Issue |
A&A
Volume 513, April 2010
|
|
---|---|---|
Article Number | A66 | |
Number of page(s) | 6 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200913732 | |
Published online | 30 April 2010 |
Chandra detection of diffuse X-ray emission from the globular cluster Terzan 5
P. Eger1 - W. Domainko2 - A.-C. Clapson2
1 - Erlangen Centre for Astroparticle Physics, Universität
Erlangen-Nürnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
2 - Max-Planck-Institut für Kernphysik, PO Box 103980, 69029
Heidelberg, Germany
Received 24 November 2009 / Accepted 19 January 2010
Abstract
Aims. Terzan 5, a globular cluster (GC)
prominent in mass and population of compact objects, is searched for
diffuse X-ray emission, as proposed by several models.
Methods. We analyzed the data of an archival Chandra
observation of Terzan 5 to search for extended diffuse X-ray
emission outside the half-mass radius of the GC. We removed detected
point sources from the data to extract spectra from diffuse regions
around Terzan 5. The Galactic background emission was modeled
by a 2-temperature thermal component, which is typical for Galactic
diffuse emission.
Results. We detected significant diffuse excess
emission above the particle background level from the whole
field-of-view. The surface brightness appears to be peaked at the GC
center and decreases smoothly outwards. After the subtraction of
particle and Galactic background, the excess spectrum of the diffuse
emission between the half-mass radius and 3' can be described by a
power-law model with photon index
and a surface flux of
erg cm-2 s-1 sr-1
in the 1-7 keV band. We estimated the contribution from
unresolved point sources to the observed excess to be negligible. The
observations suggest that a purely thermal origin of the emission is
less likely than a non-thermal scenario. However, from simple modeling
we cannot identify a clearly preferred scenario.
Key words: globular clusters: individual: Terzan 5 - acceleration of particles - X-rays: diffuse background
1 Introduction
Terzan 5 (stellar luminosity ,
Harris 1996) is
the Galactic globular cluster (GC) with the largest population of known
millisecond pulsars (33 MSPs, Ransom
2008). Furthermore, a high production rate for low-mass X-ray
binaries (LMXBs) is expected in the extremely dense core of
Terzan 5 (Ivanova
et al. 2005). The GC is located at a distance of
5.5 kpc (Ortolani
et al. 2007) or 8.7 kpc (Cohn et al. 2002),
only
1.7 deg
above the Galactic plane (RA: 17
48
04
0,
Dec:
). Its core (
)
and half-mass radii (
)
are 0.18' and 0.83', respectively (Harris
1996).
GCs are expected to contain intracluster gas originating
from the mass loss of evolved stars. Since GCs move through the
Galactic halo medium with typical velocities of 200 km s-1,
bow shocks should form in front of them in the direction of their
proper motion (Krockenberger
& Grindlay 1995). These shocks have the ability to
both accelerate particles and heat the gas behind them. In this
scenario, electrons accelerated at the bow shock could produce diffuse
non-thermal X-ray emission because of either inverse Compton (IC)
scattering on ambient photon fields (Krockenberger
& Grindlay 1995) or non-thermal bremsstrahlung
resulting from the deflection of the electrons by inter-stellar medium
(ISM) nuclei (Okada
et al. 2007, henceforth Ok07).
Also, diffuse thermal X-ray emission can be emitted from shock-heated
material trailing
behind the moving GC. For a detailed discussion of the bow-shock
scenario, see Ok07.
The first unresolved diffuse X-ray emission from GCs
(47 Tuc, Cen,
and M 22) was reported by Hartwick
et al. (1982) using the Einstein
observatory, which was later confirmed by Krockenberger & Grindlay
(1995) with ROSAT. Recently, using Chandra
data, Ok07 detected significant diffuse X-ray emission from the GCs
47 Tuc, NGC 6752, M 5, and
Cen.
They find that the diffuse source at the position of
Cen
is likely to be a background cluster of galaxies.
A similar extra-Galactic nature is proposed for 47 Tuc by Yuasa et al. (2009).
