Issue |
A&A
Volume 514, May 2010
|
|
---|---|---|
Article Number | A76 | |
Number of page(s) | 14 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361/201014126 | |
Published online | 26 May 2010 |
GMRT observations of the Ophiuchus galaxy cluster
M. Murgia1 - D. Eckert2 - F. Govoni1 - C. Ferrari3 - M. Pandey-Pommier4 - J. Nevalainen5 - S. Paltani6
1 - INAF - Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy
2 - INAF - IASF-Milano, via Bassini 15, 20133 Milano, Italy
3 -
UNS, CNRS UMR 6202 Cassiopée, Observatoire de la Côte d'Azur, Nice, France
4 - CRAL - Observatoire de Lyon, 9 avenue Charles André, 69230 Saint-Genis-Laval, France
5 - Department of Physics, PO Box 48, 00014 University of Helsinki, Finland
6 -
ISDC, Université de Genève, 16, Ch. d'Ecogia, 1290 Versoix, Switzerland
Received 25 January 2010 / Accepted 10 February 2010
Abstract
Aims. Observations with the Very Large Array telescope at
1477 MHz revealed the presence of a radio mini-halo surrounding
the faint central point-like radio source in the Ophiuchus cluster of
galaxies. In this work we present a study of the radio emission from
this cluster of galaxies at lower radio frequencies.
Methods. We observed the Ophiuchus cluster at 153, 240, and 614 MHz with the Giant Metrewave Radio Telescope.
Results. The mini-halo is clearly detected at 153 and
240 MHz, the frequencies at which we reached the best sensitivity
to the low-surface brightness diffuse emission, while it is not
detected at 610 MHz because of the too low signal-to-noise ratio
at this frequency. The most prominent feature at low frequencies is a
patch of diffuse steep spectrum emission located at about 5 south-east
from the cluster centre. By combining these images with that at
1477 MHz, we derived the spectral index of the mini-halo.
Globally, the mini-halo has a low-frequency spectral index of
and an high-frequency spectral index of
.
Moreover, we measure a systematic increase of the high-frequency spectral index with radius: the azimuthal radial average of
increases from about 1.3, at the cluster centre, up to about 2.0 in the mini-halo outskirts.
Conclusions. The observed radio spectral index agrees with that
obtained by modelling the non-thermal hard X-ray emission in this
cluster of galaxies. We assume that the X-ray component arises from
inverse-Compton scattering between the photons of the cosmic microwave
background and a population of non-thermal electrons, which are
isotropically distributed and whose energy spectrum is a power law with
an index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of
up to a maximum Lorentz factor of
with an index
.
The volume-averaged strength for a completely disordered intra-cluster magnetic field is
G.
Key words: galaxies: clusters: individual: Ophiuchus - radio continuum: galaxies
1 Introduction
Galaxy clusters, the largest gravitationally bound structures in the
Universe, are still forming at the present epoch by a merging of nearly
equal-mass systems or accretion of groups and field galaxies. They are
excellent laboratories to study the baryonic cosmic fraction as well as
the interplay between baryonic and dark matter in the formation and
evolution process of large scale structures (e.g. Arnaud et al. 2009;
Kravtsov et al. 2009).
In the last twenty years important progress has been made in the study
of galaxy clusters, of the thermal intra-cluster medium (ICM) and of
their interaction (e.g. Boselli & Gavazzi 2006; Markevitch & Vikhlinin 2007). Much less is known though about the physical properties and the
origin of a non-thermal intra-cluster component (relativistic electrons
with energies of 10 GeV spiralling in magnetic fields of few
Gauss) that was discovered and studied mostly through deep
radio observations (see e.g. Ferrari et al. 2008, and references therein).
However, it is now clear that the impact of the non-thermal component
in the physics and thermo-dynamical evolution of galaxy clusters cannot
be neglected anymore (e.g. Dursi & Pfrommer 2008; Parrish et al. 2009).
Intra-cluster relativistic electrons radiate through synchrotron emission in the radio domain, but also through inverse Compton scattering of cosmic microwave background (CMB) photons in the hard X-ray (HXR) band. The diffuse non-thermal component is now well detected at radio wavelengths in about 30 clusters (Giovannini et al. 2009). Only a few X-ray satellites allowed possible but controversial detection of a hard tail in the X-ray spectrum of about ten clusters (see e.g. Fusco-Femiano et al. 2003; Nevalainen et al. 2004; Rephaeli et al. 2008). Very recent results either agree with an HXR non-thermal detection (e.g. Eckert et al. 2008) or suggest a possible thermal origin of the detected HXR emission (e.g. Kawano et al. 2009).
Table 1: Details of the GMRT observations.
The Ophiuchus cluster (z=0.028, Johnston et al. 1981) is one of
the brightest clusters of galaxies in the X-ray band. It is an
extremely interesting target for non-thermal cluster studies, because it
shows evidence of both radio and possibly HXR emission (Eckert et al.
2008; Govoni et al. 2009; Murgia et al. 2009; Nevalainen et al. 2009).
The dynamical state of the Ophiuchus cluster has been strongly debated
in the last years. A recent Chandra study (Million et al. 2009) shows
evidence of a recent merger event in the central region of the cluster
of
arcmin2 (but see also Fujita et al. 2008, for an opposite conclusion
based on Suzaku data). In addition several clusters and groups
of galaxies have been detected within a distance of 8
from
the cluster centre, indicating that Ophiuchus is in a supercluster
environment (Wakamatsu et al. 2005).
By analysing Very Large Array (VLA) data of Ophiuchus at 1477 MHz, Govoni et al. (2009) recently detected a radio mini-halo surrounding the faint central
point-like radio source. Radio
mini-halos are diffuse steep-spectrum (
;
)
sources, permeating the central regions of relaxed,
cool-core galaxy clusters. They usually surround a radio galaxy. These
diffuse radio sources are extended on a moderate scale (typically
500 kpc) and, in common with large-scale halos observed in
merging clusters of galaxies, have a steep spectrum and a very low
surface brightness. As a consequence of their relatively small angular
size and possibly strong radio emission of the central radio galaxy,
radio mini-halos are very elusive sources, and our current observational
knowledge of mini-halos is limited to only a handful of well-studied
clusters.
Based on current observational and theoretical analyses, radio emission from mini-halos would be due to a population of relativistic electrons ejected by the central AGN and re-accelerated by MHD turbulence, whose energy is, in turn, supplied by the cluster cooling-core (Gitti et al. 2004). Recent analysis of the most X-ray luminous cluster (RX J1347-1145) suggests that additional energy for electron re-acceleration in mini-halos might be provided by sub-cluster mergers that have not been able to destroy the central cluster cooling-core (Gitti et al. 2007). Ophiuchus is the second known cluster showing a radio mini-halo, and also a cool-core that has survived a possible recent merging event (Nevalainen et al. 2009; Million et al. 2009). Indeed, Burns et al. (2008) simulated the formation and evolution of galaxy clusters, and showed that cool-core clusters can accrete mass over time and grow slowly via hierarchical mergers; when late mergers occur, the cool-cores survive the collisions.
