Namir E. Kassim

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

Abstract : 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 3C 84 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 to the 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 10 MHz. 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(exp 44) and 10(exp 45) erg s (exp-1), depending on the filling factor of the relativistic plasma.

The Very Large Array Low-frequency Sky Survey Redux (VLSSr)

We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalog as a sky model to correct the ionospheric distortions in the data and create a new set of sky maps and corresponding catalog at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map RMS noise level of σ � 0.1 Jy beam 1 . The clean bias is 0.66 × σ and the theoretical largest angular size is 36 arcmin. Six previously unimaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalog includes 92,964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing center in the VLSS.

A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER

We present a catalog of 9017 X-ray sources identified in Chandra observations of a 2 ◦ × 0. 8 field around the Galactic center. This enlarges the number of known X-ray sources in the region by a factor of 2.5. The catalog incorporates all of the ACIS-I observations as of 2007 August, which total 2.25 Ms of exposure. At the distance to the Galactic center (8 kpc), we are sensitive to sources with luminosities of 4 × 10 32 erg s −1 (0.5–8.0 keV; 90% confidence) over an area of 1 deg 2 , and up to an order of magnitude more sensitive in the deepest exposure (1.0 Ms) around Sgr A ∗ . The positions of 60% of our sources are accurate to <1 �� (95% confidence), and 20% have positions accurate to < 0. 5. We search for variable sources, and find that 3% exhibit flux variations within an observation, and 10% exhibit variations from observation-to-observation. We also find one source, CXOUGC J174622.7−285218, with a periodic 1745 s signal (1.4% chance probability), which is probably a magnetically accreting cataclysmic variable. We compare the spatial distribution of X-ray sources to a model for the stellar distribution, and find 2.8σ evidence for excesses in the numbers of X-ray sources in the region of recent star formation encompassed by the Arches, Quintuplet, and Galactic center star clusters. These excess sources are also seen in the luminosity distribution of the X-ray sources, which is flatter near the Arches and Quintuplet than elsewhere in the field. These excess point sources, along with a similar longitudinal asymmetry in the distribution of diffuse iron emission that has been reported by other authors, probably have their origin in the young stars that are prominent at l ≈ 0. 1.

A radio counterpart to a neutron star merger

GROWTH observations of GW170817 The gravitational wave event GW170817 was caused by the merger of two neutron stars (see the Introduction by Smith). In three papers, teams associated with the GROWTH (Global Relay of Observatories Watching Transients Happen) project present their observations of the event at wavelengths from x-rays to radio waves. Evans et al. used space telescopes to detect GW170817 in the ultraviolet and place limits on its x-ray flux, showing that the merger generated a hot explosion known as a blue kilonova. Hallinan et al. describe radio emissions generated as the explosion slammed into the surrounding gas within the host galaxy. Kasliwal et al. present additional observations in the optical and infrared and formulate a model for the event involving a cocoon of material expanding at close to the speed of light, matching the data at all observed wavelengths. Science, this issue p. 1565, p. 1579, p. 1559; see also p. 1554 Radio observations constrain the energy and geometry of relativistic material ejected from a binary neutron star merger. Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultrarelativistic jet, viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will enable observers to distinguish between these models, and the angular velocity and geometry of the debris will be directly measurable by very long baseline interferometry.

A powerful bursting radio source towards the Galactic Centre

Transient astronomical sources are typically powered by compact objects and usually signify highly explosive or dynamic events. Although high-time-resolution observations are often possible in radio astronomy, they are usually limited to quite narrow fields of view. The dynamic radio sky is therefore poorly sampled, in contrast to the situation in the X-ray and γ-ray bands in which wide-field instruments routinely detect transient sources. Here we report a transient radio source, GCRT J1745–3009, which was detected during a moderately wide-field monitoring programme of the Galactic Centre region at 0.33 GHz. The characteristics of its bursts are unlike those known for any other class of radio transient. If located in or near the Galactic Centre, its brightness temperature (∼1016 K) and the implied energy density within GCRT J1745–3009 vastly exceed those observed in most other classes of radio astronomical sources, and are consistent with coherent emission processes that are rarely observed. We conclude that it represents a hitherto unknown class of transient radio sources, the first of possibly many new classes that may be discovered by emerging wide-field radio telescopes.

