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DEEP OBSERVATIONS OF THE FORNAX DWARF SPHEROIDAL GALAXY
We request 250 kse of ACIS-I time on the Fornax dwarf spheroidal galaxy, enough to reah
the ux level of quiesent neutron star X-ray binaries at its distane of 140 kp. We also request 4
nights of CTIO time to make optial observations of this galaxy for omparison with the X-ray data.
This will allow us to determine the rate of X-ray binary formation in a large sample of stars with
a well-dened star formation history and with a low stellar interation rate at a known distane.
The eld we propose for also inludes one of the galaxy's globular lusters, whih will allow us to
measure, with better ontrols than in previous measurements, the enhanement in X-ray binary
formation due to stellar interations.
Theoretial estimates of the birth rate of low mass X-ray binaries (LMXBs) are poorly onstrained due to a lak of observational data on the relevant parameters needed as inputs for models
(Kalogera & Webbink 1998). An old ontroversy over whether LMXBs are formed primarily in globular lusters (GCs) and then released into the eld through tidal destrutions and stellar ejetions
(see e.g. Grindlay 1984) or formed in situ in the eld, as is impliit in most population synthesis
models (e.g. Ghosh & White 2001) has been re-kindled through Chandra observations of external
galaxies whih hint that the former proesses are likely to be important (e.g. Maarone, Kundu &
Zepf 2003). The ontroversy is espeially interesting for an old stellar population, where high mean
mass aretion rate systems would have long sine areted their entire stellar ompanions, but
where low duty yle transients may present a way to lengthen the lifetimes of LMXBs produed
in the eld (Piro & Bildsten 2002). There are thus urrently no good onstraints on the birth and
survival rates of LMXBs outside dense stellar systems (i.e. those where dynamial interations will
not produe binary stars). A key step towards solving this problem is the establishment of a sample
of systems where the star formation history is well understood.
The best set of galaxies with old stellar populations and well understood star formation histories
is the Milky Way's dwarf spheroidal galaxy population. In these galaxies, the individual stars are
resolved by optial observations, so olor magnitude diagrams an be used to determine aurately
the sizes and ages of the dierent stellar populations, as well as the distanes to the galaxies.
Furthermore, reasonable exposure times with Chandra an reah the level of the quiesent X-ray
binaries, giving us a real handle on the number of neutron star binaries that exist in the galaxy,
without making estimations of the fration of soures whih are transients and the duty yles of
those transients.
One of the best suh systems to observe is the Fornax dwarf spheroidal galaxy. This is the
seond largest of the Milky Way's ompanion galaxies (and the largest whih is morphologially
undisturbed). It is loated as a distane of 140 kp, far from the Galati plane, so that foreground
soures and absorption will not be problemati. Its stellar luminosity is about 1.6107 L , about
7 times higher than the Sulptor dwarf spheroidal galaxy where our Cyle 5 observations have
identied several good X-ray binary andidates. Furthermore, this galaxy ontains some of its own
globular lusters. By looking at two populations of stars, one with dynamial interations and one
without them, but at the same distane, and with the same metalliity, we an truly isolate the
eets of dynamial interations in produing X-ray binaries. This is not possible in general in the
Milky Way beause of the problems in determining distanes to individual objets in the plane of
the Galaxy.
Births and Deaths of X-ray Binaries
There are two major lasses of X-ray binaries and many paths for forming and evolving these
systems. These lasses are LMXBs, whih arete through Rohe lobe overow from a low mass
star, and high mass X-ray binaries (HMXBs), whih arete through the gravitational apture of
the stellar wind of an O or B star.
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The lifetimes of the HMXBs are the main sequene lifetimes of the mass donors (i.e. typially years), preventing these systems from existing in old or intermediate-aged stellar populations.
This is in good agreement with observations. The HMXBs in the Milky Way tend to be lustered
in the star forming regions suh as the spiral arms, and tend to have a rather small sale height
indiating that they have not traveled far from their birth sites and they hene must be young
(Brandt & Podsiadlowski 1995). There is no evidene for high mass X-ray binaries in old GCs or
in undisturbed elliptial galaxies.
