to contradict the results of the XLF slopes determined for a minisurvey of starburst galaxies with Chandra Observatory and the prediction of theoretical models. Independent of these discrepancies, comparing XLFs determined for different galaxies with XLFs determined via evolution models of the parent galaxies will help to understand the nature of the X-ray sources and the parent galaxy stellar population (see [22]).

In E and S0 galaxies, XRBs could not be detected directly with pre-Chandra Observatory telescopes, because of the distance of these galaxies and the limited angular resolution of the telescopes. Already [72] predicted the presence of XRBs in E and S0 galaxies based on an analogy with the bulge of M 31. The detection of hard spectral components in early type galaxies supported this prediction. Chandra Observatory images now resolve many point-like sources in E and S0 galaxies, which often correlate with globular clusters and therefore can be identified as LMXBs (see e.g. [1,61]).

0 0

Post a comment