Annu. Rev. Astron. Astrophys. 2006. 44:
323-366 Copyright © 2006 by Annual Reviews. All rights reserved |
At variance with most previous reviews of X-ray observations of galaxies, which tend to concentrate first on nearby well-studied spiral and irregular galaxies, I will begin by discussing the X-ray populations of old stellar systems: E and S0 galaxies. By comparison with spirals, these galaxies present fairly homogeneous stellar populations, and therefore one can assume that their XRB populations are also more uniform, providing a "cleaner" baseline for population studies.
I begin this Section with a historical note (Section 3.1), followed by a summary of the detection of ubiquitous discrete X-ray source populations in spheroids and their spectral and variability properties (Section 3.2), which point to LMXB populations. I then address the characterization of these populations by means of XLFs (Section 3.3), and give an overview of the association of these sources with globular clusters (GCs) and of the properties of GC sources in comparison with field sources (Section 3.4). Finally, I summarize the discussion generated by these results for the dependence of LMXB formation in GCs on the metallicity and dynamical properties of the cluster (Section 3.5), and address the current debate on the formation and evolution of the entire LMXB populations, including both formation in GCs and evolution of field native binary systems (Section 3.6). I conclude this Section with a short discussion of results that may suggest evolution of the X-ray populations of some E and S0 galaxies (Section 3.7).
3.1. Low-Mass X-Ray Binaries in Early-Type Galaxies: There They Are - Past and Present
In the 1989 review (Fabbiano 1989) I argued that LMXBs should be present in E and S0s and might even dominate the X-ray emission of some of these galaxies. This was a controversial issue at the time, because LMXBs could not be detected individually, and their presence was supported only by statistical considerations (e.g., Trinchieri & Fabbiano 1985). Although the spectral signature of LMXBs was eventually detected (Kim, Fabbiano & Trinchieri 1992; Fabbiano, Kim & Trinchieri 1994; Matsushita et al. 1994), uncontroversial detection of samples of these sources in all early-type galaxies has become possible only with the subarcsecond resolution of Chandra (such a population was first reported in NGC 4697, where 80 sources were detected by Sarazin, Irwin & Bregman 2000; see Figure 2).
A statistical analysis of a large sample of early-type galaxies observed with Chandra is still to come, but the results so far confirm the early conclusion (see Fabbiano 1989; Kim, Fabbiano & Trinchieri 1992; Eskridge, Fabbiano & Kim 1995a, b) that LMXBs account for a very large fraction of the X-ray emission of some early-type galaxies (those formerly known as "X-ray faint," i.e., devoid of large hot gaseous halos): for example, in NGC 4697 (Sarazin, Irwin & Bregman 2000) and NGC 1316 (Kim & Fabbiano 2003) the fraction of detected counts attributable to the hot ISM is ~ 23% and ~ 50%, respectively. In both cases, given the harder spectrum of LMXBs, these sources dominate the integrated luminosity in the 0.3-8 keV range. In NGC 1316 the integrated LMXB emission, including nondetected LMXBs with luminosities below threshold, could be as high as 4 × 1040 erg s-1. Sivakoff, Sarazin & Irwin (2003) reach similar conclusions for NGC 4365 and NGC 4382.
Although this review focuses on the X-ray binary populations, I cannot help remarking that the Chandra results demonstrate unequivocally that ignoring the contribution of the hidden emission of LMXBs was a source of error in past analyses. In particular, estimates of galaxy dynamical mass were affected, as discussed in the 1989 review (see also Trinchieri, Fabbiano & Canizares 1986). NGC 1316 (Kim & Fabbiano 2003) provides a very clear illustration of this point. In this galaxy the LMXBs are distributed like the optical light and dominate the emission at large radii. Instead, the hot ISM follows a steeper profile (see Figure 3, left), with temperature possibly decreasing at larger radii, suggestive of galactic winds. Use of lower-resolution Einstein data, with the assumption that the entire detected emission originated from a hot ISM in hydrostatic equilibrium, resulted in a large mass estimate for this galaxy (2.0 × 1012 M; Forman, Jones & Tucker 1985). This result is not sustained by the present data; because the gaseous halo is less extended than assumed in the Einstein paper, its temperature is lower (because the Einstein spectrum was clearly contaminated by the harder LMXB emission), and the halo may not be in hydrostatic equilibrium.
Figure 3. Left: Radial profile of the low-mass X-ray binary distribution (red dots) in NGC 1316 compared with the profiles of the optical light (green) and diffuse hot interstellar medium emission (black data points and best-fit model); Right: X-ray luminosity function (XLF) before (squares are the binned data and the dotted line gives the unbinned XLF) and after completeness correction (filled dots; the solid line gives the best-fit power-law model, binned to resemble the data). These figures are figures 6 and 10 of Kim & Fabbiano (2003). |
Estimates of the metal abundance of the hot ISM also must be reconsidered. In NGC 1316, spectral analysis of the integrated X-ray emission obtained with ASCA suggested extremely subsolar values (0.1 solar, Iyomoto et al. 1998). This extremely low metallicity is typical of ASCA results for E and S0 galaxies, and cannot be reconciled with the predictions of stellar evolution (e.g., Arimoto et al. 1997). Spectral analysis of the NGC 1316 Chandra data (Kim & Fabbiano 2003), after subtraction of the detected LMXBs, and taking into account the unresolved LMXB component, allows larger metallicities of the hot ISM (up to 1.3 Z), more in keeping with the expected values.
