31 research outputs found
The redshift dependence of spectral index in powerful radio galaxies
We present and discuss in this paper the rest frame radio spectra (1-25 GHz) of a sample of fourteen radio galaxies at z > 2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028.8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies at z > 2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 < z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario
Low frequency follow up of radio halos and relics in the GMRT Radio Halo Cluster Survey
We performed GMRT low frequency observations of the radio halos, relics and
new candidates belonging to the GMRT Radio Halo Cluster Sample first observed
at 610 MHz. High sensitivity imaging was performed using the GMRT at 325 MHz
and 240 MHz. The properties of the diffuse emission in each cluster were
compared to our 610 MHz images and/or literature information available at other
frequencies, in order to derive the integrated spectra over a wide frequency
range.Beyond the classical radio halos, whose spectral index is in the
range (S), we found sources with
. This result supports the idea that the spectra of the
radiating particles in radio halos is not universal, and that inefficient
mechanisms of particle acceleration are responsible for their origin. We also
found a variety of brightness distributions, i.e. centrally peaked as well as
clumpy halos. Even though the thermal and relativistic plasma tend to occupy
the same cluster volume, in some cases a positional shift between the radio and
X-ray peaks of emission is evident. Our observations also revealed the
existence of diffuse cluster sources which cannot be easily classified either
as halos or relics. New candidate relics were found in A1300 and in A1682, and
in some clusters "bridges" of radio emission have been detected, connecting the
relic and radio halo emission. Combining our new data with literature
information, we derived the LogL-LogP correlation for
radio halos, and investigated the possible trend of the spectral index of radio
halos with the temperature of the intracluster medium.Comment: 19 pages, 10 figures, 5 tables, accepted for publication on A&
Low-frequency radio observations of Seyfert galaxies: A test to the unification scheme
Aims. We present low-frequency radio imaging and spectral properties of a
well defined sample of Seyfert galaxies using GMRT 240/610 MHz dual frequency
observations. Radio spectra of Seyfert galaxies over 240 MHz to 5.0 GHz are
investigated using 240 MHz, 610 MHz flux densities derived from GMRT, and 1.4
GHz and 5.0 GHz flux densities mainly from published VLA data. We test the
predictions of Seyfert unification scheme by comparing the radio properties of
Seyfert type 1s and type 2s. Methods. We choose a sample such that the two
Seyfert subtypes have matched distributions in parameters that are independent
to the orientation of AGN, obscuring torus and the host galaxy. Our sample
selection criteria allow us to assume that the two Seyfert subtypes are
intrinsically similar within the framework of the unification scheme. Results.
The new observations at 240/610 MHz, together with archival observations at 1.4
GHz, 5.0 GHz show that type 1s and type 2s have statistically similar radio
luminosity distributions at 240 MHz, 610 MHz, 1.4 GHz and 5.0 GHz. The spectral
indices at selected frequency intervals as well as index measured over 240 MHz
to 5.0 GHz for the two Seyfert subtypes have similar distributions with median
spectral index -0.7, consistent with the synchrotron emission from
optically thin plasma. In our snap-shot 240/610 MHz GMRT observations, most of
the Seyfert galaxies show primarily an unresolved central radio component,
except a few sources in which faint kpc-scale extended emission is apparent at
610 MHz. Our results on the statistical comparison of the multifrequency radio
properties of our sample Seyfert galaxies are in agreement with the predictions
of the Seyfert unification scheme.Comment: 21 pages, 5 figures, Accepted for publication in Astronomy &
Astrophysic
On the nature of Off-pulse emission from pulsars
In Basu et al. 2011 we reported the detection of Off-pulse emission from two
long period pulsars B0525+21 and B2045-16. The pulsars were observed at a
single epoch using the 325 MHz frequency band of the Giant Meterwave Radio
Telescope (GMRT). In this paper we report a detailed study of the Off-pulse
emission from these two pulsars using multiple observations at two different
frequencies, 325 MHz and 610 MHz bands of GMRT. We report detection of
Off-pulse emission during each observation and based on the scintillation
effects and spectral index of Off-pulse emission we conclude a magnetospheric
origin. The magnetospheric origin of Off-pulse emission gives rise to various
interesting possibilities about its emission mechanism and raises questions
about the structure of the magnetosphere.Comment: 13 pages, 6 figures, accepted for publication in the Ap
Detection of off-pulse emission from PSR B0525+21 and PSR B2045-16
We report the detection of off-pulse emission from two long period pulsars
PSR B0525+21 and PSR B2045-16 using the Giant Metrewave Radio Telescope (GMRT).
