arXiv subjects

N. E. Kassim

Publications and source records attributed to N. E. Kassim.

54 records · Page 3Linked to original sources

arXiv2005

Spatially Resolved Low Frequency VLA observations of the Supernova Remnant 3C 391

We present VLA images of the supernova remnant (SNR) 3C~391 at 74, 330, and 1465 MHz. This remnant has been known for some time to exhibit a turnover in its integrated radio continuum spectrum at frequencies < 100 MHz, indicative of free-free absorption from thermal ionized gas along the line of sight. For the first time, our data reveal the spatially resolved morphology of the low frequency free-free absorption with a resolution of ~70 arcsec. Contrary to the expectation that such absorption arises from unrelated low density HII regions (or their envelopes) along the line of sight, these data suggest that in this case the absorbing medium is directly linked to the SNR itself. 3C~391 has been shown in a number of recent papers to be interacting with a molecular cloud. Indeed, it exhibits a number of signposts of SNR/molecular cloud shocks including OH (1720 MHz) masers and broad molecular emission lines. Comparison of the regions of strongest 74 MHz absorption with existing X-ray, IR, and molecular data suggests that the free-free absorption originates from the SNR/molecular cloud shock boundaries due to ionized gas created from the passage of a J-type shock with a speed of ~100 km/s. This makes only the second SNR for which such (extrinsic) spatially resolved absorption has been measured, and the only one for which the absorption is thought to arise from a SNR/molecular cloud interface region.

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arXiv2005

Resolving the Steep Spectrum Emission in the Central Radio Source in ZwCl 0735.7+7421

It has long been know that an extremely steep spectrum radio source lies at the center of ZwCl 0735.7+7421, a galaxy cluster with high X-ray luminosity and a cooling core. In this Letter, we present VLA observations of this radio source at both 1425 and 325 MHz. With a resolution below 21'' (75 kpc) for both 1425 and 325 MHz, we show the morphology of the central source, and find that it is most likely a large (400 kpc) radio galaxy rather than diffuse cluster emission. We estimate a steep spectral index of $α_{325}^{1400} = -1.54$ for the core (although it may be contaminated by lobe emission), while the outer lobes are extremely steep objects, both with $α_{325}^{1400} < -3.1$. We also find evidence for restarted core activity in the form of a set of inner lobes oriented at a somewhat different angle from the outer lobes. A spectral analysis extending the frequency range down to 74 MHz appears to show a turnover at very low frequencies. Comparison of the minimum energy radio pressures with the average thermal pressure surrounding the radio cavities from McNamara et al.(2005) shows that the radio lobes appear to be roughly in pressure balance with the thermal gas.

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arXiv2005

First Results of the 74 MHz VLA-Pie Town Link. Hercules A at Low Frequencies

We present the results of the first successful observations of the Pie Town link with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement in resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like features seen at higher frequencies. We also present new high dynamic range images of this powerful radio galaxy at 325 MHz. Our low frequency observations confirm the multiple outburst interpretation of the spectral index differences at high frequencies. Comparison between our radio and ROSAT X-ray data does not reveal any association between the X-ray emission from the cluster and the radio lobes. There are no extra regions of radio emission at 74 MHz.

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arXiv2003

A Deep, High-Resolution Survey at 74 MHz

We present a 74 MHz survey of a 165 square degree region located near the north galactic pole. This survey has an unprecedented combination of both resolution (25'' FWHM) and sensitivity (RMS noise as low as 24 mJy/beam). We detect 949 sources at the 5-sigma level in this region, enough to begin exploring the nature of the 74 MHz source population. We present differential source counts, spectral index measurements and the size distribution as determined from counterparts in the high resolution FIRST 1.4 GHz survey. We find a trend of steeper spectral indices for the brighter sources. Further, there is a clear correlation between spectral index and median source size, with the flat spectrum sources being much smaller on average. Ultra-steep spectrum objects (alpha < -1.2) are identified, and we present high resolution VLA follow-up observations of these sources which, identified at such a low frequency, are excellent candidates for high redshift radio galaxies.

