arXiv subjects

Namir E. Kassim

Publications and source records attributed to Namir E. Kassim.

At least 19 recordsLinked to original sources

arXiv2025

Radio-Continuum Spectra of Pulsars with Free-Free Thermal Absorption

The radio continuum spectra of pulsars (PSRs) exhibit a wide variety of shapes, that are interpreted as pure and broken power laws, power laws with turnovers or cut-offs, and logarithmic-parabolic profiles. A notable fraction of these have well-defined power laws with $ν^{-2.1}$ exponential turnovers, indicative of free-free thermal absorption along the line-of-site. We analyse a sample of 63 PSRs with such spectral shapes, compiled from four previously published studies, to investigate their statistical properties. We normalise each spectrum to a characteristic frequency and flux density of its own, facilitating a consistent treatment across the four sub-samples. We show these two fitted parameters are correlated by a power law, with its slope reflecting the median spectral index ($α\sim -2.0$) of PSR emission. We found that the turnover frequencies in our sample are typically high, clustering around 558 MHz, implying notably high emission measures ($EM \sim 10^{5}$ pc cm$^{-6}$) for an inferred thermal absorbing medium with electron temperature of $T_{\mathrm{e}}=8000$ K. Moreover, by combining these $EM$ with dispersion measures (DM) derived from pulse time delays, we break the degeneracy between electron density and path length of the absorbers. This reveals a discrete near-in population of absorbers characterised by small sizes ($L \sim 0.1\,\text{pc}$) and high electron densities $(n_{\mathrm{e}} \sim 10^{3}\,\text{cm}^{-3} $)), which exhibit a clear size-density anticorrelation reminiscent of that observed in Galactic and extragalactic H$_\rm{II}$ regions.

astro-ph.GA↗

arXiv2025

A Radio Flaring, Chromospherically-Inactive K Dwarf

We report on an unusual radio source J180526-292953, initially identified as a steep spectrum, polarized point source toward the Galactic bulge and found to coincide with the nearby K dwarf HD317101A. We conducted a multi-wavelength radio study utilizing new GMRT observations and archival data from ASKAP, MeerKAT, and the VLA. At 1.5 GHz, HD317101A exhibits highly polarized coherent emission with variable activity lasting several hours with an apparent period of 3.7 days, which is consistent with electron cyclotron maser (ECM) emission. The behavior at 3 GHz is distinctive, with a short burst lasting tens of seconds to minutes, a flat spectrum, and no detected polarization, possibly suggesting gyro-synchrotron emission. High-resolution optical spectroscopy from CHIRON/SMARTS confirms HD317101A as a mature, chromospherically inactive K7V star, while Gaia astrometry, combined with speckle imaging from Zorro/Gemini-S, indicates the presence of a close-in M5.5V companion. We evaluated three possible origins for the combined radio behavior: chromospheric activity, auroral emission (possibly from a star-planet interaction), or an ultra-long-period transient. The bulk of the evidence favors an auroral origin, but the dominant stellar source of the ECM emission remains uncertain. Future VLBI observations, long-term TESS monitoring, high resolution spectroscopy and further radio characterization will be key to distinguishing between various scenarios.

astro-ph.SR↗

arXiv2024

An Image-Based Search for Pulsar Candidates in the MeerKAT Bulge Survey

We report on the results of an image-based search for pulsar candidates toward the Galactic bulge. We used mosaic images from the MeerKAT radio telescope, that were taken as part of a 173 deg**2 survey of the bulge and Galactic center of our Galaxy at L band (856-1712 MHz) in all four Stokes I, Q, U and V. The image root-mean-square noise levels of 12-17 uJy/ba represent a significant increase in sensitivity over past image-based pulsar searches. Our primary search criterion was circular polarization, but we used other criteria including linear polarization, in-band spectral index, compactness, variability and multi-wavelength counterparts to select pulsar candidates. We first demonstrate the efficacy of this technique by searching for polarized emission from known pulsars, and comparing our results with measurements from the literature. Our search resulted in a sample of 75 polarized pulsar candidates. Bright stars or young stellar objects were associated with 28 of these sources, including a small sample of highly polarized dwarf stars with pulsar-like steep spectra. Comparing the properties of this sample with the known pulsars, we identified 30 compelling candidates for pulsation follow-up, including two sources with both strong circular and linear polarization. The remaining 17 sources are either pulsars or stars, but we cannot rule out an extragalactic origin or image artifacts among the brighter, flat spectrum objects.