Eckert et al. (2008) measured a high confidence level (6.4)
HXR excess in Ophiuchus through INTEGRAL
observations. This emission may be of non-thermal origin, caused
presumably by Compton scattering of cosmic microwave background
radiation by the relativistic electrons responsible for the mini-halo
emission (see e.g., Rephaeli et al. 2008; Petrosian et al. 2008, and references therein for reviews). Alternative
explanations have also been put forward (Profumo 2008; Pérez-Torres et al. 2009; Colafrancesco & Marchegiani 2009). The HXR excess
detection in Ophiuchus was recently confirmed by Nevalainen et al.
(2009). In addition, their joint INTEGRAL and XMM analysis partly reconciled the previous discrepancy between the results by Eckert
et al. (2008) and the upper limits on HXR flux obtained by Ajello et al. (2009) through Swift/BAT data.
Ophiuchus is thus one of the few clusters of galaxies in which the non-thermal component is revealed both in the radio and in the HXR bands. For this reason it is particularly interesting to investigate the radio spectrum of the mini-halo. By combining this information with the observed properties of the HXR emission it would be possible to derive important constraints on the energy spectrum of the synchrotron electrons. In particular, by assuming that the synchrotron emission and the HXR excess are co-spatial and produced by the same population of relativistic electrons, their comparison would allow the determination of the cluster magnetic field (Nevalainen et al. 2009).
In this work we present a study of the radio emission from the Ophiuchus
cluster of galaxies at low radio frequencies. We observed the Ophiuchus
cluster at 153, 240, and 614 MHz with the giant metrewave radio
telescope (GMRT). Throughout this paper we assume a CDM cosmology with H0 = 71 km s-1 Mpc-1,
,
and
.
At the distance of Ophiuchus (z=0.028), 1
corresponds to 0.55 kpc.
2 GMRT observations
We observed the cluster of galaxies Ophiuchus using the GMRT at the
frequencies of 153, 240, and 614 MHz (program OJ1712). The GMRT
antennas were pointed at RA = 1712
28
and Dec = -23
22
06
(J2000). The visibilities were acquired in
spectral line mode in order to reduce the bandwidth smearing effect and
to facilitate the excision of narrow band radio frequency interferences (RFIs).
We summarize the details of the observations in Table 1, where
we provide the frequency and total bandwidth, observation date, total
time on source, beamwidth (FWHM) of the full array, and rms
level (1)
in the full resolution images. Calibration and imaging
were performed with the NRAO astronomical image processing system
(AIPS).
The Ophiuchus cluster has already been observed with the GMRT at 240 and 610 MHz by Pérez-Torres et al. (2009), who did not detect the diffuse mini-halo emission at the noise level of their images. The observations presented here, however, have on average an exposure time about a factor 4.5 longer and hence they permit us to go deeper in sensitivity.
![]() |
Figure 1:
Top left: GMRT radio iso-contours at 153 MHz at full
resolution overlaid to the ROSAT PSPC X-ray image of the Ophiuchus
galaxy cluster. The radio image has an FWHM beam of
|
Open with DEXTER |
2.1 153 MHz
The observations were performed using a central frequency of 153 MHz and a
bandwidth of 8 MHz for both the upper and lower sideband (USB and LSB).
The data were collected in spectral line mode with 128 spectral channels
of 62.5 kHz in width. The observations consist of two distinct runs of
about 5.2 and 4.7 h on source, performed on 2008 Aug. 21 and 22,
respectively. The flux density scale and the bandpass were calibrated
for both runs with the primary calibrator 3C 286.
An initial amplitude solution for the bandpass calibrators was first
obtained for a central channel free of RFIs. This channel was then
used as reference in task BPASS to derive a bandpass solution.
The bandpass solution was visually inspected
and the most obvious RFIs were carefully excised. This
procedure was repeated several times until a refined bandpass solution
was obtained. Task FLGIT was applied to the whole dataset and about 30%
of the data were automatically removed because of the contamination from
strong RFIs. We ran task SPLAT to apply the bandpass calibration and to
reduce the number of channels from 128 to six channels of 1 MHz width
each. We calibrated the final data set in both phase and amplitude. The
phase calibration was completed with the secondary calibrators
1830-360 and 1833-210, observed at intervals of 30 min.
Low-level residual RFIs were carefully removed from the six-channels
dataset by visual inspection, and finally the Ophiuchus
data were extracted with task SPLIT and imaged.
Several cycles of self-calibration were applied to remove residual phase
variations.
We ran task IMAGR in 3D mode over a mosaic of slightly overlapping
fields to account for the non-coplanarity of the incoming wavefront
within the large primary beam of
.
![]() |
Figure 2:
Central panel: GMRT radio iso-contours at 240 MHz of the Ophiuchus galaxy cluster at full resolution. The radio image has a FWHM beam of
|
Open with DEXTER |
We first calibrated the 153 MHz observations from the two runs independently
and then combined the two datasets with task DBCON,
performing a final self-calibration run. In the left panel of Fig. 1 we
present the radio image at 153 MHz resulting from the combination of the
data sets from the two observing days. The radio image has a FWHM beam
of
with PA = 35.1
and a noise level of 5 mJy/beam (1
).
2.2 240 MHz
The 240 MHz observations consist of two data sets of 4.6 and 4.3 h on
source, taken on 2008 Aug. 23 and 28, respectively. The Aug. 23
observations were performed for both the LSB and the USB with a total
bandwidth of 8 MHz splitted into 128 channels of 62.5 kHz in width. The
Aug. 28 observations were performed for the USB with a total bandwidth
of 8 MHz splitted into 64 channels of 125 kHz in width. The flux density
scale was calibrated by the source 3C 286. The source 1830-360
was observed at intervals of 30 min
and used as secondary phase and gain calibrator. The bandpass was
calibrated using the sources 3C 286 and 1830-360. Task FLGIT was
applied to the whole dataset, and about 26% of the data were eliminated
because of RFIs. Both data sets were averaged in frequency to six
channels of 1 MHz in width to reduce noise while still keeping the
bandwidth smearing effect under control. We processed the two data sets
separately applying several cycles of imaging and self-calibration. The
datasets were then combined with task DBCON, and we performed a final
self-calibration run. The contour levels of the full resolution
240 MHz image are shown in the central panel of Fig. 2. The radio image has a FWHM beam of
with PA = -25.7
and a noise level of
1.1 mJy/beam (1
).
