75 research outputs found
A compact steep spectrum radio source in NGC1977
A compact steep spectrum radio source (J0535-0452) is located in the sky
coincident with a bright optical rim in the HII region NGC1977. J0535-0452 is
observed to be mas in angular size at 8.44 GHz. The spectrum for the
radio source is steep and straight with a spectral index of -1.3 between 330
and 8440 MHz. No 2 \mu m IR counter part for the source is detected. These
characteristics indicate that the source may be either a rare high redshift
radio galaxy or a millisecond pulsar (MSP). Here we investigate whether the
steep spectrum source is a millisecond pulsar.The optical rim is believed to be
the interface between the HII region and the adjacent molecular cloud. If the
compact source is a millisecond pulsar, it would have eluded detection in
previous pulsar surveys because of the extreme scattering due to the HII
region--molecular cloud interface. The limits obtained on the angular
broadening along with the distance to the scattering screen are used to
estimate the pulse broadening. The pulse broadening is shown to be less than a
few msec at frequencies \gtsim 5 GHz. We therefore searched for pulsed
emission from J0535-0452 at 14.8 and 4.8 GHz with the Green Bank Telescope
(GBT). No pulsed emission is detected to 55 and 30 \mu Jy level at 4.8 and 14.8
GHz. Based on the parameter space explored by our pulsar search algorithm, we
conclude that, if J0535-0452 is a pulsar, then it could only be a binary MSP of
orbital period \ltsim 5 hrs.Comment: Accepted for publication in A&A (3pages, 1 fig
Carbon Recombination Lines toward the Riegel-Crutcher Cloud and other Cold HI Regions in the inner Galaxy
We report here, for the first time, the association of low frequency CRRL
with \HI\ self-absorbing clouds in the inner Galaxy and that the CRRLs from the
innermost of the Galaxy arise in the Riegel-Crutcher (R-C)
cloud. The R-C cloud is amongst the most well known of \HI\ self-absorbing
(HISA) regions located at a distance of about 125 pc in the Galactic centre
direction. Taking the R-C cloud as an example, we demonstrate that the physical
properties of the HISA can be constrained by combining multi-frequency CRRL and
\HI\ observations. The derived physical properties of the HISA cloud are used
to determine the cooling and heating rates. The dominant cooling process is
emission of the \CII\ 158 \mum line whereas dominant heating process in the
cloud interior is photoelectric emission. Constraints on the FUV flux (G0
4 to 7) falling on the R-C cloud are obtained by assuming thermal
balance between the dominant heating and cooling processes. The H formation
rate per unit volume in the cloud interior is 10 -- 10
s \cmthree, which far exceeds the H dissociation rate per unit
volume. We conclude that the self-absorbing cold \HI\ gas in the R-C cloud may
be in the process of converting to the molecular form. The cold \HI\ gas
observed as HISA features are ubiquitous in the inner Galaxy and form an
important part of the ISM. Our analysis shows that combining CRRL and \HI\ data
can give important insight into the nature of these cold gas. We also estimate
the integration times required to image the CRRL forming region with the
upcoming SKA pathfinders. Imaging with the MWA telescope is feasible with
reasonable observing times.Comment: 11 pages, 4 figures, 5 tables, accepted by MNRA
Helium Ionization in the Diffuse Ionized Gas surrounding UCHII regions
We present measurements of the singly ionized helium to hydrogen ratio
() toward diffuse gas surrounding three Ultra-Compact HII
(UCHII ) regions: G10.15-0.34, G23.46-0.20 \& G29.96-0.02. We observe radio
recombination lines (RRLs) of hydrogen and helium near 5 GHz using the GBT to
measure the ratio. The measurements are motivated by the low
helium ionization observed in the warm ionized medium (WIM) and in the inner
Galaxy diffuse ionized regions (DIR). Our data indicate that the helium is not
uniformly ionized in the three observed sources. Helium lines are not detected
toward a few observed positions in sources G10.15-0.34 \& G23.46-0.20 and the
upper limits of the ratio obtained are 0.03 and 0.05
respectively. The selected sources harbor stars of type O6 or hotter as
indicated by helium line detection toward the bright radio continuum emission
from the sources with mean value 0.060.02. Our data
thus show that helium in diffuse gas located a few pc away from the young
massive stars embedded in the observed regions is not fully ionized.We
investigate the origin of the non-uniform helium ionization and rule out the
possibilities : (a) that the helium is doubly ionized in the observed regions
and (b) that the low values are due to additional hydrogen
ionizing radiation produced by accreting low-mass stars (Smith 2014). We find
that selective absorption of ionizing photons by dust can result in low helium
ionization but needs further investigation to develop a self-consistent model
for dust in HII regions.Comment: 43 pages, 11 figures, 5 tables accepted to Ap
Hydrogen 2p--2s transition: signals from the epochs of recombination and reionization
We propose a method to study the epoch of reionization based on the possible
observation of 2p--2s fine structure lines from the neutral hydrogen outside
the cosmological H {\sc ii} regions enveloping QSOs and other ionizing sources
in the reionization era. We show that for parameters typical of luminous