3.2. Source Spectra and Variability
Populations of several tens to hundreds of sources have been detected in E and S0 galaxies with Chandra (see review by Fabbiano & White 2006), and their number is growing as more galaxies are observed and the depth of the observations increase. With the exception of a few SSSs reported in some galaxies (see Irwin, Athey & Bregman 2003; Humphrey & Buote 2004), the X-ray colors and spectra of these sources are consistent with those expected of LMXBs, and consistent with those of the LMXBs of M31 (Blanton, Sarazin & Irwin 2001; Sarazin, Irwin & Bregman 2001; Finoguenov & Jones 2002; Irwin, Athey & Bregman 2003; Kim & Fabbiano 2003; Sivakoff, Sarazin & Irwin 2003; Humphrey & Buote 2004; Jordan et al. 2004; Kim & Fabbiano 2004; Randall, Sarazin & Irwin 2004; Trudolyubov & Priedhorsky 2004; David et al. 2005).
The most extensive spectral study to date is that of Irwin, Athey & Bregman (2003), who studied 15 nearby early-type galaxies observed with Chandra.They found that the average spectrum of sources fainter than 1039 erg s-1 is remarkably consistent from galaxy to galaxy, irrespective of the distance of the sources from the center of the galaxy. These spectra can be fitted with either power laws with photon index = 1.56 ± 0.02 (90%) or with bremsstrahlung emission with kT = 7.3 ± 0.3 keV. Sources with luminosities in the (1-2) × 1039 erg s-1 range instead have softer spectra, with power law ~ 2, consistent with the high-soft emission of black-hole binaries (with masses of up to 15 M expected for these luminosities, based on the Eddington limit). Within the errors, these results are consistent with those reported in other studies, although sources in different luminosity ranges are usually not studied separately in these works. Jordan et al. (2004) confirm the luminosity dependence of the average source spectrum in M87; their color-color diagram suggests a spectral softening for sources more luminous than 5 × 1038 erg s-1.
Relatively little is known about the time variability of these sources, because repeated monitoring of the same galaxy is not generally available. Type I X-ray bursts have been detected in some GC sources in M31, identifying these sources as neutron star LMXBs (Pietsch & Haberl 2005). Time variable sources and at least five transients (dimming by a factor of at least 10) have been detected in NGC 5128, with two Chandra observations (Kraft et al. 2001). Variable sources are also detected with two observations of NGC 1399, taken two years apart (Loewenstein, Angelini & Mushotzky 2005). Sivakoff, Sarazin & Irwin (2003) report time variability in a few sources in NGC 4365 and NGC 4382 within 40ks Chandra observations; Humphrey & Buote (2004) report two variable sources in NGC 1332. Sivakoff, Sarazin & Jordan (2005) report short-timescale X-ray flares from 3 out of 157 sources detected in NGC 4697; two of these flares occur in GC sources and are reminiscent of the superbursts found in Galactic neutron star binaries, while the third could originate from a black-hole binary. Maccarone (2005) suggests that these flares may be periodic events resulting from periastron accretion of eccentric binaries in dense globular clusters. The spectral characteristics of the point sources detected in E and S0 galaxies, their luminosities, and their variability, show that these sources are compact accreting X-ray binaries.
3.3. X-Ray Luminosity Functions of Low-Mass X-Ray
Binary Populations
The luminosities of individual sources range from the detection
threshold (typically a few 1037 erg s-1, depending
on the distance of the galaxy and the observing time) up to ~ 2 ×
1039 erg s-1. XLFs have been derived in most
Chandra studies of early-type galaxies, and modeled to
characterize their functional shape (power-law slopes, eventual breaks)
and normalization. In the following I first review the work on the shape
of the XLF and then discuss the drivers of the normalization (i.e., the
total LMXB content of a galaxy).
The high luminosity (LX > a few 1037 erg
s-1) shape of the XLF has been parameterized with models
consisting of power laws or broken power laws. The overall shape (in a
single power-law approximation in the range of ~ 7 ×
1037 to a few 1039 erg s-1) is fairly
steep, i.e., with a relative dearth of high luminosity sources, when
compared with the XLFs of star-forming galaxies
(Section 4.2; see also
Kilgard et al. 2002,
Colbert et al. 2004,
Fabbiano & White
2006),
but the details of these shapes and the related presence of breaks have
been a matter of some controversy.