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arXiv2003

A Low Frequency Survey of the Galactic Plane Near l=11 degrees: Discovery of Three New Supernova Remnants

We have imaged a 1 deg^2 field centered on the known Galactic supernova remnant (SNR) G11.2-0.3 at 74, 330, and 1465 MHz with the Very Large Array radio telescope (VLA) and 235 MHz with the Giant Metrewave Radio Telescope (GMRT). The 235, 330, and 1465 MHz data have a resolution of 25 arcsec, while the 74 MHz data have a resolution of 100 arcsec. The addition of this low frequency data has allowed us to confirm the previously reported low frequency turnover in the radio continuum spectra of the two known SNRs in the field: G11.2-0.3 and G11.4-0.1 with unprecedented precision. Such low frequency turnovers are believed to arise from free-free absorption in ionized thermal gas along the lines of site to the SNRs. Our data suggest that the 74 MHz optical depths of the absorbing gas is 0.56 and 1.1 for G11.2-0.3 and G11.4-0.1, respectively. In addition to adding much needed low frequency integrated flux measurements for two known SNRs, we have also detected three new SNRs: G11.15-0.71, G11.03-0.05, and G11.18+0.11. These new SNRs have integrated spectral indices between -0.44 and -0.80. Because of confusion with thermal sources, the high resolution (compared to previous Galactic radio frequency surveys) and surface brightness sensitivity of our observations have been essential to the identification of these new SNRs. With this study we have more than doubled the number of SNRs within just a 1 deg^2 field of view in the inner Galactic plane. This result suggests that future low frequency observations of the Galactic plane of similar quality may go a long way toward alleviating the long recognized incompleteness of Galactic SNR catalogs.

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arXiv2003

Hydra A at Low Radio Frequencies

We present new, low-frequency images of the powerful FR I radio galaxy Hydra A (3C 218). Images were made with the Very Large Array (VLA) at frequencies of 1415, 330, and 74 MHz, with resolutions on the order of 20". The morphology of the source is seen to be more complex and even larger than previously known, and extends nearly 8' (530 kpc) in a North-South direction. The southern lobe is bent to the east and extends in that direction for nearly 3' (200 kpc). In addition, we find that the northern lobe has a flatter spectral slope than the southern lobe, consistent with the appearance of greater confinement to the south. We measure overall spectral indices α^{330}_{74} = -0.83 and α^{1415}_{330} = -0.89.

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arXiv2003

The Low Frequency Radio Counterpart of the XMM Large Scale Structure Survey

The XMM Large Scale Structure Survey (XMM-LSS) is a major project to map the large scale structure of the universe out to cosmological distances. An 8 degree by 8 degree region will be surveyed by XMM with planned optical follow-up to produce a three-dimensional map of many hundreds of clusters out to a redshift of z = 1. To explore the relation of the large scale structure to the location and properties of extragalactic radio sources, the XMM-LSS project also includes a low frequency radio survey of this region. This combination will provide unprecedented insight into how the radio source formation and evolution are affected by the local environment. Here, we present preliminary results from our 325 MHz and 74 MHz surveys in this region. At 325 MHz, we have a flux limit of 4 mJy/beam, a resolution of 6.3", and a total of 256 source detections over 5.6 square degrees. At 74 MHz, we have a flux limit of 275 mJy/beam, a resolution of 30", and a total of 211 source detections over 110 square degrees. We describe these results and explore what they tell us about the population of extra-galactic low frequency radio sources. The 74 MHz survey represents the first presentation of a deep, sub-arcminute resolution survey at such a low frequency. This was made possible by recent advances in both hardware and data reduction algorithms which we describe in detail.

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arXiv2002

3C 129 at 90cm: Evidence for a Radio Relic?

We present a new wide-field map of the radio galaxy 3C 129 and its companion galaxy 3C 129.1 at lambda=90 cm. We see a distinct steep-spectrum feature near the head of 3C 129, extending in a direction perpendicular to the radio tails. We propose that this Crosspiece might consist of fossil radio plasma, which has been re-energized by the compression of the bow shock of the supersonically moving galaxy 3C 129. One possible origin of the fossil radio plasma could be the tail of a nearby head-tail radio galaxy. We discuss the implications of, and give testable predictions for this scenario.