The lifetimes of LMXBs are not as well understood. It is lear that the LMXBs \die" when the
donor star is ompletely evaporated. Given a onstant luminosity over the lifetime of the system
and an aretion eÆieny suh that L = m
_ 2 , the lifetime is then Mdonor 2 =L, whih is 100 million years for a luminosity of 5 1037 erg se 1 , a 1M donor and 10% aretion eÆieny;
thus the age limit for persistently bright LMXBs should not be muh larger than that for HMXBs,
espeially in an old stellar population suh as an elliptial galaxy where there are no stars more
massive than 1M .
Three senarios exist, however, where older stellar populations an ontain some LMXBs. The
rst is delayed aretion. Given a suÆiently wide binary system, aretion may start not immediately after a supernova explosion produes a ompat objet in a binary system, but rather only
after the donor star has evolved o the main sequene, beoming larger and hene lling a Rohe
lobe substantially bigger than its main sequene radius. The typial timesale for this proess is 2
Gyrs (White & Ghosh 1998), with only a very small range of parameter spae produing high mean
mass aretion rate binary systems more than 5 Gyrs after the initial supernova explosion (see e.g.
Podsidlowski, Rappaport & Pfahl 2002); this senario thus ould not explain a large population of
old, bright X-ray binaries.
An exeption is the lass of ultraompat X-ray binaries { X-ray binaries where the donor star
itself is a ompat star (i.e. a white dwarf, sine neutron stars will not overow their Rohe lobes).
These systems are espeially interesting, sine they are potentially strong gravitational radiation
soures. It has been suggested that they may represent a substantial fration of the X-ray binaries in
elliptial galaxies, as their expeted luminosity funtion mathes well to the observations (Bildsten
& Deloye 2004).
Detetion of a gravitational wave soure at a known distane would be espeially interesting.
Suggestions exist for using the period derivatives of gravitational wave signals to onstrain some of
the unknowns in the systems, suh as the relative masses of the two omponents and the distanes
to the systems. These depend on the angular momentum evolution of the binary being solely due to
gravitational radiation losses, a suggestion whih is learly not in agreement with the observations
of 4U 1820-30, the only known X-ray binary should be detetable by LISA. What is not known is
whether the unaounted for eets are due to aeleration by the gravitational potential of the
globular luster in whih this system is loated, or irradiation indued stellar winds (Tavani et al.
1991). Detetion of suh a system in a nearby dwarf spheroidal galaxy, would allow for a lean test
of whether gravitational radiation is the only soure of angular momentum evolution in areting
ultraompat binaries.
In the seond senario, a low duty yle transient population makes up the long-lasting LMXBs
(Piro & Bildsten 2002). Suh systems would have low mean mass aretion rates, allowing them
to survive for a Hubble time, but would have oasional outbursts where their luminosities rise
above the threshold of detetability in the typial observations of elliptial galaxies (i.e. 3 1037
erg se 1 ). Suh a senario does not seem too unreasonable given that the bulk of the Milky Way's
bright aretors are either transient systems or high mass X-ray binaries, and all of its LMXBs
thought to ontain blak holes are transients (see Liu, van Paradijs & van den Heuvel 2000; 2001
and referenes within). Furthermore, additional evidene exists that many binary systems should
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have duty yles of order 2% or less - the low quiesent ux of the neutron star transient KS 1730260 indiates that its mean aretion rate must be about two orders of magnitude lower than its
mean luminosity in outburst or its rust would show more evidene of strong heating (Rutledge et
al. 2002) and observations of numerous X-ray transients' outburst durations and reurrene yles
show that the typial duty yles are about 2%, with some systems having muh lower duty yles
(Chen et al. 1997).
The nal mehanism for making bright LMXBs in old stellar populations is to produe new
binary systems from existing ompat and normal stars through stellar interations. In a GC, the
stellar densities are suÆient to allow for tidal apture (Fabian et al. 1975) or three-body aptures
(Hills 1975) of neutron stars by normal stars. These ollisions an our throughout the lifetime of
the globular luster, hene allowing short lifetime systems to exist today. In both the Milky Way
and elliptial galaxies, GCs are known to have 100 times as many bright LMXBs per unit mass
as the eld.
In fat, theoretial models have some diÆulties produing LMXBs outside globular lusters.