Two breaks have been reported in the XLFs of E and S0 galaxies: the
first is a break at ~ 2-5 × 1038 erg s-1,
near the Eddington limit of an accreting neutron star, first reported by
Sarazin, Irwin &
Bregman (2000)
in NGC 4697, which may be related to the transition in
the XLF between neutron stars and black-hole binaries
(Blanton, Sarazin &
Irwin 2001
in NGC 1553;
Finoguenov & Jones
2002
in M84;
Kundu, Maccarone &
Zepf 2002
in NGC 4472;
Jordan et al. 2004
in M87;
Gilfanov 2004,
Kim & Fabbiano
2004,
and also
Di Stefano et al. 2003
for the XLF of the Sa Sombrero galaxy, NGC 4594); the second is a high
luminosity break at ~ 1039 erg s-1, first reported
by in NGC 720 by Jeltema et al.
(2003;
see also
Sivakoff et al. 2003,
Jordan et al. 2004).
Both breaks are somewhat controversial, because the interpretation of
the observed XLFs is crucially dependent on a proper completeness
correction (see Section 2.2).
Kim & Fabbiano
(2003,
2004)
show that incompleteness effects are particularly relevant for the
detection of the Eddington break, because the typical exposure times of
the data and the distances of the target galaxies in most cases conspire
to produce a spurious break at just this value (see
Figure 3, right, for an
example). Interestingly, no break was required in the case of NGC 5128
(Kraft et al. 2001),
where the proximity of this galaxy rules out incompleteness near the
neutron star Eddington luminosity. An apparent Eddington break that
disappears a fter correction for completeness is also found by
Humphrey & Buote
(2004)
for the XLF of NGC 1332. Similarly, Eddington breaks are absent in
NGC 4365 and NGC 4382
(Sivakoff, Sarazin
& Irwin 2003),
whereas a high luminosity cut-off at 0.9-3.1 × 1039 erg
s-1 could be allowed; these researchers also consider the
effect of incompleteness in their results.
Other recent papers, however, do not discuss, or do not apply,
completeness corrections to the XLFs, so their conclusions on the
presence of Eddington breaks need to be confirmed.
Randall, Sarazin &
Irwin (2004)
report a break at ~ 5 × 1038 erg s-1 in
NGC 4649, with large uncertainties, but do not
discuss the derivation of the XLF.
Jordan et al. (2004)
derive and fit the XLF of M87, and compare it with their own fit of those of NGC 4697 and M49 (NGC 4472), using the data from
Sarazin, Irwin &
Bregman (2001)
and
Kundu, Maccarone &
Zepf (2002)
respectively. However, completeness corrections are not applied,
although the low-luminosity data are not fitted.
Jordan et al. (2004)
report breaks at 2-3 × 1038 erg s-1 in all
cases, or a good fit with a single power law truncated at
1039 erg s-1.
Note that these results are not consistent with those of
Kim & Fabbiano
(2004)
where the corrected XLFs of NGC 4647 and NGC 4472 are well fitted with
single unbroken power laws.
Kim & Fabbiano
(2004)
derive corrected luminosity functions for a sample of 14 E and S0
galaxies, including some with previously reported breaks, and find that
all the individual corrected XLFs are well fitted with single power laws
with similar
differential slopes (-1.8 to -2.2; cumulative slopes are -0.8 to -1.2)
in the observed luminosity range. None of these fits require an
Eddington break. However, a break may be hidden by the po
or statistics in each case. The statistical consistency of the
individual power laws justifies coadding the data to obtain a high
significance composite XLF of early-type galaxies
(Figure 4 left). This composite XLF is
not consistent with a single power law, suggesting a break at (5 ±
1.6) × 1038 erg s-1. The best-fit
differential slope is -1.8 ± 0.2 in the few 1037 to 5
× 1038 erg s-1 luminosity range for the
coadded XLF; at higher luminosity, above the break, the differential
slope is steeper (-2.8 ± 0.6). These results are confirmed by the
independent work of
Gilfanov (2004),
who analyzes four early-type galaxies, included in the
Kim & Fabbiano
(2004)
sample (Figure 4 right); however,
Gilfanov's differential slope for the high luminosity portion of the XLF
is somewhat steeper (-3.9 to
-7.3). Both the Kim & Fabbiano and Gilfanov analyses are consistent
with a cut-off of the XLF of LMXBs at a few 1039 erg
s-1. A more recent paper
(Xu et al. 2005)
is in agreement with the above conclusions, reporting a consistent
Eddington break in the corrected XLF of NGC 4552; on the basis of a
simulation, this paper concludes that
the break may or may not be detected in any individual galaxy XLF,
given the relatively small number of sources present in each case.
Figure 4. Left: Cumulative X-ray luminosity
function (XLF) of 14 E and S0 galaxies (figure 7 of
Kim & Fabbiano
2004),
with the single power-law best fit (dashed line), and the broken
power-law model (solid line); the M31 and Milky Way low-mass
X-ray binary (LMXB) XLFs are sketched in the left lower corner. Right:
Cumulative LMXB XLFs from figure 5 of
Gilfanov (2004).
Note the similarity of the XLFs and the break at ~ 5 ×
1038 erg s-1 in the E/SO XLF.
