C. J. Law

A Neutron Star with a Massive Progenitor in Westerlund 1

We report the discovery of an X-ray pulsar in the young, massive Galactic star cluster Westerlund 1. We detected a coherent signal from the brightest X-ray source in the cluster, CXO J164710.2–455216, during two Chandra observations on 2005 May 22 and June 18. The period of the pulsar is 10.6107(1) s. We place an upper limit to the period derivative of u P 1M⊙. Taken together, the properties of the pulsar indicate that it is a magnetar. The rarity of slow X-ray pulsars and the position of CXO J164710.2–455216 only 1.6 ′ from the core of Westerlund 1 indicates that it is a member of the cluster with >99.97% confidence. Westerlund 1 contains 07V stars with initial masses Mi�35M⊙ and >50 post-main-sequence stars that indicate the cluster is 4±1 Myr old. Therefore, the progenitor to this pulsar had an initial mass Mi>40M⊙. This is the most secure result among a handful of observational limits to the masses of the progenitors to neutron stars. Subject headings: X-rays: stars — neutron stars — open clusters and associations: individual (Westerlund 1)

Observing pulsars and fast transients with LOFAR

Low frequency radio waves, while challenging to observe, are a rich source of information about pulsars. The LOw Frequency ARray (LOFAR) is a new radio interferometer operating in the lowest 4 octaves of the ionospheric radio window: 10-240 MHz, that will greatly facilitate observing pulsars at low radio frequencies. Through the huge collecting area, long baselines, and flexible digital hardware, it is expected that LOFAR will revolutionize radio astronomy at the lowest frequencies visible from Earth. LOFAR is a next-generation radio telescope and a pathfinder to the Square Kilometre Array (SKA), in that it incorporates advanced multi-beaming techniques between thousands of individual elements. We discuss the motivation for low-frequency pulsar observations in general and the potential of LOFAR in addressing these science goals. We present LOFAR as it is designed to perform high-time-resolution observations of pulsars and other fast transients, and outline the various relevant observing modes and data reduction pipelines that are already or will soon be implemented to facilitate these observations. A number of results obtained from commissioning observations are presented to demonstrate the exciting potential of the telescope. This paper outlines the case for low frequency pulsar observations and is also intended to serve as a reference for upcoming pulsar/fast transient science papers with LOFAR.

A direct localization of a fast radio burst and its host

Fast radio bursts are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts and tidal disruption events. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.

The Host Galaxy and Redshift of the Repeating Fast Radio Burst FRB 121102

The precise localization of the repeating fast radio burst (FRB 121102) has provided the first unambiguous association (chance coincidence probability p ≲ 3 × 10‑4) of an FRB with an optical and persistent radio counterpart. We report on optical imaging and spectroscopy of the counterpart and find that it is an extended (0.″6–0.″8) object displaying prominent Balmer and [Oiii] emission lines. Based on the spectrum and emission line ratios, we classify the counterpart as a low-metallicity, star-forming, mr‧ = 25.1 AB mag dwarf galaxy at a redshift of z =0.19273(8), corresponding to a luminosity distance of 972 Mpc. From the angular size, the redshift, and luminosity, we estimate the host galaxy to have a diameter ≲4 kpc and a stellar mass of M* ∼ (4–7) × 107 M⊙, assuming a mass-to-light ratio between 2 to 3 M⊙L⊙‑1. Based on the Hα flux, we estimate the star formation rate of the host to be 0.4 M⊙yr‑1 and a substantial host dispersion measure (DM)depth ≲324 pc cm‑3. The net DM contribution of the host galaxy to FRB 121102 is likely to be lower than this value depending on geometrical factors. We show that the persistent radio source at FRB 121102’s location reported by Marcote et al. is offset from the galaxy’s center of light by ∼200 mas and the host galaxy does not show optical signatures for AGN activity. If FRB121102 is typical of the wider FRB population and if futureinterferometric localizations preferentially find them in dwarf galaxies with low metallicities and prominent emission lines, they would share such a preference with long gamma-ray bursts and superluminous supernovae.