The standard sheme for produing an X-ray binary, a Type II supernova in a binary star system,
has serious problems produing LMXBs. There is a lear deit of low mass ompanions for
spetrosopi binary O stars (Garmany et al. 1980), and tentative evidene that this orrelation
holds up even when the visual binaries are inluded in the sample (Mason et al. 1998); low mass
ompanions to stars undergoing supernova explosions may hene be rare. Furthermore, when a
supernova explosion removes more than half the mass of a binary system (as should generally be
the ase for a supernova leaving behind a low mass star and a neutron star), the binary is disrupted;
the diret supernova senario an only work through a tuning of the birth kik veloity and diretion
of the neutron star (Kalogera 1998). This problem may be alleviated by invoking ommon envelope
evolution to remove muh of the mass of the supernova progenitor prior to the explosion (e.g. van
den Heuvel 1983; Kalogera & Webbink 1998) or by senarios suh as making many LMXBs the
desendants of \Intermediate Mass X-ray Binaries" - systems where the donor star has an initial
mass of 4 M - and having a rapid unstable aretion phase whih redues the donor star's mass
into the LMXB range (e.g. Podsiadlowski & Rappaport 2000).
Motivated in part by the diÆulties in produing eld LMXBs, or at least by the unertainties
in preditions about the number density, it has been suggested that a substantial fration of eld
LMXBs may have atually been produed in GCs both in the Milky Way (Grindlay 1984) and
in elliptial galaxies (e.g. White, Sarazin & Kulkarni 2002). The LMXBs ould then esape to
the eld either through tidal destrution of the GCs (e.g. Vesperini 2000) or through dynamial
ejetions (e.g. Phinney & Sigurdsson 1991; Colpi, Possenti & Gualandris 2002 - CPG). Some reent
observations bolster the idea that this senario may be important. Numerous milliseond pulsars
in binary systems are now being seen outside the ores of globular lusters (see CPG and referenes
within) and an X-ray binary is found only about 5 arminutes from a GC in the Milky Way (Jonker
et al. 2003). At the present, though, the evidene for ejetion of LMXBs from GCs is still largely
anedotal; better observations are needed to determine whether this is the dominant mehanism
for produing LMXBs in an old stellar population, or if it produes only a small fration of the
non-luster soures.
What an we expet to see and to learn?
To begin, we should identify the types of soures we are likely to identify with these observations.
The ux limit reahed in 250 kiloseonds should be 4:5 10 16 erg se 1 (assuming a 0.2 keV
blakbody spetrum, as generally provides a good phenomenologial t to the data on quiesent
neutron star aretors). This limit orresponds to a bit less than 1033 erg se 1 (0.5-8 keV).
Hardness ratios or other rude spetrosopy should be possible for soures above 1034 erg se 1 .
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Suh sensitivity limits should be suÆient for deteting any outbursting blak hole binary we might
be luky enough to ath, most neutron star X-ray binaries, some of the brighter, longer-period
blak hole X-ray binaries in quiesene (see e.g. Garia et al. 2001), and the brightest atalysmi
variable stars in outburst (Cordova 1995 and referenes within).
How many soures are expeted?
To estimate the number of soures we expet to detet, we draw from our ongoing work on the
Sulptor dwarf spheroidal galaxy. We nd one very strong X-ray binary andidate (based on its
X-ray spetrum, and its optial ounterpart's properties whih are onsistent with a main sequene
star in the Sulptor galaxy, but inonsistent with the optial-to-X-ray ux ratios seen from the AGN
whih make up the X-ray bakground).We nd 5 other good andidates based on X-ray spetra
and ux levels, and we also estimate that there should be a few exess soures ompare with the
expetation value from the X-ray bakground. As the observations were just reently nished, we
have not ompleted the optial follow-up analysis.
The Fornax galaxy is about 7 times as luminous as the Sulptor galaxy, so we would naively
expet to nd about 40 X-ray soures in it (in addition to the bakground soures). In fat, sine
our eld will inlude one of the globular lusters assoiated with this galaxy, we expet to nd
many more X-ray binaries, with the exess assoiated with the globular lusters.
Tehnial Justiation
We are requesting one observation of 250 kiloseonds eah with ACIS-I. It will be entered on
the ore of the Fornax dwarf spheroidal galaxy (and will inlude \Fornax 4", one of the smaller
globular lusters). The loation of the galaxy and the globular luster are marked in Figure 1.
The diuse X-ray bakground from this galaxy is known to be weak based on ROSAT observations
(Gizis, Mould & Djorgovski 1993). Although the observations we propose here will be muh deeper
than the ROSAT data, it is unlikely that any gas hot enough to emit X-rays would be retained in a
galaxy with suh a shallow potential well. As a result, we assume that the instrumental bakground
will dominate.