The (5 ± 1.6) × 1038 erg s-1 break is
at somewhat higher luminosity than would be expected from the Eddington
luminosity of normal neutron star binaries. It may be consistent with
the luminosity of the most massive neutron stars
(3.2 ± 1 M;
see
Ivanova & Kalogera
2006),
He-enriched neutron star binaries (1.9 ± 0.6
M; see
Ivanova & Kalogera
2006),
or low-mass black-hole binaries. This break may be caused by the
presence of both neutron star and black-hole binary populations in
early-type galaxies; it may
also be the consequence of a true high luminosity break in the XLF (e.g.,
Sivakoff, Sarazin &
Irwin 2003).
Whatever the cause, the shape of the XLF points to a dearth of very
luminous sources in E and S0 galaxies. Note that a high luminosity
cut-off is also present in the XLF of Galactic LMXBs
(Figure 1).
With the exception of NGC 5128 (Cen A), for which the XLF has been
measured down to ~ 2 × 1036 erg s-1
(Kraft et al. 2001,
Voss & Gilfanov
2006),
the available Chandra data does not allow the detection of LMXBs
in E and S0 galaxies with luminosities below the mid or high
1037 erg s-1 range. By including Cen A and the
LMXB (bulge) population of nearby spirals (Milky Way; see
Figure 1, M31, M81) in his study,
Gilfanov (2004)
suggests that the XLF flattens below 1037 erg
s-1. A recent reanalysis of the Cen A data confirms this
result
(Voss & Gilfanov
2006).
Direct deep observations of "normal" early-type galaxies are needed to
see if this suggestion is generally valid; a legacy Chandra
program will provide the necessary data for NGC 3379 and NGC 4278 by the
end of 2007. These future studies may show complex behavior in the
low-luminosity XLFs. For example, in M31 a radially dependent XLF break
has been reported in the bulge, which could be related to an
increasingly older population at the inner radii
(Kong et al. 2002).
Also, the GC XLF of M31 has a distinctive break at 2-5 ×
1037 erg s-1
(Kong et al. 2003,
Trudolyubov &
Priedhorsky 2004).
The discovery of a similar break in the E and S0 XLFs may argue for a
GC-LMXB connection in these galaxies. The "outburst peak luminosity -
orbital period" correlation
(King & Ritter
1998)
predicts a break at this luminosity if a large fraction of the
sources are short-period neutron star systems. This is intriguing,
because the formation of ultracom pact LMXBs is favored in Milky Way GCs
(Bildsten & Deloye
2004;
see also Section 3.6).
The normalization of the XLF is related to the number of LMXBs in a
given galaxy. X-ray-optical/near-IR correlations in bulge-dominated
spirals observed with Einstein
(Fabbiano, Gioia &
Trinchieri 1988;
Fabbiano & Shapley
2002)
had suggested a connection between the number of LMXBs and the overall
stellar content of a galaxy. This connection has now been demonstrated
to hold for the LMXB populations of E and S0 galaxies
(Gilfanov 2004;
Kim & Fabbiano
2004).
That stellar mass is the main regulator of the number of LMXBs in a
galaxy is not surprising, considering that LMXBs are long-lived systems,
but there may be other effects.
White, Sarazin &
Kulkarni (2002)
suggested a link with GC specific frequency (the number of GC per unit
light in a galaxy) using low-resolution ASCA data.
Kim & Fabbiano
(2004;
see also
Humphrey & Buote
2004
for general agreement with this correlation in the case of NGC 1332) find
a correlation between K-band luminosity (which is proportional to
stellar mass) and integrated LMXB luminosity, but also note that this
correlation has more scatter than would be expected in terms of
measurement errors. This scatter appears correlated with the GC-specific
frequency, confirming a role of GCs in LMXB evolution.
3.4. Association of Low-Mass X-Ray Binaries with
Globular Clusters: The Facts
In virtually all E and S0 galaxies with good coverage of GCs, both from
the ground and better from Hubble, a fraction of the LMXBs is
found in GCs (see earlier reviews by
Verbunt & Lewin
2006,
Fabbiano & White
2006).
Sarazin, Irwin &
Bregman (2000)
first reported this association in NGC 4697 and speculated on a leading
role of GCs in LMXB formation, revisiting the original suggestion of
Grindlay (1984)
for the evolution of bulge sources in the Milky Way. Below, I summarize
the observational results on the association of LMXBs with GCs from the
large body of papers available in the literature. In
Sections 3.5 and 3.6, I will
discuss the implications of these results.
3.4.1. Statistics. It appears that
in general ~ 4-5% of the GCs in a given galaxy are likely to be
associated with a LMXB (e.g., NGC 1399 -
Angelini, Loewenstein
& Mushotzky 2001;
NGC 4472 -
Kundu, Maccarone &
Zepf 2002;
NGC 1553, NGC 4365, NGC 4649, NGC 4697 -
Sarazin et al. 2003;
NGC 1339 -
Humphrey & Buote
2004;
M87 -
Jordan et al. 2004,
Kim et al. 2006).