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arXiv2001

The properties of the X-ray holes in the intracluster medium of the Perseus cluster

High resolution X-ray and low frequency radio imaging now allow us to examine in detail the interaction and physical properties of the radio source 3C84 and the surrounding thermal gas. The radiative and dynamical properties of the inner X-ray holes, which coincide with the radio lobes, indicate that the ratio of the energy factor k and filling factor f is in the range 180<k/f<500. We define k to be the ratio of the total particle energy to that of the electrons radiating above a fiducial frequency of 10MHz. The relativistic plasma and magnetic field are not in equipartition, since the field must be a factor of 4 or more lower than required for pressure balance. Unexpected steep-spectrum spurs in the low-frequency radio maps point to outer X-ray holes, which are plausibly buoyant old radio lobes. The evidence that the inner lobes are currently expanding subsonically, yet have not detached due to buoyancy, and the requirement that the synchrotron cooling time must exceed the age of the hole, enable us to constrain the jet power of the nucleus to between 10^44 and 10^45 ergps, depending on the filling factor of the relativistic plasma.

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arXiv2001

Radio Spectral Index and Expansion of 3C58

We present new observations of the plerionic supernova remnant 3C58 with the VLA at 74 and 327 MHz. In addition, we re-reduced earlier observations at 1.4 and 4.9 GHz taken in 1973 and 1984. Comparing these various images, we find that: 1. the remnant has a flat and relatively uniform spectral index distribution, 2. any expansion of the remnant with time is significantly less than that expected for uniform, undecelerated expansion since the generally accepted explosion date in 1181 A.D., and 3. there is no evidence for a non-thermal synchrotron emission shell generated by a supernova shock wave, with any such emission having a surface brightness of <1 x 10^(-21) W / (m^2 Hz sr) at 327 MHz.

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arXiv2001

Low Frequency VLA Observations of Abell 754: Evidence for a Cluster Radio Halo and Possible Radio Relics

We present 74 MHz and 330 MHz VLA observations of Abell 754. Diffuse, halo-like emission is detected from the center of the cluster at both frequencies. At 330 MHz the resolution of 90'' distinguishes this extended emission from previously known point sources. In addition to the halo and at a much lower level, outlying steep-spectrum emission regions straddle the cluster center and are seen only at 74 MHz. The location, morphology and spectrum of this emission are all highly suggestive of at least one, and possibly two cluster radio relics. Easily obtained higher resolution, higher sensitivity VLA observations at both frequencies are required to confirm the extended nature of the halo-like emission and the 74 MHz relic detections. However, since there is prior evidence that this cluster is or has recently been in the process of a major merger event, the possible discovery of relics in this system is of great interest in light of recent observational and theoretical evidence in favor of a merger-relic connection. We discuss the possible role the merger shock waves, which are seen in the X-ray emission, may have played in the formation of the halo and radio relics in A754.

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arXiv2000

Extended Extragalactic Radio Emission

Extended radio emission and its relation to parent galaxy properties is briefly reviewed. Our current understanding of the relation between absolute radio and optical luminosity, radio morphology and linear size is discussed. The impact of radio jets on dense cluster cores is discussed using M87 as an example. Finally, the relation of AGN's to star-bursting galaxies at high redshift is considered.

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arXiv2000

M87 at 90cm: A Different Picture

We report new radio imaging of the large scale radio structure of M87 with the VLA at 90 cm. These new images show the complex structure of the radio emission more clearly than previous attempts, some of which date back to the 1940's. The images suggest that the outward flow from the M87 nucleus extends well beyond the 2 kpc jet. Two ``bubbles'' of synchrotron emission appear to be inflated by this flow. A simple model of the emission, combined with our knowledge of the inner jet, suggests that the energy input into this region from the M87 nucleus exceeds the energy being radiated away as X-rays. This argues that the region within 40 kpc of the center of M87 is currently dominated by energy input from the M87 nucleus. The gas in the region is expanding, not flowing inward as is envisioned in the cooling flow model.