astro-ph.HE↗

arXiv2021

Radio observations of SN2004dk with VLITE confirm late-time re-brightening

The study of stripped-envelope core-collapse supernovae (SNe), with evidence for strong interaction of SN ejecta with the circumstellar medium (CSM), provides insights into the pre-supernova progenitor, and a fast-forwarded view of the progenitor mass-loss history. In this context, we present late-time radio observations of SN2004dk, a type Ibc supernova located in the galaxy, NGC 6118, at a distance of $d_L \approx 23$ Mpc. About 15 years after explosion, SN2004dk has shown evidence for H$α$ emission, possibly linked to the SN ejecta interacting with an H-rich CSM. Using data from the VLA Low Band Ionosphere and Transient Experiment (VLITE), we confirm the presence of a late-time radio re-brightening accompanying the observed H$α$ emission. We model the SN2004dk radio light curves within the (spherically symmetric) synchrotron-self-absorption (SSA) model. Within this model, our VLITE observations combined with previously collected VLA data favor an interpretation of SN2004dk as a strongly CSM-interacting radio SN going through a complex environment shaped by a non-steady mass-loss from the SN progenitor.

astro-ph.HE↗

arXiv2019

A First Search for Prompt Radio Emission from a Gravitational-Wave Event

Multimessenger observations of the binary neutron star merger GW170817 have enabled the discovery of a diverse array of electromagnetic counterparts to compact binary mergers, including an unambiguous kilonova, a short gamma-ray burst, and a late-time radio jet. Beyond these counterparts, compact binary mergers are additionally predicted to be accompanied by prompt low-frequency radio emission. The successful observation of a prompt radio counterpart would be immensely valuable, but is made difficult by the short delay between the gravitational-wave and prompt electromagnetic signals as well as the poor localization of gravitational-wave sources. Here, we present the first search for prompt radio emission accompanying a gravitational-wave event, targeting the binary black hole merger GW170104 detected by the Advanced LIGO and Virgo gravitational-wave observatories during their second (O2) observing run. Using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA), we search a $\sim900\,\mathrm{deg}^2$ region for transient radio emission within approximately one hour of GW170104, obtaining an upper limit of $2.5\times10^{41}\,\mathrm{erg}\,\mathrm{s}^{-1}$ on its equivalent isotropic luminosity between 27-84 MHz. We additionally discuss plans to target binary neutron star mergers in Advanced LIGO and Virgo's upcoming O3 observing run.

astro-ph.HE↗

arXiv2017

The LWA1 Low Frequency Sky Survey

We present a survey of the radio sky accessible from the first station of the Long Wavelength Array (LWA1). Images are presented at nine frequencies between 35 and 80 MHz with spatial resolutions ranging from $4.7^\circ$ to $2.0^\circ$, respectively. The maps cover the sky north of a declination of $-40^\circ$ and represent the most modern systematic survey of the diffuse Galactic emission within this frequency range. We also combine our survey with other low frequency sky maps to create an updated model of the low frequency sky. Due to the low frequencies probed by our survey, the updated model better accounts for the effects of free-free absorption from Galactic ionized Hydrogen. A longer term motivation behind this survey is to understand the foreground emission that obscures the redshifted 21 cm transition of neutral hydrogen from the cosmic dark ages ($z>10$) and, at higher frequencies, the epoch of reionization ($z>6$).