Not surprisingly, as first noticed by
Maccarone, Kundu &
Zepf (2003),
the number of LMXBs associated with GCs varies, depending on the
GC-specific frequency of the galaxy, which is also a function of the
morphological type.
Sarazin et al. (2003)
point to this dependence on the galaxy Hubble type, with the fraction of
LMXBs associated with GCs increasing from spiral bulges (MW, M31) ~
10-20%, to S0s ~ 20% (NGC 1553,
Blanton, Sarazin &
Irwin 2001;
see also NGC 5128, where 30% of the LMXBs are associated with
GCs,
Minniti et al. 2004),
E ~ 50% (NGC 4697 -
Sarazin, Irwin &
Bregman 2000;
NGC 4365 -
Sivakoff, Sarazin &
Irwin 2003;
NGC 4649 -
Randall, Sarazin &
Irwin 2004;
see also NGC 4552, with 40% of sources in GCs -
Xu et al. 2005),
cD ~ 70% (in NGC 1399 -
Angelini, Loewenstein
& Mushotzky 2001;
see also M87, where 62% of the sources are associated with GCs -
Jordan et al. 2004).
3.4.2. Dependence on low-mass X-ray binary and
globular cluster luminosity. In NGC 1399
(Angelini, Loewenstein
& Mushotzky 2001)
the most luminous LMXBs are associated with GCs. No significant LMXB
luminosity dependence of the LMXB-GC association is instead seen in
NGC 4472
(Kundu, Maccarone &
Zepf 2002)
or in the four galaxies studied by
Sarazin et al. (2003);
if anything, a weak trend is present in the opposite sense. The reverse
is, however, consistently observed: more luminous GCs are more likely to
host a LMXB
(Angelini, Loewenstein
& Mushotzky 2001;
Kundu, Maccarone &
Zepf 2002;
Sarazin et al. 2003;
Minniti et al. 2004;
Xu et al. 2005;
Kim et al. 2006);
this trend is also observed in M31
(Trudolyubov &
Priedhorsky 2004).
Kundu, Maccarone &
Zepf (2002)
suggest that this effect is just a consequence of the larger number of
stars in optically luminous GCs.
Sarazin et al. (2003)
estimate that the probability per optical luminosity of LMXBs to be
found in a GC is ~ 2.0 × 10-7 LMXBs per
L,I for
LX 3 ×
1037 erg s-1.
This probability is consistent with past estimates based on the Milky
Way and is a few hundred times larger than the probability of LMXBs
occurring in the field per unit integrated stellar light in a galaxy,
in agreement with the conclusion that dynamical
interactions in GCs favor LMXB formation
(Clark 1975).
3.4.3. Dependence on globular cluster color.
The probability that a GC hosts a LMXB is not a function of
the GC luminosity alone. GC color is also an important variable, as
first reported by
Angelini, Loewenstein
& Mushotzky (2001)
in NGC 1399 and
Kundu, Maccarone &
Zepf (2002,
see also
Maccarone, Kundu &
Zepf 2003)
in NGC 4472, and confirmed by subsequent studies (e.g.,
Sarazin et al. 2003,
Jordan et al. 2004,
Minniti et al. 2004,
Xu et al. 2005,
Kim et al. 2006).
In particular, the GC populations in these galaxies tend to be bi-modal
in color (e.g.,
Zepf & Ashman
1993),
and LMXBs preferentially are found in red, younger and/or metal-rich
clusters (V-I > 1.1), rather than in blue, older and/or metal-poor
ones. The association of LMXBs with high metallicity GCs was observed in
the Galaxy and M31
(Bellazzini et al. 1995,
Trudolyubov &
Priedhorsky 2004).
In NGC 4472, red GCs are three times more likely to
host a LMXB than blue ones
(Kundu, Maccarone &
Zepf 2002).
Similarly, in M87, which has a very rich LMXB population, the fraction
of red GCs hosting a LMXB is 5.1% ± 0.7% versus 1.7% ±
0.5% for blue GCs
(Jordan et al. 2004),
also a factor of three discrepancy. In a sample of six ellipticals
yielding 285 LMXB-GC associations
(Kim et al. 2006),
the mean probability for a LMXB-GC association is 5.2%, the probability
of a blue GC to host a LMXB is ~ 2% for all galaxies except NGC 1399
(where it is 5.8%), while that of LMXB-red GC association is generally
larger, but varies from one galaxy to another (2.7% to 13%).
3.4.4. X-ray colors.
Maccarone, Kundu &
Zepf (2003)
reported that in NGC 4472 LMXBs associated with blue GCs have harder
"stacked" X-ray spectra than those in red GCs. However, this result is
not confirmed by the analysis of the much larger sample of sources
assembled by
Kim et al. (2006),
where no significant differences are found in the X-ray colors of LMXBs
associated with either red or blue GCs. Also, no significant differences
are found in the X-ray colors of LMXBs in the field or in GCs
(Sarazin et al. 2003;
Kim et al. 2006).