The remaining potentially GC-associated diffuse X-ray emission, from
M 5 and NGC 6752, could arise from different
scenarios. The emission in M 5 features an arclike morphology
and exhibits a thermal spectrum (kT<0.1 keV),
possibly from shock-heated gas. The clumpy structure seen near
NGC 6752 presents a hard non-thermal spectrum (
2) and a radio
counterpart, maybe from non-thermal bremsstrahlung emission by
shock-accelerated electrons hitting nearby gas clouds.
Non-thermal emission in the X-ray band may also be associated with compact objects, either directly, as detected from isolated low-mass X-ray binary systems (see for instance Wijnands et al. 2005, for such a candidate system in Terzan 5), or as secondary emission from the population of high-energy particles they would generate. This was modeled for millisecond pulsars by Venter & de Jager (2008, henceforth VJ08) for the synchrotron radiation mechanism. The second case may translate into larger physical scales, due to the diffusion of the energetic particles away from their source. The populations of compact objects in Terzan 5 may provide an opportunity to test these scenarios. Apparent, extended X-ray emission from the direction of GCs might also arise from a population of faint unresolved point-like sources below the detection limit of the observing instrument.
In this work we analyzed an archival Chandra observation to search for a diffuse emission component above the Galactic background associated with Terzan 5. We characterized the X-ray signal using spatially resolved spectral analyses, after a careful study of the diffuse Galactic background, and briefly discuss different scenarios for the emission.
2 X-ray analysis and results
2.1 Observation and data preparation
To search for extended diffuse X-ray emission from Terzan 5,
we analyzed the Advanced CCD Imaging Spectrometer
(ACIS, Garmire
et al. 2003) data of an archival 40 ks Chandra
(Weisskopf et al.
2002) observation (ObsID 3798), which was originally
performed to characterize the faint X-ray point-source population of
this GC (Heinke
et al. 2006, henceforth H06). Only the ACIS-S3 chip
was switched on, so we were only able to search for diffuse X-ray
emission at angular distances
from the cluster core. A comprehensive study of diffuse X-ray emission
seen from a number of Galactic GCs was performed by Ok07. However,
these authors excluded Terzan 5 from their work because the
only available Chandra dataset at that time
suffered from serious pile-up effects because of a bright binary
outburst in the field-of-view (FoV). In this paper we analyzed data
from a newer observation where such an event did not occur.
For the X-ray analysis we used the CIAO software
version 4.1, supported by tools from the FTOOLS package and
XSPEC version 12.5.0 for spectral modeling (Arnaud 1996). The event1
data were reprocessed with the latest position and energy calibration
(CTI correction, v4.1.3) using bad pixel files generated by acis_run_hotpix.
The good-time-interval (GTI) file supplied by the standard processing,
which was used by H06, screens out a 4.0 ks interval of strong background
flaring at the end of the observation. To remove an additional time
period of 4.3 ks with a slightly increased background level,
we used the light curve in the 0.5-7.0 keV energy band after
the core region of the cluster and additional bright sources were
removed from the data. A screening threshold of 1.0 cts/s
yielded a net exposure of 31.0 ks. We chose these stricter
criteria with respect to H06 because understanding the background is
crucial for analyzing faint extended sources.
2.2 Extraction regions
To detect and remove point-like X-ray sources from the event-list, we
ran wavdetect on the GTI-screened dataset in
three energy bands (0.5-2.0 keV, 2.0-7.0 keV and
0.5-7.0 keV). H06 used pwdetect for the
detection within
and wavdetect for outer regions. In this paper we
only analyzed areas outside
,
so results should be comparable. We estimated a point-source detection
limit of
erg cm-2 s-1
in the 0.5-7.0 keV band. For the most part our results are
compatible with the sources listed by H06, Table 2. However,
the shorter exposure time compared to the analysis of H06 led to a
higher point-source detection threshold. Therefore we did not detect
the faintest seven sources from H06 that we introduced manually into
our source list. Sources were removed from the dataset using the 3
radius of the point spread function. Additionally, all events within
were disregarded.