2.3 614 MHz
The observations were performed on 2008 Aug. 28 for a total of
4.3 h on source. Data were recorded for both the USB and the LSB
with a 16 MHz bandwidth splitted in 128 channels of
125 kHz in width. The central frequencies of the USB and LSB are
606 and 622 MHz, respectively. The flux density scale was
calibrated by using the source 3C 286. The phase 1830-360 was
observed at intervals of 30 min. The bandpass was
calibrated using the sources 3C 286 and 1830-360. In order to improve the
signal to noise ratio, data were averaged in frequency by collapsing the
bandwidth to six spectral channels of 2 MHz in width. The six-channels
dataset was carefully inspected to excise the RFIs. Several
cycles of imaging and self-calibration were applied to remove the
residual phase variations.
The USB and the LSB were imaged separately and then averaged
together to produce a final image at a frequency of 614 MHz with a FWHM
beam of
and a noise level of 0.25 mJy/beam
(1
). Cut-outs of the 614 MHz image are shown in the insets of
Fig. 2.
3 Results
We analyse the results of the GMRT observations with particular emphasis on the determination of the radio spectrum of the cluster discrete sources and of the mini-halo.
3.1 Optical and radio properties
The Ophiuchus cluster is one of the most luminous X-ray galaxy clusters in the local Universe, but
its optical properties are not very well known because of its
unfortunate line-of-sight. The cluster lies in projection at only about 10 from the Galactic Centre and hence
it is highly obscured.
Recently, Ophiuchus was studied in the optical band
by Wakamatsu et al. (2005), who derived spectroscopic redshifts for about 200 galaxies
to within 5
from the cD galaxy at the centre of the cluster core.
The velocity dispersion of the Ophiuchus cluster is found to be
km s-1. Such a
large velocity dispersion is consistent with its high X-ray luminosity. Moreover, several
clusters and groups of galaxies are observed to within a distance of 8
from
the cluster centre, indicating that the Ophiuchus concentration may be
a supercluster comparable in richness to the Coma-A1367 system, as
early suggested by Djorgovski et al. (1990).
In the top left panel Fig. 1,
the overlay of the GMRT radio iso-contours at 153 MHz to the ROSAT
PSPC X-ray image of the Ophiuchus galaxy cluster is shown. The X-ray
image is in the 0.1-2.4 keV energy band and has been background
subtracted, divided by the exposure image, and smoothed with a Gaussian
kernel with
.
![]() |
Figure 3:
Radio images of the Ophiuchus cluster of galaxies at
|
Open with DEXTER |
The most relevant radio features we detected are
labeled A through E. At the centre of the cluster we detected a faint
point-source (labeled A), whose position is coincident with the peak of
the X-ray emission. This is the same point source detected with the VLA
at 1477 MHz by Govoni et al. (2009), see Fig. 1 top right panel. Radio source A is associated with the prominent cD galaxy, which lies at the centre of
the cluster (Fig. 1, bottom left panel).
Source A appears point-like at our resolution
and sensitivity. The brightest radio source in the field is source B, which is
located about 10 north to the centre. Source B is an extended
source with an angular size of
,
corresponding
to a projected linear size of about
kpc (see top-left
inset in Fig. 2). There is no obvious optical identification
for this radio source, whose rather peculiar morphology makes its
classification very uncertain. In fact, it lacks of any of the typical
features observed in ordinary radio galaxies, like core, jets, or lobes.
Source B is not a head-tail radio galaxy either. Rather, the source has an
amorphous filamentary structure. The south-east part is characterized by
several threads of radio emission emerging perpendicular to the source's
major axis. The north-west part of the source is composed by a single
filamentary feature whose tip bends backwards to form what resembles a
vortex-like structure. Instead, sources C and D are the typical cluster
tailed radio galaxies. Source C is about 300
long (165 kpc) and
is pointing north-east to south-west. Source D has a faint tail about
230
long (127 kpc) pointing from south to north.
The apparent difference in radial velocity between sources C and D and
the Ophiuchus cluster is
1000 km s-1. We assume here a mean
recession velocity of 9063 km s-1 for the central region of the
Ophiuchus cluster (Wakamatsu et al. 2005) and that the two radio
sources are associated to galaxies 2MASX J17115542-2309423 (cz = 8050 km s-1)
and 2MASX J17120908-2328263 (cz = 7469 km s-1) (Hasegawa et al. 2000).
These properties are further confirmed by the spectral ageing analysis presented in Sect. 3.3 and indicate that the ram-pressure model can easily explain the radio jet deflection.
The two galaxies could have this
high velocity with respect to the ICM either because they are individually infalling
towards the cluster centre or because they are part of
merging sub-clusters. The merger-induced bulk motion of the galaxies in
the ICM would then be responsible for bending the radio jets. The latter
scenario would agree with the results by Bliton et al. (1998),
who derived that narrow-angle tailed radio galaxies are preferentially
found in dynamically complex clusters.
Finally, we detected an extended patch of diffuse emission at about
5 south-west from the cluster centre. This feature, labelled E in
Fig. 1,
is the only part of the mini-halo visible in the 153 MHz GMRT
image at full resolution. This patch is also present in the
240 MHz image at full resolution shown in Fig. 2,
where few other ``fragments'' of the mini-halo can be observed all
around the cluster centre. At 614 MHz the diffuse emission of the
mini-halo is too faint to be detected at full resolution.
The VLA image at 1477 MHz shown in the top-right panel of Fig. 1 has a
very good sensitivity to the extended emission. Given the noise level
of about 0.1 mJy and the relatively high resolution of
provided by the VLA in D configuration, the 3
sensitivity level on the mini-halo diffuse emission is of 0.07
Jy/arcsec2. For comparison,
the sensitivities of the full resolution GMRT images are of 15.8, 8.9, and 13.5
Jy/arcsec2 at 153, 240, and 614 MHz. It should be considered that the observed surface brightness at 1477 MHz
of the mini-halo is at best
0.5
Jy/arcsec2 and hence the
minimum spectral index required to detect the mini-halo at 153 and 240 MHz is
.
The detection turns out to be prohibitive at 614 MHz because the required mini-halo spectral index should be as high as
.
Thus to improve the signal to noise ratio of the GMRT data,
we realized a set of images with natural weighting (ROBUST = 5 in IMAGR) and by tapering the
longest baselines in order to smooth the angular resolution to
.
This is the resolution adopted in the VLA study of the mini-halo at 1477 MHz by Murgia et al. (2009). The radio iso-contours of these images are presented in Fig. 3.
