Scott D. Hyman

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 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.

A New Radio Detection of the Transient Bursting Source GCRT J1745–3009

GCRT J1745-3009 is a transient bursting radio source located in the direction of the Galactic center, discovered by Hyman et al. in 330 MHz VLA observations from 2002 September 30-October 1. 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 Giant Metrewave Radio Telescope (GMRT) observations. We report a new radio detection of the source in 330 MHz GMRT data taken on 2003 September 28. Due to sparse sampling of the observation, only a single burst, already at or passed its peak, was detected. The maximum flux density detected was ~0.5 Jy, approximately 3 times weaker than the five bursts detected in 2002. We present tentative evidence suggesting that this burst is an isolated one, but we cannot rule out additional undetected bursts that may have occurred with the same ~77 minute periodicity observed in 2002 or with a different periodicity. Assuming the peak emission was detected, the decay time of the burst, ~2 minutes, is consistent with that determined for the 2002 bursts. Based on the total time for which we have observations, we estimate that the source exhibits bursting activity roughly 9% of the time.

DISCOVERY OF A METER-WAVELENGTH RADIO TRANSIENT IN THE SWIRE DEEP FIELD: 1046+59

We report the results of a low frequency radio variability and slow transient search using archival observations from the Very Long Array. We selected six 325 MHz radio observations from the spring of 2006, each centered on the Spitzer-Space-Telescope Wide-area Infrared Extragalactic Survey (SWIRE) Deep Field: 1046+59. Observations were spaced between one day to three months, with a typical single-epoch peak flux sensitivity below 0.2 mJy beam{sup -1} near the field pointing center. We describe the observation parameters, data post-processing, and search methodology used to identify variable and transient emission. Our search revealed multiple variable sources and the presence of one, day-scale transient event with no apparent astronomical counterpart. This detection implies a transient rate of 1 {+-} 1 event per 6.5 deg{sup 2} per 72 observing hours in the direction of 1046+59 and an isotropic transient surface density {Sigma} = 0.12 deg{sup -2}at 95% confidence for sources with average peak flux density higher than 2.1 mJy over 12 hr.

Low-Frequency Radio Transients in the Galactic Center

We report the detection of a new radio transient source, GCRT J1746-2757, located only 11 north of the Galactic center. Consistent with other radio transients toward the Galactic center, this source brightened and faded on a timescale of a few months. No X-ray counterpart was detected. This suggests that either there was only very faint X-ray emission, compared with highly Doppler boosted radio emission, or that GCRT J1746-2757 is a member of a class of radio transients with no associated X-ray emission. We also report new 0.33 GHz measurements of the radio counterpart to the X-ray transient source, XTE J1748-288, previously detected and monitored at higher radio frequencies. We show that the spectrum of XTE J1748-288 steepened considerably during a period of a few months after its peak. We also discuss the need for a more efficient means of finding additional radio transients.

Radio Continuum Imaging of the Spiral Galaxy NGC 4258

We analyze 35 resolution, high-sensitivity radio continuum images of the nearby spiral galaxy NGC 4258 at 6 and 20 cm derived from multiple observations used to monitor the radio supernova SN 1981K by Van Dyk et al. Seven bright H II regions and five supernova remnant candidates are identified. Extinctions to the H II regions are estimated for the first time from a comparison of radio flux densities to new optical fluxes derived from Hα observations by Dutil & Roy. The bright end of the H II region luminosity function is established at each wavelength. The luminosity functions are best fit by power laws consistent with the shape of previously published radio and optical luminosity functions for a number of galaxies. The supernova remnants are all about 2-3 times the radio luminosity of Cas A. In addition, the galaxys nucleus is shown to have been variable over the SN 1981K monitoring period. The spectral index (α) distribution of the anomalous radio arms is investigated and found to be relatively uniform at α = -0.65 ± 0.10.

Circularly Polarized Emission from the Transient Bursting Radio Source GCRT J1745 – 3009

We report on the detection of strong circularly polarized emission from the transient bursting source GCRT J1745 – 3009 based on new analysis of 325 MHz Giant Metrewave Radio Telescope observations conducted on 2003 September 28. We place 8 R ☉ 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 ≤4 kpc away could have given rise to the GCRT radio emission.

A faint, steep-spectrum 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. The 2004 flux density was ~0.05 Jy, ~10× weaker than the single 2003 burst and ~30× weaker than the five bursts detected in 2002. We derive a very steep spectral index, α = -13.5 ± 3.0, across the bandpass, a new result previously not obtained in the 2002 and 2003 observations. Also, the burst was detected for only ~2 minutes, in contrast to the ~10 minute 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 completely rule out additional undetected bursts that may have occurred with the same ~77 minute 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.

A Search for the Near-Infrared Counterpart to GCRT?J1745?3009

We present an optical/near-infrared search for a counterpart to the perplexing radio transient GCRT J1745?3009, a source located ~1? from the Galactic center. Motivated by some similarities to radio bursts from nearby ultracool dwarfs, and by a distance upper limit of -->70 pc for the emission to not violate the -->1012 K brightness temperature limit for incoherent radiation, we searched for a nearby star at the position of GCRT J1745?3009. We found only a single marginal candidate, limiting the presence of any late-type star to >1?kpc (spectral types earlier than M9), >200?pc (spectral types L and T0-T4), and >100?pc (spectral types T4-T7), thus severely restricting the possible local counterparts to GCRT J1745?3009. We also exclude any white dwarf within -->1 kpc or a supergiant star out to the distance of the Galactic center as possible counterparts. This implies that GCRT J1745?3009 likely requires a coherent emission process, although whether or not it reflects a new class of sources is unclear.

EXPLORING THE TRANSIENT RADIO SKY WITH VLITE: EARLY RESULTS

We report on a search for radio transients at 340 MHz with the Jansky Very Large Array (VLA) Low band Ionospheric and Transient Experiment (VLITE). Between 2015 July 29 and 2015 September 27, operating in commensal mode, VLITE imaged approximately 2800 pointings covering 12,000 deg