To measure the level of diffuse X-ray emission around Terzan 5, we extracted spectra from eight concentric annular regions centered on the cluster core with radii from 1.1' to 3.9' (Fig. 1). Each ring has a width of 0.4'. We chose rings with equal width over rings with constant area to have comparable statistical quality in the spectra since the surface brightness decreases with distance from the GC. For the spectral analysis, we chose the 1-7 keV energy band. Widening the band in either direction lead to lower signal-to-noise ratios. At lower energies an increased contribution to the signal from soft thermal Galactic diffuse emission is expected. At energies above 7-8 keV the charged particle induced background component increases significantly for instruments onboard Chandra. The mean effective area and energy response for each spectrum was calculated by weighting the contribution from each pixel by its flux using a detector map in the same energy band. To subtract the particle induced non-X-ray background (NXB), we used a background dataset provided by the calibration database, where the detector was operated in stowed position. The background spectrum for each ring was extracted from the respective region in this background dataset. To account for the time dependence of the NXB, we scaled the background by the ratio of the source and background count rates in the 9-12 keV energy band for each spectrum (as described by Markevitch et al. 2003).
To produce an image of diffuse X-ray emission above the
particle background from the direction of Terzan 5, we
extracted counts in the 1-7 keV energy band and refilled the
excluded source regions and the region inside
with dmfilth using the photon distributions from
rings around the excluded areas. We subtracted the particle background
using the respective image from the stowed dataset after correction for
the different exposures. The resulting image was corrected for relative
exposure and adaptively smoothed with asmooth. We
required a minimum significance of 3
for the kernel size of the smoothing algorithm. The resulting smoothing
radii were a few arcminutes, so that no details smaller than that scale
can be seen in the smoothed image. Figure 1 shows the
resulting image together with all extraction regions that we used in
this work.
Table 1: Extraction regions and results from spectral fitting.
![]() |
Figure 1:
Smoothed, exposure corrected and NXB subtracted Chandra
image of diffuse X-ray emission in the 1-7 keV band around
Terzan 5. Excluded regions from point sources and the region
inside |
Open with DEXTER |
Even though a significant contribution from thermal Galactic diffuse
emission is expected, the spectra from the single rings were fit well
enough by an absorbed power-law model for a preliminary flux estimate.
The resulting fit parameters are given in Table 1. We found
significant diffuse excess emission above the particle background in
all rings and derived the surface brightness for each region by
dividing the model flux by the effective extraction area, which is the
geometric ring area inside the FoV minus excluded regions and bad
pixels. Due to limited statistics, we fixed the column density at a
default value of cm-2.
Therefore, we list the observed surface fluxes in Table 1, as
opposed to the intrinsic fluxes we provide for all other spectra. The
diffuse surface flux shows a clear radial dependence (Fig. 2), which
indicates that a significant part of the excess is connected to the
cluster. At distances greater than
from the GC core, the observed surface flux seems to reach a base level
of
erg cm-2 s-1 arcsec-2
(1-7 keV). In the following section we derive the unabsorbed
surface flux for the outer region by applying a more realistic physical
model.
![]() |
Figure 2:
Radial dependence of the observed diffuse X-ray surface flux above the
particle background in the 1-7 keV band as seen with Chandra
(black crosses with error bars). Stars (red) denote the infrared
surface brightness profile from Trager
et al. (1995). The solid curves (green) show the
X-ray point-source distribution described by a generalized King-profile
(Heinke et al. 2006)
for the two extreme cases |
Open with DEXTER |
2.3 Galactic diffuse background
Terzan 5 is close to the Galactic plane where diffuse Galactic
emission becomes an important component. However, the Chandra
blank-sky datasets are composed of observations towards high Galactic
latitudes, which would underestimate the sky background in our case. To
test whether the spectrum observed from the outer three rings is
compatible with thermal Galactic diffuse emission, we used a more
physically reasonable model. Similar to Kaneda et al. (1997)
and Ebisawa
et al. (2005), who modeled the diffuse Galactic
ridge emission as observed with ASCA and Chandra,
respectively, we describe the Galactic diffuse component using a
two-temperature (2-T) non-equilibrium ionization model (NEI) (Masai 1984). To
improve the statistical quality, we combined the outer three (
175-246'') rings into a single
spectrum. The spectrum was adaptively binned to a minimum of 20 excess
counts per bin. As background we again used the spectrum extracted from
the same region in the NXB dataset. We fitted a 2-T NEI model to the
outer spectrum, freezing most of the parameters to the best-fit values
from Table 8 in Ebisawa
et al. (2005). We left the surface brightnesses of
the two components and the N
free to vary to account for the difference in flux and column density
between the region around Terzan 5 and the area observed by Ebisawa et al. (2005).