The LWA1 Radio Telescope

LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instruments “all dipoles” modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and four independently steerable beams utilizing digital “true time delay” beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

The radio nebula of the soft γ-ray repeater 1806 – 20

AN important clue to the nature of soft γ-ray repeaters (SGRs), which emit recurrent bursts of γ-rays, has been provided by the association of two of the three known SGRs with supernova remnants1,2 (SNRs). Here we present radio images of the non-thermal radio nebula G10.0 – 0.3, a supernova remnant which has been associated previously2 with SGR1806 – 20 (refs 3, 4). Our images show that the nebula is a plerion (that is, the radio emission is synchrotron radiation powered by a central pulsar), as revealed by the observation of a hierarchy of nested shells and a bright central peak. The recent detection5 of an X-ray point source coincident with the radio peak and of a hard X-ray burst5,6 from G10.0 – 0.3 confirms the SGR–SNR association. We propose that SGR1806 – 20 is an isolated pulsar that emits both steady and impulsive winds of relativistic particles, which together power the nebula. We suggest that the offset from the centres of the SNRs observed for both this object and SGR0526 – 66 (ref. 1), requiring high velocities of the pulsars, provides a clue to their formation mechanism.

Spectral index maps of the radio halos in Abell 665 and Abell 2163

New radio data at 330 MHz are presented for the rich clusters Abell 665 and Abell 2163, whose radio emission is characterized by the presence of a radio halo. These images allowed us to derive the spectral properties of the two clusters under study. The integrated spectra of these halos between 0.3 GHz and 1.4 GHz are moderately steep: α 1.4 0.3 = 1.04 and α 1.4 0.3 = 1.18, for A665 and A2163, respectively. The spectral index maps, produced with an angular resolution of the order of ∼1 � , show features of the spectral index (flattening and patches), which are indication of a complex shape of the radiating electron spectrum, and are therefore in support of electron reacceleration models. Regions of flatter spectrum are found to be related to the recent merger activity in these clusters. This is the first strong confirmation that the cluster merger supplies energy to the radio halo. In the undisturbed cluster regions, the spectrum steepens with the distance from the cluster center. This is interpreted as the result of the combination of the magnetic field profile with the spatial distribution of the reacceleration efficiency, thus allowing us to set constraints on the radial profile of the cluster magnetic field.

The low-frequency radio emission and spectrum of the extended SNR W44: new VLA observations at 74 and 324 MHz

Aims. We present new Very Large Array (VLA) radio images at 74 and 324 MHz of the SNR W44. The VLA images, obtained with unprecedented angular resolution and sensitivity for such low frequencies ( HPBW 37″ at 74 MHz, and 13″ at 324 MHz), have been used in combination with existing 1442 MHz radio data, Spitzer IR data, and ROSAT and Chandra X-ray data to investigate morphological and spectral continuum properties of this SNR. Methods. The observations were carried out with the VLA simultaneously at 74 and 324 MHz in the A and B configurations and at 324 MHz in the C and D configurations. The radio continuum spectral index distribution was derived through direct comparison of the combined data at 74, 324, and 1442 MHz. In addition, to isolate and identify different spectral components, tomographic spectral analysis was performed. Results. We measured total flux densities of 634 Jy and 411 Jy at 74 and 324 MHz, respectively, for W44, and from a careful assessment of published values between 22 and 10 700 MHz derived a global integrated continuum spectral index

New Nonthermal Filaments at the Galactic Center: Are They Tracing a Globally Ordered Magnetic Field

\alpha=-0.37 \pm 0.02