The degradation of the resolution of the images results in an improved sensitivity to the diffuse emission. At
resolution, the 3
sensitivities of the GMRT images are of 5.6, 3.1, and 1.6
Jy/arcsec2 respectively at 153, 240, and 614 MHz.
The mini-halo is clearly detected at 153 and 240 MHz, the frequencies at
which we reached the best sensitivity to the low-surface brightness
diffuse emission, while it is too faint to be detected at 614 MHz even at the
resolution. The most prominent feature of the mini-halo at low
frequencies is still the patch E, which is marginally visible also in
the VLSS at 74 MHz. However, the higher signal-to-noise ratio
achieved in the 153 and 240 MHz images at low-resolution reveals a
bridge of diffuse radio emission, which is aligned along the cluster
major axis and connects patch E, the central source A, and the
amorphous source B.
Table 2: Integrated spectra for cluster discrete sources.
3.2 Integrated radio spectra of discrete sources
Determining the radio spectra of the discrete sources is important
to understand their nature and, possibly, their connection with the
mini-halo. We analysed the integrated radio spectra of sources A,
B, C, and D by complementing the GMRT measurements at 153, 240 and
614 MHz with the flux densities available in literature. In
particular, we made use of the VLSS at 74 MHz, the WISH survey at
325 MHz, the NVSS at 1400 MHz and the VLA 1477 MHz image
by Govoni et al. (2009). The flux density of source A at the different frequencies has been determined by using AIPS
task JMFIT. Since sources B, C, and D are extended, we determined the flux density
by integrating their radio brightness down to the
3
isophote. All the VLSS flux densities at 74 MHz reported in this
work have been corrected for the clean bias, following the prescription of
Cohen et al. (2007). The integrated spectra are
reported in Table 2. The flux density uncertainties include a 5% absolute calibration error.
In Fig. 4 we present the plots of the integrated radio spectra along with a power law fit to the data. Overall, the GMRT measurements agree both with the fit and with the adjacent data points taken from the literature, thus providing a positive check of the flux density scale in our images.
Source A is a compact source with a steep spectrum. The spectral index
of the central source in the considered frequency window is
0.6, a typical value for radio sources.
The amorphous source B presents a power law radio spectrum with a spectral
index of about
.
This is a
quite usual value for active radio galaxies, which makes the interpretation of this object even
more puzzling. Indeed, although the distorted morphology of this radio
source recalls that of extreme relic sources in clusters of galaxies (see
e.g. Slee et al. 2001),
on the basis of its radio spectrum it cannot be classified as an
ultra-steep spectrum source. A 5 GHz archive VLA image (not shown)
suggests a very weak point-like source but without kpc-scale jets.
Indeed, it is not clear which mechanism is powering the relativistic
electrons in this extended radio source. One possibility could be that
source B is a relic radio source
revived by the adiabatic compression caused by a shock wave or a bulk
gas motion propagating thought the ICM (Enßlin & Gopal-Krishna 2001).
However, no particular X-ray feature is visible in coincidence
of the radio source neither in the ROSAT nor in the XMM-Newton images and
hence the origin of this peculiar radio source as well as its possible
relation with the mini-halo remain, at the moment, unclear.
Finally, the tailed sources C and D have similar integrated spectra that are well described by a power law with the index
.
![]() |
Figure 4:
Integrated radio spectra for the central source A (top-left), for the amorphous source B (top-right), and for the two tailed sources C and D (bottom panels).
The triangles represent the GMRT flux density measurement from this
work, while the dots are the data taken from the literature. The lines
represent the best fit of a power law with index |
Open with DEXTER |
![]() |
Figure 5: Spectral index profile between 153 and 1400 MHz along the tail for radio sources C (left panel) and D (right panel). Data points have been sampled in circular boxes of one beam-width, as shown in the inset. The line is the expected spectral index trend for the case of constant advance speed. |
Open with DEXTER |
3.3 Spectral ageing analysis of sources C and D
Sources C and D can be classified as narrow-angle-tails (NATs). This morphology is indicative of a strong relative velocity between the host galaxy and the ICM. The ram pressure exerted by the external gas bends the radio jets that merge together forming the characteristic tail of radio plasma. The relativistic electrons at the end of the tail must have been deposited first and hence their radio spectrum should be steeper because they suffered greater energy losses.
In Fig. 5 we present the spectral index profile between 153 MHz (GMRT) and 1400 MHz (NVSS) as a function of distance
from the host galaxy for sources C and D. Both the GMRT and the NVSS images
were convolved to the same resolution of
and re-gridded to a common pixel size to ensure a proper comparison.
For both sources, the spectral index increases systematically with the
increasing distance from the host galaxy, a typical behaviour observed
in many NATs.
The spectral steepening can be interpreted in terms of radiative losses
of the relativistic electrons. In particular, we assumed that the radio
spectrum is described by a JP model (Jaffe & Perola 1972),
characterized by a low-frequency zero-age power law with an index
and an exponential cut-off beyond a high-frequency break,
.
The break frequency is related to the radiative age of the relativistic electron,
,
and to the source's magnetic field, B, through
where the magnetic field is in


Following Parma et al. (1999),
we assumed that the radio plasma in the tail separates from the host
galaxy at a constant speed. In this case, the break frequency scales
,
where d is the distance along the tail. Given this trend for
,
we computed the expected spectral index on the basis of the JP model as a function of the distance d.
Close to the host galaxy (
)
the break frequency
;
the radio spectrum is a power law with the index
.
At an increasing distance from the host galaxy,
shifts to a low frequency and the radio spectrum steepens. By fitting
the observed spectral index profile, we derived the injection spectral
index and the lowest value for the break frequency. For source C we
found
and a minimum break frequency of
MHz at a distance of 120 kpc from the host galaxy. By using standard formulas (e.g. Pacholczyk 1970), we estimated the minimum energy magnetic field,
,
of the tail by integrating the radio luminosity from 100 MHz to 10 GHz
and by assuming a ratio between the energy density of relativistic protons to that of the electrons of k=1. For source C we obtained
G and, on the basis of Eq. (1), we calculated a radiative age of
Myr.
The corresponding advancing speed of the tail is
km s-1, in agreement with the dispersion velocity of galaxies in the Ophiuchus cluster.
Source D is fainter than source C, and thus the uncertainties on the best-fit parameters are larger. However, we found
and a lowest break frequency of
MHz
at a distance of about 40 kpc from the host galaxy. We calculated
for the radio source a minimum energy magnetic field of
G, and we estimated a radiative age of
Myr, which corresponds to an advancing speed for
the tail of
km s-1, i.e. smaller than that of source C.
It is worth noting that the estimated advancing velocities should be regarded as lower limits if the tail's length has been significantly shortened by projection effects.