In addition, we allowed the Si-abundance of the soft component as a
free fit-parameter, because the low-ionized Si line at
1.8 keV
(Kaneda
et al. 1997; Ebisawa et al. 2005)
was otherwise underestimated.
The spectrum of the outer region together with the model fit
is shown in Fig. 3
(Top). To be able to compare our results to the
analysis of Ebisawa
et al. (2005), we chose an energy range of
0.7-10 keV in this specific case. The best-fit values are
given in Table 1
(Outer region). The total intrinsic surface flux
of the two components is a factor of three lower than the value for the
Galactic region observed by Ebisawa
et al. (2005). This relation is in good agreement
with the ratio between the column densities for both regions, which is 4 (Dickey & Lockman 1990).
Assuming that the Galactic column density seen from a certain direction
is directly related to the expected flux from a diffuse Galactic
component, we conclude that at least
3/4 of the total excess above particle background
observed from the outer region comes from Galactic diffuse emission.
2.4 Diffuse excess emission connected to Terzan 5
In this section we focus on the diffuse emission observed from the inner five rings ( 55-175''). In addition to the radial dependence of the diffuse excess emission, Fig. 2 shows the infrared surface brightness profile (Trager et al. 1995) and the X-ray point-source distribution (King-profile from Heinke et al. 2006). Both profiles are scaled to match the first diffuse X-ray data point, using an exponential fit in the case of the infrared data.
![]() |
Figure 3: Top: Chandra spectrum from the outer annulus ( 175-246'') with a 2-temperature non-equilibrium ionization model fit (stepped red line). All parameters are fixed to the values from Ebisawa et al. (2005) except for the surface brightnesses of the two components. Bottom: Chandra spectrum from the inner annulus ( 55-175'') with the spectrum from the outer annulus subtracted as background. The fit is an absorbed power-law model (red stepped line). The parameters for both fits are given in Table 1. |
Open with DEXTER |
To investigate the nature of the diffuse excess emission observed from
the inner region in more detail and to improve the statistical quality,
we extracted the combined spectrum from the inner five rings (
55-175''). As a first step we
fitted the same 2-T NEI model to the NXB subtracted inner spectrum,
binned to a minimum of 20 excess counts per bin, as was done for the
outer region in the previous section. The resulting surface fluxes of
the two components are listed in Table 1 (Inner
region). Following the same argument as in the previous section, we
estimate that in this case only 1/3 of the total observed emission is of diffuse
Galactic origin. Together with the surface brightness showing a clear
radial dependence with respect to the core of Terzan 5, we
conclude that a significant part of the observed flux is connected to
the GC.
As an estimate for the Galactic diffuse background component,
we subtracted the outer (
175-246'', see previous
section) from the inner spectrum. Figure 3 (Bottom)
shows the resulting excess spectrum from the inner region, binned to a
minimum of 20 excess counts per bin, together with an absorbed
power-law model fit. The spectral parameters are collected in
Table 1
(Inner region). Fitting a thermal plasma (MEKAL, )
or a thermal bremsstrahlung model (BREMSS,
=
1.4(50)) to the spectrum gave temperatures kT>17 keV
and kT>25 keV, respectively. The
most prominent feature of the Galactic thermal emission observed from
the outer region is an emission line, centered on
1.8 keV.
Introducing a Gaussian line at that energy or an additional thermal
component to the excess spectrum from the inner annulus does not
significantly improve the fit (
= 1.2). Therefore, we conclude that the spectrum from the outer region
describes the Galactic diffuse background component sufficiently. The
total unabsorbed diffuse excess flux in the 1-7 keV band
measured from the inner region above the Galactic background is
erg cm-2 s-1.
Assuming a distance of 5.5 kpc the intrinsic luminosity is
erg s-1.