We recorded high time resolution interferometric data at 325 MHz and gated the
data offline to separate the on- and off-pulse sections of the pulsar. On
imaging the two temporal sections separately, we detected a point source in the
off-pulse images of both the pulsars coincident with the pulsar locations in
the on-pulse images. The long periods (3.75 and 1.96 s respectively) and low
spin-down energies (E ~ 10exp(31) erg s-1) of the two pulsars argue against a
pulsar wind nebula (PWN) origin for the off-pulse emission. The derived
estimates of the ISM particle density required to drive a PWN do not support
such an interpretation. A magnetospheric origin for the off-pulse emission
raises questions regarding the location of the emission region, which is an
important input into pulsar emission models.Comment: 15 pages, 11 figure
A new approach to mitigation of radio frequency interference in interferometric data
Radio frequency interference (RFI) is the principal factor limiting the
sensitivities of radio telescopes, particularly at frequencies below 1 GHz. I
present a conceptually new approach to mitigation of RFI in interferometric
data. This has been used to develop a software tool (RfiX) to remove RFI from
observations using the Giant Metrewave Radio Telescope, India. However, the
concept can be used to excise RFI in any interferometer. Briefly, the
fringe-stopped correlator output of an interferometer baseline oscillates with
the fringe-stop period in the presence of RFI. RfiX works by identifying such a
pattern and subtracting it from the data. It is perhaps the only purely
software technique which can salvage the true visibility value from
RFI-corrupted data. It neither requires high-speed hardware nor real-time
processing and works best on normal correlator output integrated for 1-10s. It
complements other mitigation schemes with its different approach and the regime
it addresses. Its ability to work with data integrated over many seconds gives
it an advantage while excising weak, persistent RFI unlike most other
techniques which use high-speed sampling to localise RFI in time-frequency
plane. RfiX is also different in that it does not require RFI-free data to
identify corrupted sections. Some results from the application of RfiX is
presented including an image at 240 MHz with a Peak/noise ratio of 43000, the
highest till date at wavelengths >1m.Comment: Accepted for Publication in the Astrophysical Journa
The Molonglo reference catalog 1 Jy radio source survey. III. Identification of a complete quasar sample
We present a new complete sample of 111 radio quasars (including six BL Lac objects) selected from the Molonglo Reference Catalog (MRC) at 408 MHz. The sample, which we call the Molonglo Quasar Sample (MQS), forms part of a complete survey of 557 MRC radio sources with S408 ≥ 0.95 Jy in the declination range -30 ° < δ < -20 ° , b > 20 ° but excluding the R.A. range 14h03m-20h20m. Quasar classifications are based on high-resolution radio images, deep optical identifications, and follow-up spectroscopy of sources in the strip. The relatively low radio frequency of the finding survey and the complete optical identification of quasars to faint magnitudes ensure that the MQS is relatively free from orientation biases that affect most other samples of radio-loud quasars. The MQS is therefore particularly well suited to investigating the effects of radio axis orientation on quasar properties. This paper describes in detail the formation of the MQS and presents basic radio and optical data, including VLA images of extended radio sources in the sample and a complete set of optical finding charts
The Molonglo reference catalog/1 Jansky radio source survey. I. Radio galaxy identifications
This is the first in a series of papers discussing the properties of a complete set of radio sources selected near S408 = 1 Jy. We present optical or infrared identifications for a sample of 452 radio galaxies. The sources were selected from the 408 MHz Molonglo Reference Catalog, restricted only by flux density and position on the sky, as follows: S408 ≥ = 0.95 Jy and -30° < decl. (1950) < -20°, and 9h20m < R.A.(1950) < 14h4m or 20h20m < R.A.(1950) < 6h14m. This complete sample, the MRC/1 Jy survey, contains 558 radio galaxies or quasars, of which six are occulted by bright stars or galaxies and two are multiple confused sources. Of the remaining 550 sources, 527, or 96%, are identified to an r magnitude of 25, and eight additional sources are identified to K = 19. Of the 17 unocculted objects that remain unidentified, 15 are sources for which we have either poor radio maps or inadequate optical/IR images. This paper presents finding charts and astrometric positions for the 452 sources that are not identified as either quasars or BL Lacertae objects. Magnitudes in the r passband accurate to typically 0.1 mag are given for 353 of the radio galaxy identifications. Redshifts for 268 of the galaxies are also listed; these have been derived from 450 spectroscopic observations. The radio observations, quasar and BL Lac identifications, spectroscopy, and near- IR images will be presented in subsequent papers in this series