astro-ph.IM↗

arXiv2015

Observations of Giant Pulses from Pulsar PSR B0950+08 using LWA1

We report the detection of giant pulse emission from PSR B0950+08 in 24 hours of observations made at 39.4 MHz, with a bandwidth of 16 MHz, using the first station of the Long Wavelength Array, LWA1. We detected 119 giant pulses from PSR B0950+08 (at its dispersion measure), which we define as having SNRs at least 10 times larger than for the mean pulse in our data set. These 119 pulses are 0.035% of the total number of pulse periods in the 24 hours of observations. The rate of giant pulses is about 5.0 per hour. The cumulative distribution of pulse strength $S$ is a steep power law, $N(>S)\propto S^{-4.7}$, but much less steep than would be expected if we were observing the tail of a Gaussian distribution of normal pulses. We detected no other transient pulses in a dispersion measure range from 1 to 90 pc cm$^{-3}$, in the beam tracking PSR B0950+08. The giant pulses have a narrower temporal width than the mean pulse (17.8 ms, on average, vs. 30.5 ms). The pulse widths are consistent with a previously observed weak dependence on observing frequency, which may be indicative of a deviation from a Kolmogorov spectrum of electron density irregularities along the line of sight. The rate and strength of these giant pulses is less than has been observed at $\sim$100 MHz. Additionally, the mean (normal) pulse flux density we observed is less than at $\sim$100 MHz. These results suggest this pulsar is weaker and produces less frequent giant pulses at 39 MHz than at 100 MHz.

astro-ph.HE↗

arXiv2014

The Density and Mass of Unshocked Ejecta in Cassiopeia A through Low Frequency Radio Absorption

Characterizing the ejecta in young supernova remnants is a requisite step towards a better understanding of stellar evolution. In Cassiopeia A the density and total mass remaining in the unshocked ejecta are important parameters for modeling its explosion and subsequent evolution. Low frequency (<100 MHz) radio observations of sufficient angular resolution offer a unique probe of unshocked ejecta revealed via free-free absorption against the synchrotron emitting shell. We have used the Very Large Array plus Pie Town Link extension to probe this cool, ionized absorber at 9 arcseconds and 18.5 arcseconds resolution at 74 MHz. Together with higher frequency data we estimate an electron density of 4.2 electrons per cubic centimeters and a total mass of 0.39 Solar masses with uncertainties of a factor of about 2. This is a significant improvement over the 100 electrons per cubic centimeter upper limit offered by infrared [S III] line ratios from the Spitzer Space Telescope. Our estimates are sensitive to a number of factors including temperature and geometry. However using reasonable values for each, our unshocked mass estimate agrees with predictions from dynamical models. We also consider the presence, or absence, of cold iron- and carbon-rich ejecta and how these affect our calculations. Finally we reconcile the intrinsic absorption from unshocked ejecta with the turnover in Cas A's integrated spectrum documented decades ago at much lower frequencies. These and other recent observations below 100 MHz confirm that spatially resolved thermal absorption, when extended to lower frequencies and higher resolution, will offer a powerful new tool for low frequency astrophysics.

astro-ph.GA↗

arXiv2012

A wide-band, active antenna system for long wavelength radio astronomy

We describe an "active" antenna system for HF/VHF (long wavelength) radio astronomy that has been successfully deployed 256-fold as the first station (LWA1) of the planned Long Wavelength Array. The antenna system, consisting of crossed dipoles, an active balun/preamp, a support structure, and a ground screen has been shown to successfully operate over at least the band from 20 MHz (15 m wavelength) to 80 MHz (3.75 m wavelength) with a noise figure that is at least 6 dB better than the Galactic background emission noise temperature over that band. Thus, the goal to design and construct a compact, inexpensive, rugged, and easily assembled antenna system that can be deployed many-fold to form numerous large individual "stations" for the purpose of building a large, long wavelength synthesis array telescope for radio astronomical and ionospheric observations was met.

astro-ph.IM↗

arXiv2010

The First Station of the Long Wavelength Array

The Long Wavelength Array (LWA) will be a new multi-purpose radio telescope operating in the frequency range 10-88 MHz. Upon completion, LWA will consist of 53 phased array "stations" distributed over a region about 400 km in diameter in the state of New Mexico. Each station will consist of 256 pairs of dipole-type antennas whose signals are formed into beams, with outputs transported to a central location for high-resolution aperture synthesis imaging. The resulting image sensitivity is estimated to be a few mJy (5 sigma, 8 MHz, 2 polarizations, 1 hr, zenith) in 20-80 MHz; with resolution and field of view of (8", 8 deg) and (2",2 deg) at 20 MHz and 80 MHz, respectively. All 256 dipole antennas are in place for the first station of the LWA (called LWA-1), and commissioning activities are well underway. The station is located near the core of the EVLA, and is expected to be fully operational in early 2011.