We found no indication of a variation in the spectral index with increasing radius, when subdividing the inner region into two or more sub-regions. Furthermore, there is no evidence of a directional variation in the index and flux. Table 1 (north, east, south, and west regions) illustrates the result for directional dependence of spectra extracted from pie-shaped regions towards the north, east, south and west with respect to the cluster center (Fig. 1). Their inner and outer radii are 60'' and 120'', respectively. The latter value was chosen such that the southern region is not truncated by the FoV. As background we again used the spectrum from the outer annulus. A similar result was achieved when the regions were rotated by 45 degrees.
3 Origin of the diffuse emission
The present results indicate GC-centered diffuse hard X-ray excess
emission above Galactic background, which extends significantly beyond .
In this section we briefly discuss standard thermal and non-thermal
emission scenarios, leaving out more exotic possibilities, as described
by, e.g., Domainko
& Ruffert (2005). Throughout this section we use a
distance to Terzan 5 of 5.5 kpc. The larger distance
estimate (8.7 kpc) would increase the energy requirements for
the models by a factor of 2.5.
3.1 Contribution from unresolved point sources
The luminosity of unresolved point sources inside
has been estimated by Heinke
et al. (2006) to
erg s-1.
They furthermore determined the spatial surface distribution of X-ray
sources in Terzan 5 to
with q=1.43. From this distribution we expect to
find in the 1-3 arcmin annulus only 9% of the luminosity of unresolved
point sources within
.
The expected
erg s-1
is much lower than the measured emission (
erg s-1,
so we conclude that the contribution from unresolved point sources is
negligible.
3.2 Synchrotron radiation
One possibility for producing non-thermal emission by relativistic
electrons is synchrotron radiation emission (SR), which would radiate
at a frequency
sin
MHz,
where B is the strength of the magnetic field,
the Lorentz factor of the electrons, and
the pitch angle between the magnetic field and the electron velocity
(Ok07). Following Ok07, electrons with an energy of
1014 eV
would be needed to produce SR emission in typical Galactic magnetic
fields of a few
G
in the keV regime. The population of MSPs in the center of
Terzan 5 was suggested as a continuous source of such
highly-energetic electrons (Bednarek & Sitarek 2007;
Venter
et al. 2009). These particles propagate to the
observed extension of the diffuse emission of 3' (4.8 pc) on a
timescale of
years,
assuming Bohm diffusion (VJ08). The cooling of electrons with energies
of
eV in GCs is
dominated by SR emission with typical cooling times of
years
(VJ08). Assuming an injection spectrum with index -1, SR cooling, which
depends linearly on the energy of the electrons, should change the
index to -2. Since no such steepening of the spectrum is observed at
the 2
level,
is required, which would limit the magnetic field to
1
G or it
would indicate a faster diffusion of electrons. In this scenario, the
population of highly energetic electrons has to radiate the observed
X-ray luminosity (
erg s-1)
on a timescale of
,
so would require a total energy in these electrons of
erg.
Associated IC radiation in the TeV energy range should be detectable in
spatial coincidence in the case of low magnetic fields
few
G,
providing a test for this scenario (VJ08).
3.3 Inverse Compton emission
Non-thermal X-ray emission in GCs can also be produced by IC up-scattering of star-light photons by mildly relativistic electrons (Krockenberger & Grindlay 1995). The bow shock of the GC could provide these electrons (Ok07). The power of IC radiation









3.4 Non-thermal bremsstrahlung
One additional emission process of non-thermal X-rays is
non-thermal bremsstrahlung, which is produced when energetic electrons
are deflected by protons and nuclei. In this scenario the flux of the
emission should follow the distribution of target material. The
Galactic density profile of ISM perpendicular to the Galactic plane at
the relevant galactocentric distances of 1-3 kpc was
constrained as a single Gaussian with a full width at half maximum of
less than 200 pc and virtually no gas above 400 pc (Lockman 1984). At a
distance of 5.5 kpc, Terzan 5 would be at an offset
above the disk of 160 pc and thus in an ambient gas density of
a few times 0.1 cm-3. The total energy
in non-thermal electrons required for the emission of 2
erg s-1 of diffuse X-ray emission would be about
/0.1 cm-3) erg
if an electron energy of 20 keV is assumed (Ok07). In contrast
to the asymmetric morphology detected by Ok07 for GCs in a bow-shock
scenario, we did not find evidence of a non-uniform shape of the excess
emission from Terzan 5. This scenario could be tested by the
presence of target material in the environment of the GC. Target
material in the form of molecular clouds could be probed by carefully
examining molecular emission lines that are shifted by the relative
Galactic rotation velocity at the
physical location of Terzan 5.