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arXiv1999

74 MHz-VLA-Observation of Coma Berenices with Subarcminute Resolution -- Observation, Data Reduction, and Preliminary Results

We report on an ongoing project to map a 15 degree x 15 degree region in Coma Berenices with the new 74 MHz receiver system of the VLA. The field is centered on the Coma cluster of galaxies. Coma Berenices has been observed in all VLA configurations, which allowed us to resolve several hundred point sources with 35 arcsecond resolution. We have successfully detected the diffuse emission from the Coma cluster radio halo, radio relic, and radio bridge. Difficulties and strategies of the observation and data reduction are discussed. Preliminary results are given.

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arXiv1999

Discovery of a Non-Thermal Galactic Center Filament (G358.85+0.47) Parallel to the Galactic Plane

We report the discovery of a new non-thermal filament, G358.85+0.47, the ``Pelican'', located ~225 pc in projection from SgrA, and oriented parallel to the Galactic plane. VLA continuum observations at 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, most apparent at its ends. Observations at 6 and 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from alpha(20/6)=-0.8 to alpha(6/3.6)=-1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 rad/m2 to +500 rad/m2. The intrinsic magnetic field is well-aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the non-thermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner kiloparsec of the Galaxy.

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arXiv1999

Discovery of a New Filamentary Structure near the Galactic Center: G358.85+0.47 - The Pelican

We report the discovery of a new filamentary structure, G358.85+0.47, consisting of at least three mutually parallel but bent `strands', located about 1.5 degrees SW of the Sgr A complex. Unlike all the other known Galactic center filaments, G358.85+0.47 is oriented parallel to the galactic plane. This orientation of the filament may have implications for the large scale structure of the magnetic field near the Galactic Center. This feature was identified in a new wide-field image of the Galactic Center region at 90 cm. Further higher resolution observations at 20 cm revealed a filamentary structure which is similar to several other known linear features in the vicinity of the Galactic center. Based on its appearance in a 20 cm image, we give it the name 'the Pelican' for further reference.

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arXiv1995

Does the Crab Have a Shell?

We present deep images of a region around the Crab nebula made with the VLA, utilizing new imaging and deconvolution algorithms in a search for a faint radio shell. The existence of a high-velocity, hydrogen-rich envelope has been predicted to account for the low total mass and kinetic energy of the observed nebula. No radio emission was detected from an extended source outside the Crab nebula. Our limits on surface brightness are sufficiently low to rule out the existence of a shell around the Crab whose brightness is at least two orders of magnitude below SN\thinspace{1006}, the faintest historical shell-type supernova remnant. We consider models for the progenitor star and the pre-supernova environment and conclude that if a fast, outer shock exists then it has a sharply reduced efficiency at accelerating relativistic particles from the kinetic energy of the blast wave. We also looked for a steepening of the spectral index along the boundary of the Crab nebula itself, the signature of an outer shock. However, contrary to previous claims, no such steepening was found. The absence of any evidence at radio wavelengths that either the Crab nebula or a hypothetical shell is interacting with the ambient medium leads to an interpretation that the supernova of 1054 AD was a peculiar low energy event.

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arXiv1994

The Radio Nebula of the Soft Gamma Ray Repeater SGR 1806-20

Earlier we had suggested that G10.0-0.3, a non-thermal nebula, most likely an old supernova remnant (SNR), was associated with the soft gamma repeater (SGR) SGR 1806-20. Here we present new radio images obtained at the Very Large Array (VLA) of the non-thermal radio nebula G10.0-0.3. The nebula is a plerion with a hierarchy of nested amorphous components culminating in a peak. The recent dramatic detection of an X-ray point source coincident with the radio peak and a hard X-ray burst localized to G10.0-0.3 confirms the SNR-SGR association. We propose that the SGR is an isolated pulsar with both a steady and impulsive relativistic particle wind and these two together power the nebula. We note that both this SGR and SGR 0526-66 are offset from the centers of their SNRs which requires high space motion of the pulsar. We suggest that some natal mechanism produces high velocity and induces SGR activity.

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