3.4 The mini-halo spectral index image
The main goal of this work is to constrain the spectral index of the
mini-halo in the Ophiuchus cluster. This is a particularly hard task
because the mini-halo is very faint and extended. The best compromise
between sensitivity and resolution is obtained in the 240 MHz image
at a
resolution.
In the top-left panel of Fig. 6 we present the spectral
index image between 240 and 1477 MHz with the 240 MHz radio iso-contours overlaid.
The spectral index image is calculated only from those pixels whose brightness
is above the
level at both frequencies.
The overall radio spectrum of the mini-halo is steep. The spectral index
ranges from about
,
close to the cluster centre, up to about
in correspondence of patch E. In the top-right panel of Fig. 6 we show the spectral index uncertainty,
which is in the range from 0.05 to 0.2.
The bottom panels of Fig. 6 show the radio spectrum of the
mini-halo at four different sample positions.
In addition to the GMRT data, the spectra also include the measurements obtained from the
VLSS at 74 MHz and the VLA at 1477 MHz. All the images considered
were re-gridded to a common geometry and convolved at the same angular
resolution of
.
The radio spectra between 74 and 1477 MHz indicate no deviation from a
power law model, although this could be due to the comparatively small frequency range considered.
Patch E is also detected at 74 MHz in the VLSS and has a spectral index of
(see Fig. 6,
bottom-left spectrum). Patch E appears as an isolated feature at a
comparatively large distance from the centre of the cluster. It is
important to stress again though that the VLA image at 1477 MHz
has a much higher dynamic range compared to the GMRT image at
240 MHz. Indeed, there are regions of the mini-halo that are
clearly detected at 1477 MHz but are not represented in Fig. 6 simply because their spectrum is not steep enough.
3.5 Azimuthally averaged radial profile of the mini-halo emission
The GMRT images at 153 and 240 MHz at
resolution
have enough sensitivity to allow us the analysis of the azimuthally averaged radial
profiles of the mini-halo emission.
The surface brightness of mini-halos in clusters of galaxies decreases with
increasing distance from the cluster centre, eventually falling below
the noise level of the radio images. Although deviations of the diffuse
emission from spherical symmetry are often observed, the azimuthally
averaged radial profiles are indeed quite smooth and regular. We derived the
azimuthally averaged brightness of the Ophiuchus mini-halo at 153 and 240 MHz
and compare the result with the finding obtained with the VLA at 1477
MHz by Murgia et al. (2009).
![]() |
Figure 6:
Spectral index image of the mini-halo in the Ophiuchus cluster. The top left panel
shows the spectral index image of the mini-halo in the Ophiuchus
cluster between 240 and 1477 MHz at an angular resolution of
|
Open with DEXTER |
In the top panel of Fig. 7 we show the azimuthally averaged
radio halo brightness profiles obtained from the 153, 240 and 1477 MHz images
at 92
resolution. Each data point represents the average
brightness in concentric annuli of half beam width centred on the X-ray
peak. Discrete sources were masked out and
excluded from the statistics. We considered only data points whose observed brightness
was five times above the error on the radial average.
Following Murgia et al. (2009), in order
to carefully separate the contribution of the mini-halo from that of the
central radio galaxy, we fitted the total brightness profiles with a
central point source plus the radio mini-halo diffuse emission
The profile of the central point source was characterized by a Gaussian of the form
while the brightness profile of the mini-halo was characterized by an exponential law of the form
The best fit of the total model in Eq. (2) is represented by the solid lines in the top panel of Fig. 7. The contribution from the mini-halo exponential disc alone is represented by the dotted lines. The fit was performed in the image plane as described in Murgia et al. (2009). In order to properly take into account the resolution, the exponential model was first calculated in a 2-dimensional image, with the same pixel size and field of view as the observed one, and was then convolved with the same beam by means of a Fast Fourier Transform. The resulting image was masked exactly in the same regions as the observations. Finally, the model was azimuthally averaged with the same set of annuli used to obtain the observed radial profile. All these functions were performed at each step during the fit procedure. As a result, the values of the central brightness,


At 153 MHz the best fit of the exponential model yields a central
brightness of
Jy/arcsec2 and
kpc, while
at 240 MHz the best fit yields
Jy/arcsec2 and
kpc.
In the bottom panels of Fig. 7 we trace the spectral index radial
profile of the mini-halo between 153 and 240 MHz and between 240 and 1477 MHz.
The radial profile of the low-frequency spectral index between 153 and 240 MHz is fairly
constant to a value of about
.
On the other hand, the e-folding radius of the mini halo at low
frequency is more than four times larger than at 1477 MHz. This
results in a progressive steepening of the spectral index with the
increase of the distance from the cluster centre. The spectral index
between 240 and 1477 MHz increases steadily from
at the cluster centre, and up to
at the mini-halo periphery.
![]() |
Figure 7: Azimuthally averaged radial profiles of the mini-halo radio intensity at 153, 240 and 1477 MHz ( top panel). We considered only data points whose observed brightness is five times above the error on the radial average. The solid lines represent the best fit of the exponential disc model plus the contribution by the central point source. The dotted lines represent the exponential disc model alone. The mid and bottom panels show the radial profiles of the spectral index between 153 and 240 MHz and between 240 and 1477 MHz, respectively. |
Open with DEXTER |
Table 3: Properties of the Ophiuchus mini halo derived from the fit procedure.
We calculated the model flux densities in an area with a radius of
(corresponding to about 230 kpc) from the cluster centre. This is the extraction region of the HXR emission
used by Nevalainen et al. (2009).
The mini-halo flux densities were obtained by the formula (Murgia et al. 2009):
![]() |
(5) |
where we set

We obtained
S153=2900 mJy,
S240=1560 mJy, and
S1477=85 mJy.
The mini-halo is not detected at 74 and 614 MHz, the upper limit to the flux density
in a 7 area are of
S74<48 500 mJy and
S614<900 mJy, respectively. These limits were
calculated with the assumption that the average surface brightness of
the diffuse emission is lower than the
noise level of the corresponding radio image.
The global radio spectrum of the mini-halo is shown in Fig. 8. The low and high frequency spectral indices are
and
respectively. Indeed, a hint of spectral steepening is seen at high
frequency, although the two values are still compatible to within the
errors.
It is worthwhile to note that the global low-frequency radio spectral index is fully consistent with the reported range of 1.2-1.5 obtained in the XMM-Newton/INTEGRAL analysis by Nevalainen et al. (2009), see the next section for further details.
4 Discussion
The Ophiuchus galaxy cluster is one of the very rare clusters where the non-thermal component is revealed both in the radio and X-ray bands.