astro-ph.IM↗

arXiv2010

Circularly polarized emission from the transient bursting radio source GCRT J1745-3009

We report detection of strong circularly polarized emission from the transient bursting source GCRT J1745-3009 based on new analysis of 325 MHz GMRT observations conducted on 28 September 2003. We place 8 Solar radius as the upper limit on the size of the emission region. The implied high brightness temperature required for an object beyond 1 pc and the high fraction of circular polarization firmly establish the emission as coherent. Electron cyclotron or plasma emission from a highly subsolar magnetically dominated dwarf located less than 4 kpc away could have given rise to the GCRT radio emission.

astro-ph.GA↗

arXiv2008

GCRT J1742-3001: A New Radio Transient Towards the Galactic Center

We report the detection of a new transient radio source, GCRT J1742-3001, located ~1 degree from the Galactic center. The source was detected ten times from late 2006 to 2007 May in our 235 MHz transient monitoring program with the Giant Metrewave Radio Telescope (GMRT). The radio emission brightened in about one month, reaching a peak observed flux density of ~100 mJy on 2007 January 28, and decaying to ~50 mJy by 2007 May when our last monitoring observation was made. Two additional faint, isolated 235 MHz detections were made in mid-2006, also with the GMRT. GCRT J1742-3001 is unresolved at each epoch, with typical resolutions of ~20 arcsec x 10 arcsec. No polarization information is available from the observations. Based on nondetections in observations obtained simultaneously at 610 MHz, we deduce that the spectrum of GCRT J1742-3001 is very steep, with a spectral index less than about -2. Follow-up radio observations in 2007 September at 330 MHz and 1.4 GHz, and in 2008 February at 235 MHz yielded no detections. No X-ray counterpart is detected in a serendipitous observation obtained with the X-ray telescope aboard the Swift satellite during the peak of the radio emission in early 2007. We consider the possibilities that GCRT J1742-3001 is either a new member of an existing class of radio transients, or is representative of a new class having no associated X-ray emission.

astro-ph↗

arXiv2008

A Bursting Radio Transient in the Direction of the Galactic Center

The radio sky is poorly sampled for rapidly varying transients because of the narrow field-of-view of most imaging radio telescopes at cm and shorter wavelengths. The emergence of sensitive long wavelength observations with intrinsically larger fields-of-view are changing this situation, as partly illustrated by our ongoing meter-wavelength monitoring observations and archival studies of the Galactic Center. In this search, we discovered a transient, bursting, radio source in the direction of the Galactic Center, GCRT J1745-3009, with extremely unusual properties. Its flux and rapid variability imply a brightness temperature >10^12 K if it is at a distance >70 pc, implying that it is a coherent emitter. I will discuss the discovery of the source and the subsequent re-detections, as well as searches for counterparts at other wavelengths, and several proposed models.

astro-ph↗

arXiv2007

A Faint, Steep Spectum Burst from the Radio Transient GCRT J1745-3009

GCRT J1745-3009 is a transient bursting radio source located in the direction of the Galactic center. It was discovered in a 330 MHz VLA observation from 2002 September 30--October 1 and subsequently rediscovered in a 330 MHz GMRT observation from 2003 September 28 by Hyman et al. Here we report a new radio detection of the source in 330 MHz GMRT data taken on 2004 March 20. The observed properties of the single burst detected differ significantly from those measured previously, suggesting that GCRT J1745-3009 was detected in a new physical state. The 2004 flux density was ~0.05 Jy, ~10x weaker than the single 2003 burst and ~30x weaker than the five bursts detected in 2002. We derive a very steep spectral index, alpha = -13.5 +/- 3.0, across the bandpass, a new result previously not detectable due to limitations in the analysis of the 2002 and 2003 observations. Also, the burst was detected for only ~2 min., in contrast to the 10 min. duration observed in the earlier bursts. Due to sparse sampling, only the single burst was detected in 2004, as in the 2003 epoch, and we cannot rule out additional undetected bursts that may have occurred with the same ~77 min. periodicity observed in 2002 or with a different periodicity. Considering our total time on source throughout both our archival and active monitoring campaigns, we estimate the source exhibits detectable bursting activity ~7% of the time.