3.4.5. Spatial distributions of field and globular
cluster low-mass X-ray binaries. To obtain
additional constraints on LMXB formation and evolution, the radial
distributions of the LMXBs have been compared with those of the GCs and
of the field stellar light. Some of these comparisons have used the
entire sample of detected LMXBs, irrespective of GC counterpart; others
have also investigated differences between the LMXBs associated with GCs
and those in the field.
Investigating the overall LMXB distribution in NGC 4472,
Kundu, Maccarone &
Zepf (2002)
suggest that it follows more closely the distribution of the GCs than
the stellar light (which differ, with the GC one being more extended)
and infer an evolutionary connection of all LMXBs with GCs (see
Section 3.6). Other authors instead conclude that
overall the LMXB distribution and the stellar light trace each other in
E and S0 galaxies (NGC 1316,
Kim & Fabbiano
2003,
shown in Figure 3 left; NGC 1332,
Humphrey & Buote
2004).
As for the XLFs, incompleteness may affect these comparisons and account
for some of the discrepant reports: sources may be missed in the crowded
inner parts of a galaxy, resulting in an apparently more extended
distribution than the real one (see
Kim & Fabbiano
2003,
Gilfanov 2004).
Comparisons of the radial distributions of field and GC X-ray sources do
not reveal any measurable differences
(Sarazin et al. 2003;
Jordan et al. 2004;
Kim et al. 2006).
A first comparison of these LMXB distributions with those of the stellar
light and GCs was attempted in M87, but was inconclusive, given the
statistical uncertainties
(Jordan et al. 2004).
With their significantly larger LMXB and GC samples,
Kim et al. (2006)
instead find that the LMXB radial profiles, regardless of association
with either red or blue GCs, are closer to the more centrally peaked
field stellar surface brightness distribution, than to the overall
flatter GC distributions (Figure 5). The
implication of this result for the GC sources is that the probability of
a GC being assoc iated with a LMXB increases at smaller galactocentric
radii.
Figure 5. Radial distributions of low-mass
X-ray binaries (LMXBs) in the field (green), in red GCs
(red) and in blue GCs (blue), compared with the best-fit
GC distributions (red and blue at the bottom of the
figure), plotted versus the radius normalized to R25 for a
sample of six galaxies. The flattening of the distributions at small
radii is likely to be an incompleteness effect. The dotted blue and red
lines are the best-fit models of the red and blue GC distributions. The
solid lines show the best fits of the LMXB distributions. The black
dashed lines represent the stellar light distribution
(Kim et
al. 2006).
3.4.6. X-ray luminosity functions of field and
globular cluster low-mass X-ray binaries. No
significant differences have been found in the XLFs of LMXBs in the
field and in GCs
(Kundu, Maccarone &
Zepf 2002;
Jordan et al. 2004).
The coadded XLFs of field and GC LMXBs in six ellipticals
(Kim et al. 2006)
are also consistent within the errors, with a similar percentage of high
luminosity sources with LX > 1039 erg
s-1.
This similarity of field and GC XLFs does not extend, however, to the
X-ray populations of the Sombrero galaxy
(Di Stefano et al. 2003)
and M31 (from a comparison of the XLFs of bulge and GC
sources;
Trudoyubov &
Priedhorsky 2004).
In both cases, the GC XLFs show a more pronounced high luminosity break
than the field (bulge) XLFs. In M31 the XLF of GC sources is relatively
more prominent at the higher luminosities than that of field LMXBs; in
the Sombrero galaxy, GC sources dominate the emission in
the 1-4 ×
1038 erg s-1 range, but there is a high luminosity
tail in the field XLF, which, however, could
be due to contamination from a younger binary system belonging to the
disk of this galaxy (see
Di Stefano et al. 2003).
3.5. Metallicity and Dynamical Effects in Globular
Cluster Low-Mass X-Ray Binary Formation
The preferential association of LMXBs with red clusters could be either
an age or a metallicity effect. A correlation between the number density
of binaries and the metallicity of GCs was first suggested by
Grindlay (1987),
who ascribed this effect to a flatter IMF in higher metallicity GCs,
resulting in a larger number of neutron stars and thus LMXBs.
Kundu et al. (2003)
argue that metallicity is the main driver, based on the absence of any
correlations of LMXB association with different age GC populations in
NGC 4365.
Maccarone, Kundu &
Zepf (2004)
propose irradiation-induced winds in metal-poor stars to speed up
evolution and account for the observed smaller numbers of LMXBs in blue
GCs. These winds, however, would cause absorption and thus harder X-ray
spectra. Although these authors tentatively reported this spectral
effect in NGC 4472, studies of a larger sample of sources do
not confirm
this conclusion (see Section 3.4.4).