Assuming that the X-ray component arises from inverse-Compton (IC)
scattering from the non-thermal electrons with the photons of the CMB,
the volume-averaged cluster magnetic field, ,
can be derived
essentially from the ratio between the power emitted through synchrotron and IC
(see e.g. Blumenthal & Gould 1970).
Using XMM-Newton and INTEGRAL spectra of the cluster and fixing the spectral
index of the power law to 1.4 (this work), we extracted the fluxes of
the non-thermal component in five different X-ray energy bands (0.6-2, 2-5, 5-10,
20-40, and 40-80 keV). For the details of the data analysis and the
modelling of the different thermal components, we refer to
Nevalainen et al. (2009). The total non-thermal flux in the 20-80 keV band is
ergs s-1 cm-2, in agreement with the upper
limits derived from Swift (Ajello et al. 2009) and Suzaku
(Fujita et al. 2008) data.
Using these measurements together with the radio fluxes presented in Table 3, we constructed a spectral energy distribution (SED) of the non-thermal emission (see Fig. 8). We also added to the SED the upper limits at 74 MHz (VLSS) and 607 MHz (GMRT), as well as the Swift upper limit (Ajello et al. 2009).
![]() |
Figure 8:
Spectral energy distribution of the non-thermal emission
(plotted as
|
Open with DEXTER |
4.1 SED modelling
To model the SED, we used the exact derivation of the synchrotron and IC
spectrum for a single electron from Blumenthal & Gould (1970), and numerically
convolved the resulting spectrum with the differential distribution of
non-thermal electrons
.
Here
is the electron's Lorentz factor, while
is the
pitch angle between its velocity and the local direction of the magnetic
field. Indeed, d
d
d
is the number
density of non-thermal electrons with a Lorentz factor between
and
and a pitch angle between
and
.
We assumed that the energy spectrum of the non-thermal electrons is described by a power law
with the index p, with a high-energy cut-off
and a low-energy
cut-off
where we consider an isotropic distribution of pitch angles.
The radio emissivity as a function of the pitch angle in a uniform
magnetic field of strength B is given by
where

The constant

Due to the high-beaming of the synchrotron radiation pulses, only relativistic
electrons with a pitch angle
will be observed
if the magnetic field direction shows an angle
with respect to the line-of-sight.
In this work, we suppose that the intra-cluster magnetic field is completely tangled in an infinitesimally
small scale compared to the mini-halo size. With this assumption, the synchrotron
emissivity averaged over all the possible magnetic field directions
is
By using standard formulas, we calculated the HXR emission deriving from the inverse Compton scattering of the CMB photons by the same population of relativistic electrons in Eq. (6):
where





The synchrotron and inverse Compton emissivities are converted to flux densities by
multiplying Eqs. (9) and (10) by a volume of
cm3 (that is the volume of a sphere whose radius corresponds to a projected distance of
from the cluster centre) and then by dividing by the cluster's luminosity distance, DL, i.e.
.
We then fitted our model to the data and extracted the relevant physical parameters. The
resulting best-fit model is shown as a line in Fig. 8, while
the best-fit parameters are reported in Table 4 along with their 1
uncertainties.
The two upper limits at 74 and 614 MHz and that in the
20-60 keV band were not considered in the fit because they have no
influence on the
statistics.
Unfortunately, the available HXR data did not allow us to precisely constrain the value
of the low-energy cut-off, but only to place an upper limit at
.
We indeed decided to fix this parameter to the arbitrary value of
,
see Sect. 4.2.
Table 4: Best fit parameters of the spectral energy distribution of the non-thermal emission from the Ophiuchus cluster.
The radio and HXR data are consistent with a slope for the energy spectrum of the relativistic electrons of
and with a high-energy cut off
.
The power law index p is related to the radio spectral index via the relation
.
Indeed, the global mini-halo radio spectrum can be described by a
low-frequency power law spectral index of
followed by a high-frequency break at
MHz.
The derived volume-average magnetic field value
is found to be
![]() |
(11) |
This estimate depends only weakly on the values of the spectral parameters





An important point to discuss is that given a magnetic field strength of
G,
only high-energy electrons with
can emit at radio frequencies of 153 MHz and above (see Eq. (8)).
On the other hand, the HXR emission would be tracing relativistic electrons
of lower energy, with characteristic Lorentz factors in the range 103-104.
Indeed, the radio and HXR emissions are not tracing exactly the same
particles. Nevertheless, the low-frequency spectral index measured in
this work,
,
is consistent with the scenario in which the energy spectrum of the
synchrotron electrons most probably belongs to the extrapolation at
higher energies of the power law energy distribution of the electrons
radiating in the HXR band through the inverse Compton process.
4.2 Energetics and particle life-time
The total pressure from the non-thermal electrons is given by
![]() |
(12) |
The non-thermal energy pressure depends critically on the low energy cut-off of the particle distribution. With the choice of







Another important consideration is that the energy density of the relativistic
electrons is four orders of magnitude higher than the energy density of the magnetic field,
erg cm-3.
This result would imply that the mini-halo is not in a minimum energy
condition, which requires instead that the energy densities of
particles and field should be nearly equipartited.
This result holds even in the case where the energy distribution is
truncated at a
,
that is the upper limit we can place
on the basis of the current HXR data.
The radiative life-time of the relativistic electrons can be estimated as
where the magnetic field is in


Since
,
the radiative losses are dominated by the inverse Compton process, which
cools down the high-energy electrons with
(i.e. those radiating at
153 MHz) in a time-scale of
yrs. This is a relatively short time compared to
the diffusion time needed by the relativistic electrons to cross the mini-halo.
If we suppose (Melrose 1968) that the relativistic electrons diffuse at the Alfvén speed
![]() |
(14) |
where the thermal gas density is expressed in units of 10-3 cm-3, the characteristic diffusion length of the synchrotron electrons radiating at 153 MHz results in just

It is important to stress that the interpretation of the SED
presented above relies on a simplified cluster model, in which the
relevant physical properties,
and B,
represent volume-averaged quantities. It is clear, however, that a much
more detailed modelling is needed to explain the spatial variations of
the spectral index observed across the mini-halo. Particularly, the
systematic increase of the mini-halo spectral index with the radius
shown in the bottom panel of Fig. 7 may indicate that the high-frequency break
,
and hence either
and/or the magnetic field B (see Eq. (8)), decrease from the cluster centre outward. Another possibility is that just the slope, p, and the normalisation, K0,
of the energy spectrum of the non-thermal electrons are changing with
radius. This could be supported by the radio spectrum of patch E, which
shows no evidence for a strong spectral curvature (see Fig. 7).
Disentangling these scenarios is not easy with the current data. Future
spatially resolved analyses of both the radio and HXR emissions could
shed light on this important issue.