astro-ph↗

arXiv2006

GBT Observations of IC 443: the Nature of OH(1720 MHz) Masers and OH Absorption

We present results of spectral line observations of the ground state transitions of hydroxyl(OH) toward supernova remnant IC 443 carried out with the Green Bank Telescope. At a spatial resolution of 7.2 arcminutes we detect weak, extended OH(1720 MHz) maser emission with OH(1667/5,1612 MHz) absorption along the southern extent of the remnant, where no bright compact maser sources have been observed previously. These newly detected SNR-type masers are coincident with known molecular clumps and a ridge of shocked molecular hydrogen indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission including excitation temperature, OH column density and filling factor. Observed line profiles suggest the shock is largely propagating toward the line-of-sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.

astro-ph↗

arXiv2006

74 MHz Discrete HII Absorption Regions Towards The Inner Galaxy

At low radio frequencies (< 100 MHz), classical HII regions may become optically thick (optical depth > 1) and can be observed as discrete absorption regions against the Galactic nonthermal background emission created by Galactic cosmic ray electrons spiraling around magnetic fields. In this work we present 74 MHz observations in the region 26>l>-15, -5<b<5, and report the detection of 92 absorption features associated with known HII regions, and derive the brightness temperature of the Galactic cosmic ray electron synchrotron emission emanating from the column behind these regions. For the 42 HII regions with known distances, the average emissivity of the column behind the HII region is derived. 74 MHz emissivity values range between 0.3 and 1.0 Kelvin per parsec for a model assuming uniform distribution of emissivity. Methods for utilizing this type of data to model the 3-dimensional distribution of cosmic ray emissivity and the possibility of using this method to break the HII region kinematic distance degeneracy are discussed.

astro-ph↗

arXiv2005

A New Radio Detection of the Bursting Source GCRT J1745-3009

GCRT J1745-3009 is a transient bursting radio source located in the direction of the Galactic center, discovered in 330 MHz VLA observations from 2002 September 30--October 1 by Hyman et al. We have searched for bursting activity from GCRT J1745-3009 in nearly all of the available 330 MHz VLA observations of the Galactic center since 1989 as well as in 2003 GMRT observations. We report a new radio detection of the source in 330 MHz GMRT data taken on 2003 September 28. A single ~0.5 Jy burst was detected, approximately 3x weaker than the five bursts detected in 2002. Due to the sparse sampling of the 2003 observation, only the decay portion of a single burst was detected. We present additional evidence indicating that this burst is an isolated one, but we cannot completely rule out additional undetected bursts that may have occured with the same ~77 min. periodicity observed in 2002 or with a different periodicity. Assuming the peak emission was detected, the decay time of the burst, ~2 min, is consistent with that determined for the 2002 bursts. Based on the total time for which we have observations, we estimate that the source has a duty cycle of roughly 10%.

astro-ph↗

arXiv2005

Canonical Particle Acceleration in FRI Radio Galaxies

Matched resolution multi-frequency VLA observations of four radio galaxies are used to derive the asymptotic low energy slope of the relativistic electron distribution. Where available, low energy slopes are also determined for other sources in the literature. They provide information on the acceleration physics independent of radiative and other losses, which confuse measurements of the synchrotron spectra in most radio, optical and X-ray studies. We find a narrow range of inferred low energy electron energy slopes, n(E)=const*E^-2.1 for the currently small sample of lower luminosity sources classified as FRI (not classical doubles). This distribution is close to, but apparently inconsistent with, the test particle limit of n(E)=const*E^-2.0 expected from strong diffusive shock acceleration in the non-relativistic limit. Relativistic shocks or those modified by the back-pressure of efficiently accelerated cosmic rays are two alternatives to produce somewhat steeper spectra. We note for further study the possiblity of acceleration through shocks, turbulence or shear in the flaring/brightening regions in FRI jets as they move away from the nucleus. Jets on pc scales and the collimated jets and hot spots of FRII (classical double) sources would be governed by different acceleration sites and mechanisms; they appear to show a much wider range of spectra than for FRI sources.

astro-ph↗