Jordan et al. (2004)
revisit the IMF-metallicity effect, because the resulting increase in
the number of neutron stars agrees with their conclusion that the
probability that a GC contains a LMXB is driven by the dynamical
properties of the cluster. Based on their study of M87, these
researchers propose that the probability pX for a
given GC to generate a LMXB has the form pX ~
0-0.42 ± 0.11 (Z /
Z)0.33
± 0.1, where
is a parameter
related to the tidal capture and binary-neutron star exchange rate and
0
is the central density of the cluster. This conclusion agrees with
three-dimensional hydrodynamical calculations of the dynamical formation
of ultracompact binaries in GCs, from red giant and neutron star
progenitors
(Ivanova et al. 2005).
Kim et al. (2006)
also invoke dynamical effects to explain the increasing probability of
LMXB-GC association at smaller galactocentric radii. They suggest that
the GCs nearer to the galaxy centers are likely to have more compact
cores and higher central densities to survive tidal disruption, compared
with the GCs at the outskirts, characteristics that would also increase
the chance of dynamical LMXB formation.
The formation processes of LMXBs have been debated since these s
ources were discovered in the Milky Way (see
Giacconi 1974).
LMXBs may result from the evolution of a primordial binary system, if
the binary is not disrupted when the more massive star undergoes
collapse and a supernova event, or may be formed by capture of a
companion by a compact remnant in GCs (see
Grindlay 1984,
reviews by
Verbunt 1993,
Verbunt & van den
Heuvel 1995).
The same scenarios are now being debated for the LMXB populations of E
and S0 galaxies. If GCs are the principal (or sole) birthplaces,
formation kicks or evaporation of the parent cluster have been suggested
as an explanation for the existence of field LMXBs in these galaxies
(see e.g.,
Kundu, Maccarone &
Zepf 2002).
The correlation of the total LMXB luminosity in a galaxy with the GC
specific frequency
(White, Sarazin &
Kulkarni 2002;
Kim & Fabbiano
2004;
see Section 3.3) suggests that GCs are important in
the formation of LMXBs.
White, Sarazin &
Kulkarni (2002)
proposed formation in GCs as the universal LMXB formation mechanism in
early-type galaxies. Other authors have supported this hypothesis,
because of the similarity of field and GC LMXB properties (see
Section 3.4; e.g.,
Maccarone, Kundu &
Zepf 2003).
However, this conclusion is by no means certain or shared by all. Beside
uncertainties in the correlations
(Kim & Fabbiano
2004),
the relationship between the fraction of LMXBs found in GCs and the GC
specific frequency (see Section 3.4.1) is
consistent with the simple relationship expected
if field LMXBs originate in the field while GC LMXBs originate in GCs
(Juett 2005;
Irwin 2005).
This picture would predict different spatial distributions of field and
GC LMXBs, an effect not seen so far, although, as
Juett (2005)
notes, the prevalence of LMXBs in red (more centrally concentrated) GCs
and the effect of supernova kicks in the distribution of binaries may
make the two distributions less distinguishable.
Piro & Bildsten
(2002)
and
Bildsten & Deloye
(2004)
compare the observational results with theoretical predictions for the
evolution of field and GC binaries. Piro & Bildsten remark that the
large X-ray luminosities of the LMXBs detected in early-type galaxies
(> 1037 and up to 1039 erg s-1)
imply large accretion rates (> 10-9
M
yr-1). In an old stellar population these sources are likely
to be fairly detached binaries that accumulate large accretion disks
over time, and undergo transient X-ray events when accretion is
triggered by disk instabilities. These transients would have recurrence
times greater than 100 years and outbursts of 1-100 years duration. In
this picture field binaries should be transient, a prediction that is
supported by the detection of transients in the NGC 5128 LMXB population
(Kraft et al. 2001)
and by the discovery of a population of quiescent X-ray binaries in the
Sculptor dwarf spheroidal galaxy
(Maccarone et
al. 2005).
Piro & Bildsten also point out that GC sources tend to have shorter
orbital periods and would be persistent sources, reducing the fraction
of transients in the LMXB population. Interestingly,
Trudolyubov &
Priedhosky (2004)
report only one recurrent transient in their study of GC sources in M31, although 80% of these sources show some
variability; however, they also find six persistent sources in the
1038 erg s-1 luminosity range.
Bildsten & Deloye
(2004)
instead look at ultracompact binaries formed in GCs to explain the bulk
of the LMXBs detected in E and S0 galaxies. A motivation for this work
is the large probability of finding LMXBs in GCs (per unit optical
light, see Section 3.4.2), which makes formation in
GCs more efficient than in the field. Ultra-compact binaries would be
composed of an evolved low-mass donor star (a white dwarf), filling its
Roche lobe, in a 5-10 minute orbit around a neutron star or a black
hole. The entire observable life of such a system is ~ 107
years, much shorter than the age of the galaxies and the GCs, therefore
their total number would be indicative of their birth rate. From this
consideration Bildsten & Deloye derive a XLF with a functional slope
in excellent agreement with the measurements of
Kim & Fabbiano
(2004)
and
Gilfanov (2004).