We then alulate the expeted sensitivity level using PIMMS. The bakground level will be
about 8.410 6 ts/se, so in 250 kse, a detetion at the 10 7 signane level an be made in
the full band with 11 soure photons, whih orresponds to 1033 ergs/se for a blakbody spetrum
with 0.2 keV temperature. In fat, for soft spetrum soures, a narrower bandpass should be more
sensitive than the full band, beause muh bakground and few soure photons will be lost by
restriting the searh to the 0.2-2 keV range. As a result, 250 kse is appropriate for deteting all
but the faintest quiesent X-ray binaries at 140 kp.
Based on our failure to detet any bright X-ray transients in Sulptor, we think it is also unlikely
that we will detet any in Fornax. For this reason, we do not plae any onstraints on how the
observing time should be sheduled.
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Optial Observations
We also request 4 nights of gray time on the CTIO 4m telesope. Aording to the exposure time
alulator, this will enable us to reah S/N of 5 at 27th magnitude for B,V, and R, and to reah
an emission measure level of about 10 4 in H. The requested exposure times will be about 15000
seonds in B and V, about 25000 in R, and one night in H. This will allow for all the data to
be taken in three nights with enough time left over for overheads like readout and taking standard
star observations.
Going this deep in many lters is essential for getting the optial ounterparts to the X-ray
soures. Most of the soures we detet will be bakground ative galati nulei. Multi-olor
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Figure 1: The eld we plan to observe. The box shows the Chandra eld of view, while the irle
shows the loation of the globular luster. The optial data are from the DSS. The roll angle is not
onstrained; an arbitrary angle is plotted.
photometry will help to identify these soures based on their harateristi olors (e.g. Rihards et
al. 2004) and X-ray to optial ux ratios (see e.g. Maaaro et al. 1988). Three olor photometry
is typially suÆient for separating out AGN from foreground stars, but onsiderable errors will be
possible with only two olors.
Understanding the nature of the faint X-ray binaries also requires good photometry. The two
most likely types of X-ray binaries in old stellar populations are very low duty yle transients
areting from a red giant star, and ultraompat systems (i.e. systems where the donor star is
a white dwarf). Obviously, these should have very dierent photometri signatures, and any red
giant donors will already be known from previous surveys of this galaxy. At distanes of 140
kp, the optial emission from the ultraompat binaries should be dominated by their aretion
disks, rather than by their donor stars, even at very low X-ray luminosities.
The depth of our photometry is set to ensure that any undeteted system is an ultraompat
X-ray binary. Even quiesent X-ray binaries should have outer aretion disks whih radiate as
blakbodies at temperatures just slightly below the ionization temperature of hydrogen (about 104
K for typial densities of an aretion disk). The size of an aretion disk should be about half the
orbital separation (i.e. a few times the irularization radius, sine orbital anglular momentum will
be transported outwards to allow the disk to exist past the irularization radius). This leads to
2=3
(T=104 K)4 , where M is the mass of the aretor and Phr
a luminosity, in solar units of M 1=3 Phr
is the orbital period in hours, and T is the disk temperature. Sine a solar luminosity orresponds
to a magnitude of about R = 25:8 at 140 kp, going to R=27 ensures that we will detet any
objet brighter than about 1/3 L , whih orresponds to a period of about 5 minutes for a 1.4 M
neutron star mass donor. Additionally, ultraompat binaries should stand out as being too hot
for their magnitudes when plotted in a olor magnitude diagram. We note that arhival HST data
an be used in the regions where the stellar densities are highest, but that HST does not over the
lower density regions whih omprise the bulk of our eld.
The H emission will be useful for two reasons. Firstly, at low X-ray luminosities, X-ray binaries
generally show high equivalent widths of H (e.g. Garia et al. 1996), whih we should reah with
one night of H imaging. This would obviously not be expeted for an ultraompat binary, sine
these systems have white dwarf donors with no hydrogen. H emission detetions an thus help
to break the degeneray between bright ultraompat X-ray binary disks and faint normal donor
stars. Seondly, gas in dwarf spheroidal galaxies has been notoriously diÆult to detet. One night
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of H imaging on the Fornax dwarf would reah an emission measure of about 10 4 , whih would
be suÆient for deteting the presene of ionized gas with a density of about 10 4 H m 3 , three
orders of magnitude less than the level found from pulsar dispersion measure variations in globular
lusters (e.g. Freire et al. 2001).
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