sources observed at the strength of this signal, which is
proportional to the H {\sc i} fraction, has a brightness temperature for a fully neutral medium. The fine structure line from this redshift
is observable at and we discuss prospects for the
detection with several operational and future radio telescopes. We also compute
the characteristics of this signal from the epoch of recombination: the peak
brightness is expected to be ; this signal appears in the
frequency range 5-10 MHz. The signal from the recombination era is nearly
impossible to detect owing to the extreme brightness of the Galactic emission
at these frequencies.Comment: 17 pages, 1 figure, to appear in Ap
Hydrogen recombination lines near 327 MHz. I. distribution of low-density ionized gas in the galactic disk
We present the results of a low-resolution (2° × 2°) survey of radio recombination lines (RRLs) near 327 MHz in the Galactic plane made with the Ooty Radio Telescope (ORT). Although the angular resolution is coarse, these observations represent the first contiguous survey of low-frequency RRL emission in the longitude range l = 330°-0°-89° (inner Galaxy). Hydrogen RRLs were detected in almost all directions in the inner Galaxy and carbon lines in several positions. In the outer Galaxy (l = 172°-252°), an unbiased set of 14 positions were observed and lines were detected toward three of them. To study the extent of the ionized gas above the Galactic disk, we have observed RRLs along the Galactic latitude at two specific longitudes (l = 0°.0 and 13°.9). RRLs were detected up to b = ±3°. The l-ν diagram and the radial distribution, obtained from RRL emission near 327 MHz, show good similarity with that of RRL emission near 1.4 GHz, "intense" 12CO emission and to some extent with the RRLs observed near 3 cm from normal H II region. These distributions are distinctly different from those of Hα and H I emission from the Galactic disk. Based on a comparison of the radial distribution of different components in the Galactic disk, we conclude that the diffuse RRL emission is associated with star-forming regions and possibly with a low-density component of known H II regions in the inner Galaxy
A new technique to improve RFI suppression in radio interferometers
Radio interferometric observations are less susceptible to radio frequency
interference (RFI) than single dish observations. This is primarily due to :
(1)fringe-frequency averaging at the correlator output and (2) bandwidth
decorrelation of broadband RFI. Here, we propose a new technique to improve RFI
suppression of interferometers by replacing the fringe-frequency averaging
process with a different filtering process. In the digital implementation of
the correlator, such a filter should have cutoff frequencies times
the frequency at which the baseband signals are sampled. We show that filters
with such cutoff frequencies and attenuation 40 dB at frequencies above the
cutoff frequency can be realized using multirate filtering techniques.
Simulation of a two element interferometer system with correlator using
multirate filters shows that the RFI suppression at the output of the
correlator can be improved by 40 dB or more compared to correlators using a
simple averaging process.Comment: 12 pages, 7 figures; Invited talk given at IVS Symposium in Korea --
New Technologies in VLBI, Korea, Nov 2002; to appear in the conference
proceedings (Added answers to the questions during the discussion session
Hydrogen recombination lines near 327 MHz. III. physical properties and origin of the low-density ionized gas in the inner galaxy
We present constraints on the physical properties of the ionized gas responsible for hydrogen radio recombination lines (RRLs) near 327 MHz detected in a recent Galactic plane survey made with the Ooty Radio Telescope. To obtain these constraints, we combined the data at 327 MHz with previously published RRL observations near 1.4 GHz. The density of the ionized gas is well constrained and is in the range of 1 to 10 cm-3. The data implies upper limits to the temperature and size of the line emitting regions of ~12,000 K and ~500 pc, respectively. Assuming an electron temperature of 7000 K, the derived path lengths of the line emitting region are in the range of 20 to 200 pc. The derived properties of the ionized gas responsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 µm emission and a considerable fraction of the [C II] 158 µm emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The Hα emission from these ionized gases is mostly undetected in the existing Hα surveys because of large interstellar extinction. About 50% of the free-free absorption of the Galactic nonthermal radiation observed at frequencies less than 100 MHz can be accounted for by the same ionized gas. We also discuss the origin of this low-density ionized gas in the inner Galaxy. The derived low line-of-sight filling factor (<1%) for this ionized gas indicates that it does not form a pervasive medium. On the basis of the similarity of the distribution of this gas in the Galactic disk with that of the star-forming regions and the range of derived physical properties, we support the earlier suggestion that the low-frequency RRL emission originates from low-density ionized gas, which forms envelopes of normal H II regions
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