Bildsten & Deloye also predict a break at LX ~
1037 erg s-1 in the XLF, which would correspond to
the luminosity below which such a system would be a transient. As
discussed in Section 3.3, there is some evidence of a
low-luminosity break in the composite XLF of
Gilfanov (2004),
which, however, includes data from spiral bulges as well.
Confirmation of this break in a number of E and S0 populations by itself
would not be proof of the Bildsten & Deloye scenario, because the
break may occur from the evolution of field binaries. For example, a
flattening of the XLF at the lower luminosities is found in the
population synthesis of Pfahl, Rappaport & Podsiadlowski
(2003,
their figure 3), if irradiation of the donor star from the X-ray
emission of the compact companion is considered in the model. More
recently,
Postnov & Kuranov
(2005)
have proposed that the mean shape of the XLF of
Gilfanov (2004)
can be explained by accretion on neutron star from Roche lobe overflow
driven by gravitational wave emission, below ~ 2 ×
1037 erg s-1, and by
magnetic stellar winds at higher luminosities. Optical identification of
X-ray sources with GCs and an estimate of the transient fraction at
different luminosities would help to discriminate among possible
scenarios; planned deep time-monitoring Chandra observations may
provide the observational constraints.
The nature of the most luminous sources in E and S0 galaxies (those with
LX above the 5 × 1038 erg s-1
break,
Kim & Fabbiano
2004)
is the subject of a recent paper by
Ivanova & Kalogera (2006).
These researchers point out that only a small fraction of these luminous
sources are associated with GCs (at least in M87, see
Jordan et al. 2004)
and that they are too luminous to be explained easily with accreting
neutron star systems that may form in GCs
(Kalogera, King &
Rasio 2004).
With the assumption that these sources are accreting black-hole
binaries, these authors explore their nature from the point of view of
the evolution of f ield native binaries. In this picture most donor
stars would be of low enough mass (< 1-1.5
M given the age
of the stellar populations in question) that the binary would be a
transient (see
Piro & Bildsten
2002)
and therefore populate the XLF only when in outburst emitting at the
Eddington luminosity; this would happen from main-sequence, red-giant,
and white-dwarf donors. In this case the XLF is a footprint of the
black-hole mass spectrum
in these stellar populations, which is an important ingredient for
linking the massive star progenitors with the resulting black
hole. Ivanova & Kalogera derive a differential slope of ~ -2.5 for
the black-hole mass spectrum, and an upper black-hole mass cut-off at ~ 20
M, to be
consistent with the observed cumulative XLF of
Kim & Fabbiano
(2004)
and
Gilfanov (2004).
Depending on the magnetic breaking prescription adopted, either
red-giant donors or main-sequence donors would dominate the source
population. A word of caution is in order here, because the similar
shape of GC and field LMXB XLFs
(Kim et al. 2006,
see Section 3.4.6) suggests that high-luminosity
black-hole sources may also be found in GCs, at odds with theoretical
discussions (e.g.,
Kalogera, King &
Rasio 2004).
3.7. Young Early-Type Galaxies and Rejuvenation
There
have been some puzzling and somewhat controversial results suggesting
that the stellar populations of some early-type galaxies may not be
uniformly old, as implied by their optical characteristics, but may hide
a small fraction of younger stars, which give rise to luminous and
easily detectable X-ray binaries. Rejuvenation (e.g., by a merger event
or close encounter with a dwarf galaxy) has been suggested to explain
the presence of very luminous and asymmetrically distributed X-ray
source populations in some galaxies [NGC 720 -
Jeltema et al. 2003;
NGC 4261 (shown in
Figure 6) and NGC 4697 -
Zezas et al. 2003].
Sivakoff, Sarazin &
Carlin (2004)
report an exceptionally luminous population of 21 sources with
LX > 2 × 1039 erg s-1 (in the
ULX regime, see Section 6) in the X-ray
bright elliptical NGC 1600, which is twice the number of sources that
would be expected from background AGNs and suggests an XLF slightly
flatter than in most ellipticals. In all these cases, however, both
cosmic variance affecting the background AGN density and distance
uncertainties may play a role. Moreover,
Giordano et al. (2005)
report the identification of the NGC 4261 sources with GCs, undermining
the suggestion that they may be linked to a rejuvenation event.
Figure 6. The left panel shows a
Chandra image of NGC 4261; note the distribution of the luminous
point sources, which clearly do not follow the optical light shown in
the right panel; Both images are from
http://chandra.harvard.edu/photo/category/galaxies.html;
credit NASA/CXC;
Zezas et al. (2003).
The behavior opposite the one just discussed is reported in an X-ray and
optical study of the nearby lenticular galaxy NGC 5102
(Kraft et al. 2005).
In this galaxy, where the stellar population is young (< 3 ×
109 years old), and where there is evidence of two recent
bursts of star formation, a
definite lack of X-ray sources is observed. NGC 5102 has also a very low
specific frequency of GC (~ 0.4). Kraft et al. speculate that the lack
of LMXBs may be related either to insufficient time for the evolution of
a field binary and/or to the lack of GCs.