5 Conclusions
In a search for diffuse radio emission in relaxed, cool-core galaxy clusters at 1.4 GHz, Govoni et al. (2009) found the presence of a mini-halo surrounding the faint central point-like radio source in the Ophiuchus cluster of galaxies. Murgia et al. (2009) analysed the radio properties of this diffuse radio source in comparison to other mini-halos and radio halos known in the literature and found that Ophiuchus is characterized by a brightness and size very similar to that of the smaller halos rather than to that of the prototypical mini-halo in the Perseus cluster (e.g. Burns et al. 1992). In this work we presented a study of the radio emission of the Ophiuchus cluster of galaxies at low radio frequencies performed at 153, 240, and 614 MHz with the GMRT.
The mini-halo is clearly detected at 153 and 240 MHz, the frequencies at which we reached the best sensitivity to the low-surface brightness diffuse emission, while at 614 MHz we only derived an upper limit to the mini-halo emission. By combining these images with the VLA data at 1477 MHz from Govoni et al. (2009) and with the VLSS upper limit at 74 MHz, we derived the spectral index of the mini-halo.
Globally, the mini-halo has a low-frequency spectral index of
,
with a hint of steepening at higher frequencies.
Moreover, we found indications that the high-frequency spectral index
increases with the increasing distance from the cluster centre.
The most prominent feature at low frequencies is a patch of
diffuse steep spectrum emission located at
about 5
south-east from the cluster centre.
The observed radio spectral index agrees with that obtained by
modelling the non-thermal HXR emission in this cluster of galaxies. We
assume that the X-ray component arises from inverse-Compton scattering
between the photons of the cosmic microwave background and a population
of non-thermal electrons, which are isotropically distributed and whose
energy spectrum is a power law with the index p. We derive that the electrons energy spectrum should extend from a minimum Lorentz factor of
up to a maximum Lorentz factor of
with an index
and that the volume-averaged strength for a completely disordered intra-cluster magnetic field is
G.
Given this magnetic field strength, only high-energy electrons with
can emit in the observed radio frequency window, while the HXR emission would be tracing relativistic electrons
of lower energy, with characteristic Lorentz factors in the range 103-104.
Indeed, the radio and HXR emissions are not tracing exactly the same
particles. Nevertheless, the low-frequency radio spectral index
measured in this work is consistent with the scenario in which
the energy spectrum of the synchrotron electrons most probably belongs
to the extrapolation at higher energies of the power law energy
distribution of the electrons radiating in the HXR band through the
inverse Compton process.
In addition to the mini-halo spectrum, we also analysed the
properties of the cluster discrete sources, specifically source A, the
radio source associated to the central cD galaxy, source B, the
brightest radio source in the field located at about 10 north to the cluster centre, and the two NATs C and D located in the cluster outskirts.
Source A is point-like at our highest resolution of 7
(corresponding to about 3.8 kpc)
and its spectral index is
,
a typical value for radio sources.
Source B is an extended source with no obvious optical identification and a
rather peculiar morphology, which makes its classification very uncertain.
The global spectral index of the source is
.
This is a
quite usual value for active radio galaxies, which makes the interpretation of this object even
more puzzling. In fact, although the distorted morphology of this radio
source recalls that of extreme relic sources in clusters of galaxies (see
e.g. Slee et al. 2001), on the basis of its radio spectrum it cannot be
classified as an ultra-steep spectrum source. A possibility could be that
source B is a relic radio source revived by the adiabatic compression caused by a
shock wave or a bulk gas motion propagating thought the ICM (Enßlin & Gopal-Krishna 2001).
However, the relation of this peculiar radio source with the mini-halo
remains at the moment unclear. Finally, the analysis of the radio
morphology and spectral properties of the tailed sources C and D
confirm that their host galaxies are moving at high velocity with
respect to the ICM either because they are individually infalling
towards the cluster centre or because they are part of merging
sub-clusters.
We acknowledge the anonymous referee for helpful comments that improved the paper. We thank the staff of the GMRT, who made these observations possible. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This work is part of the ``Cybersar'' Project, which is managed by the COSMOLAB Regional Consortium with the financial support of the Italian Ministry of University and Research (MUR), in the context of the ``Piano Operativo Nazionale Ricerca Scientifica, Sviluppo Tecnologico, Alta Formazione (PON 2000-2006)''. This research was partially supported by ASI-INAF I/088/06/0 - High Energy Astrophysics and PRIN-INAF 2008. C.F. acknowledges financial support by the Agence Nationale de la Recherche through grant ANR-09-JCJC-0001-01. J.N. is supported by the Academy of Finland. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
References
- Ajello, M., Rebusco, P., Cappelluti, N., et al. 2009, ApJ, 690, 367 [NASA ADS] [CrossRef] [Google Scholar]
- Arnaud, M., Böhringer, H., Jones, C., et al. 2009, Technical Report, [arXiv:0902.4890] [Google Scholar]
- Bliton, M., Rizza, E., Burns, J. O., et al. 1998, MNRAS, 301, 609 [NASA ADS] [CrossRef] [Google Scholar]
- Blumenthal, G. R., & Gould, R. J. 1970, Rev. Mod. Phys., 42, 237 [NASA ADS] [CrossRef] [Google Scholar]
- Boselli, A., & Gavazzi, G. 2006, PASP, 118, 517 [NASA ADS] [CrossRef] [Google Scholar]
- Burns, J. O., Sulkanen, M. E., Gisler, G. R., & Perley, R. A. 1992, ApJ, 388, L49 [NASA ADS] [CrossRef] [Google Scholar]
- Burns, J. O., Hallman, E. J., Gantner, B., et al. 2008, ApJ, 675, 1125 [NASA ADS] [CrossRef] [Google Scholar]
- Cohen, A. S., Lane, W. M., Cotton, W. D., et al. 2007, AJ, 134, 1245 [NASA ADS] [CrossRef] [Google Scholar]
- Colafrancesco, S., & Marchegiani, P. 2009, A&A, 502, 711 [Google Scholar]
- Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693 [NASA ADS] [CrossRef] [Google Scholar]