3.5 Thermal contribution
The very high fitted temperature (>15 keV) of
the diffuse X-ray
emission would suggest a non-thermal origin. However, at least a
thermal
contribution to the total excess cannot be excluded at this point. If
thermal bremsstrahlung is presumed as the emission mechanism, the
temperature of the plasma can be estimated from the X-ray luminosity,
the
volume of the emission region, and the density of the plasma (Krockenberger & Grindlay
1995). With the radius of the emission region set to
5 pc (which corresponds to 188'' at a distance of
5.5 kpc), this leads to

Assuming that the observed emission is entirely thermal, i.e.,



To heat plasma to such high temperatures, strong shocks would be indispensable. The remnants of catastrophic events may release such strong shocks (see, e.g., Acero et al. 2007, for a remnant of a supernova Ia and Domainko & Ruffert 2005, for remnants of compact binary mergers). It was proposed that Terzan 5 may host the required mergers; e.g., Shara & Hurley (2002) for white dwarf mergers and Grindlay et al. (2006) for neutron star - neutron star mergers. However, even supernova remnants may have difficulty producing such high temperatures, because even the hot, thermal plasma in the young remnant of the type Ia supernova remnant SN 1006 reaches a temperature of about 2 keV (e.g. Acero et al. 2007), significantly cooler than the temperature found for the thermal fit to the diffuse emission in Terzan 5.
If the diffuse X-ray emission is indeed thermal, it could also originate in principle from a background galaxy cluster that by chance coincides with the core of Terzan 5. Since galaxy clusters with temperatures >10 keV are very rare (e.g. Reiprich & Böhringer 2002), such a correlation appears rather unlikely.
From the available data, a contribution from thermal emission processes to the measured flux cannot be ruled out, but it is not likely to represent the dominant fraction.
4 Conclusions
We discovered diffuse hard X-ray emission from the GC Terzan 5 with a photon index of about 1 and a peak flux density profile centered on the cluster core. The hard photon index makes a purely thermal emission scenario unlikely. Energetics would favor an SR scenario as the origin of the emission and would challenge simple IC and non-thermal Bremsstrahlung models generated by electrons accelerated by the bow shock of the GC. However, no simple model is clearly preferred to explain the observed emission, as expected from the limited statistics provided by the available X-ray dataset. Additional X-ray observations, detailed multi-wavelength informations, as well as refined modeling are needed to accurately interpret the unique properties of the diffuse X-ray radiation in Terzan 5.
AcknowledgementsThis research has made use of data obtained from the Chandra Data Archive and software provided by the Chandra X-ray Center (CXC) in the application packages CIAO and ChIPS. We thank the referee for the very constructive feedback.
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All Tables
Table 1: Extraction regions and results from spectral fitting.
All Figures
![]() |
Figure 1:
Smoothed, exposure corrected and NXB subtracted Chandra
image of diffuse X-ray emission in the 1-7 keV band around
Terzan 5. Excluded regions from point sources and the region
inside |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Radial dependence of the observed diffuse X-ray surface flux above the
particle background in the 1-7 keV band as seen with Chandra
(black crosses with error bars). Stars (red) denote the infrared
surface brightness profile from Trager
et al. (1995). The solid curves (green) show the
X-ray point-source distribution described by a generalized King-profile
(Heinke et al. 2006)
for the two extreme cases |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Top: Chandra spectrum from the outer annulus ( 175-246'') with a 2-temperature non-equilibrium ionization model fit (stepped red line). All parameters are fixed to the values from Ebisawa et al. (2005) except for the surface brightnesses of the two components. Bottom: Chandra spectrum from the inner annulus ( 55-175'') with the spectrum from the outer annulus subtracted as background. The fit is an absorbed power-law model (red stepped line). The parameters for both fits are given in Table 1. |
Open with DEXTER | |
In the text |
Copyright ESO 2010
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