- De Breuck, C., Tang, Y., de Bruyn, A. G., Röttgering, H., & van Breugel, W. 2002, A&A, 394, 59 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Djorgovski, S., Thompson, D. J., de Carvalho, R. R., & Mould, J. R. 1990, AJ, 100, 599 [NASA ADS] [CrossRef] [Google Scholar]
- Dursi, L. J., & Pfrommer, C. 2008, ApJ, 677, 993 [NASA ADS] [CrossRef] [Google Scholar]
- Eckert, D., Produit, N., Paltani, S., Neronov, A., & Courvoisier T. J.-L. 2008, A&A, 479, 27 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Enßlin, T. A., & Gopal-Krishna 2001, A&A, 366, 26 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Ferrari, C., Govoni, F., Schindler, S., Bykov, A. M., & Rephaeli, Y. 2008, SSRv, 134, 93 [Google Scholar]
- Fujita, Y., Hayashida, K., Nagai, M., et al. 2008, PASJ, 60, 1133 [NASA ADS] [Google Scholar]
- Fusco-Femiano, R., dal Fiume, D., Orlandini, M., et al. 2003, ASP Conf. Ser., 301, 109 [NASA ADS] [Google Scholar]
- Gitti, M., Brunetti, G., Feretti, L., & Setti, G. 2004, A&A, 417, 1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Gitti, M., Ferrari, C., Domainko, W., Feretti, L., & Schindler, S. 2007, A&A, 470, 25 [Google Scholar]
- Giovannini, G., Bonafede, A., Feretti, L., et al. 2009, A&A, 507, 1257 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Govoni, F., Murgia, M., Markevitch, M., et al. 2009, A&A, 499, 371 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Jaffe, W. J., & Perola, G. C. 1973, A&A, 26, 423 [NASA ADS] [Google Scholar]
- Johnston, M. D., Bradt, H. V., Doxsey, R. E., et al. 1981, ApJ, 245, 799 [NASA ADS] [CrossRef] [Google Scholar]
- Kawano, N., Fukazawa, Y., Nishino, S., et al. 2009, PASJ, 61, 377 [NASA ADS] [Google Scholar]
- Kravtsov, A., Gonzalez, A., Vikhlinin, A., et al. 2009, Technical Report, [arXiv:0903.0388] [Google Scholar]
- Markevitch, M., & Vikhlinin, A. 2007, Phys. Rep., 443, 1 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Melrose, D. B. 1968, Ap&SS, 2, 171 [NASA ADS] [CrossRef] [Google Scholar]
- Million, E. T., Allen, S. W., Werner, N., & Taylor, G. B. 2010, MNRAS, in press [arXiv:0910.0025] [Google Scholar]
- Murgia, M., Govoni, F., Markevitch, M., et al. 2009, A&A, 499, 679 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Nevalainen, J., Oosterbroek, T., Bonamente, M., & Colafrancesco, S. 2004, ApJ, 608, 166 [NASA ADS] [CrossRef] [Google Scholar]
- Nevalainen, J., Eckert, D., Kaastra, J., Bonamente, M., & Kettula, K. 2009, A&A, 508, 1161 [NASA ADS] [CrossRef] [EDP Sciences] [Google Scholar]
- Pacholczyk, A. G. 1970, Series of Books in Astronomy and Astrophysics (San Francisco: Freeman) [Google Scholar]
- Parma, P., Murgia, M., Morganti, R., et al. 1999, A&A, 344, 7 [NASA ADS] [Google Scholar]
- Parrish, I. J., Quataert, E., & Sharma, P. 2009, ApJ, 703, 96 [NASA ADS] [CrossRef] [Google Scholar]
- Petrosian, V., Bykov, A., & Rephaeli, Y. 2008, Space Sci. Rev., 134, 191 [Google Scholar]
- Pérez-Torres, M. A., Zandanel, F., Guerrero, M. A., et al. 2009, MNRAS, 396, 2237 [NASA ADS] [CrossRef] [Google Scholar]
- Profumo, S. 2008, Phys. Rev. D, 77, 103510 [NASA ADS] [CrossRef] [Google Scholar]
- Rephaeli, Y., Nevalainen, J., Ohashi, T., & Bykov, A. M. 2008, Space Sci. Rev., 134, 71 [Google Scholar]
- Rybicki, G. B., & Lightman, A. P. 1979 (New York: Wiley-Interscience), 393 [Google Scholar]
- Sarazin, C. L. 1999, ApJ, 520, 529 [NASA ADS] [CrossRef] [Google Scholar]
- Slee, O. B., Roy, A. L., Murgia, M., et al. 2001, AJ, 122, 1172 [NASA ADS] [CrossRef] [Google Scholar]
- Wakamatsu, K., Malkan, M. A., Nishida, M. T., et al. 2005, in Nearby Large-Scale Structures and the Zone of Avoidance, ASP Conf. Ser., 329, 189 [Google Scholar]
Footnotes
- ... directions
- From simple geometric considerations it follows that the probability to observe an angle
between the local direction of a random magnetic field and the line-of-sight is proportional to
.
All Tables
Table 1: Details of the GMRT observations.
Table 2: Integrated spectra for cluster discrete sources.
Table 3: Properties of the Ophiuchus mini halo derived from the fit procedure.
Table 4: Best fit parameters of the spectral energy distribution of the non-thermal emission from the Ophiuchus cluster.
All Figures
![]() |
Figure 1:
Top left: GMRT radio iso-contours at 153 MHz at full
resolution overlaid to the ROSAT PSPC X-ray image of the Ophiuchus
galaxy cluster. The radio image has an FWHM beam of
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Central panel: GMRT radio iso-contours at 240 MHz of the Ophiuchus galaxy cluster at full resolution. The radio image has a FWHM beam of
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Radio images of the Ophiuchus cluster of galaxies at
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Integrated radio spectra for the central source A (top-left), for the amorphous source B (top-right), and for the two tailed sources C and D (bottom panels).
The triangles represent the GMRT flux density measurement from this
work, while the dots are the data taken from the literature. The lines
represent the best fit of a power law with index |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Spectral index profile between 153 and 1400 MHz along the tail for radio sources C (left panel) and D (right panel). Data points have been sampled in circular boxes of one beam-width, as shown in the inset. The line is the expected spectral index trend for the case of constant advance speed. |
Open with DEXTER | |
In the text |
![]() |
Figure 6:
Spectral index image of the mini-halo in the Ophiuchus cluster. The top left panel
shows the spectral index image of the mini-halo in the Ophiuchus
cluster between 240 and 1477 MHz at an angular resolution of
|
Open with DEXTER | |
In the text |
![]() |
Figure 7: Azimuthally averaged radial profiles of the mini-halo radio intensity at 153, 240 and 1477 MHz ( top panel). We considered only data points whose observed brightness is five times above the error on the radial average. The solid lines represent the best fit of the exponential disc model plus the contribution by the central point source. The dotted lines represent the exponential disc model alone. The mid and bottom panels show the radial profiles of the spectral index between 153 and 240 MHz and between 240 and 1477 MHz, respectively. |
Open with DEXTER | |
In the text |
![]() |
Figure 8:
Spectral energy distribution of the non-thermal emission
(plotted as
|
Open with DEXTER | |
In the text |
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