The LOFAR pilot surveys for pulsars and fast radio transients

We have conducted two pilot surveys for radio pulsars and fast transients with the Low-Frequency Array (LOFAR) around 140 MHz and here report on the first low-frequency fast-radio burst limit and the discovery of two new pulsars. The first survey, the LOFAR Pilot Pulsar Survey (LPPS), observed a large fraction of the northern sky, ~1.4 x 10^4 sq. deg, with 1-hr dwell times. Each observation covered ~75 sq. deg using 7 independent fields formed by incoherently summing the high-band antenna fields. The second pilot survey, the LOFAR Tied-Array Survey (LOTAS), spanned ~600 sq. deg, with roughly a 5-fold increase in sensitivity compared with LPPS. Using a coherent sum of the 6 LOFAR Superterp stations, we formed 19 tied-array beams, together covering 4 sq. deg per pointing. From LPPS we derive a limit on the occurrence, at 142 MHz, of dispersed radio bursts of 107 Jy for the narrowest searched burst duration of 0.66 ms. In LPPS, we re-detected 65 previously known pulsars. LOTAS discovered two pulsars, the first with LOFAR or any digital aperture array. LOTAS also re-detected 27 previously known pulsars. These pilot studies show that LOFAR can efficiently carry out all-sky surveys for pulsars and fast transients, and they set the stage for further surveying efforts using LOFAR and the planned low-frequency component of the Square Kilometer Array.

Diffuse, Nonthermal X-Ray Emission from the Galactic Star Cluster Westerlund 1

We present the diffuse X-ray emission identified in Chandra observations of the young, massive Galactic star cluster Westerlund 1. After removing pointlike X-ray sources down to a completeness limit of ≈ ergs s−1, we identify ergs s−1 (2–8 keV) of diffuse emission. The spatial distribution of the emission can be described as a slightly elliptical Lorentzian core with a half-width at half-maximum along the major axis of , similar to the distribution of point sources in the cluster, plus a 5 halo of extended emission. The spectrum of the diffuse emission is dominated by a hard continuum component that can be described as a keV thermal plasma that has a low iron abundance (0.3 solar) or as nonthermal emission that could be stellar light that is inverse Compton scattered by MeV electrons. Only 5% of the flux is produced by a keV plasma. The low luminosity of the thermal emission and the lack of a 6.7 keV iron line suggest that 40,000 unresolved stars with masses between 0.3 and 2 M are present in the cluster, fewer than previously estimated. Moreover, the flux in the diffuse emission is a factor of several lower than would be expected from a supersonically expanding cluster wind, and there is no evidence for thermal remnants produced by supernovae. Less than 10−5 of the mechanical luminosity of the cluster is dissipated as 2–8 keV X-rays, leaving a large amount of energy that either is radiated at other wavelengths, is dissipated beyond the bounds of our image, or escapes into the intergalactic medium.

X-Ray Observations of Stellar Clusters Near the Galactic Center

We report the first detection of X-ray emission from the Quintuplet star cluster and compare its X-ray emission to that of the Arches star cluster. Four point sources are significantly detected near the core of the Quintuplet cluster with a total absorption-corrected luminosity of � 1 ; 10 33 ergs s � 1 . Diffuse, thermal emission is also detected near the core of the Quintuplet cluster with an absorption-corrected luminosity of � 1 ; 10 34 ergs s � 1 . We analyze the diffuse and pointlike emission from the Arches and Quintuplet clusters and discuss the possibility that they are host to cluster wind outflows. We also present the results of our search for X-ray emission from candidate star clusters in the Galactic center (GC) region. We use extinction estimated by near-IR colors and X-ray spectral fits, as well as other IR properties, to determine if the candidate clusters are new GC star clusters. We find that three of the six candidate clusters toward the GC are likely foreground clusters, two of the candidate clusters are not detected in the X-ray data but have near-IR extinctions consistent with a GC location, and one of the candidate clusters has X-ray and near-IR extinctions consistent with being in the GC. The X-ray and IR emission from the candidate clusters is compared to the known massive GC star clusters. Subject headings: Galaxy: center — Galaxy: open clusters and associations: individual (Arches, Quintuplet) — stars: Wolf-Rayet — X-rays: galaxies: clusters

The Allen Telescope Array Twenty-centimeter Survey—a 690 deg2, 12 epoch radio data set. I. Catalog and long-duration transient statistics

We present the Allen Telescope Array Twenty-centimeter Survey (ATATS), a multi-epoch (12 visits), 690 deg(2) radio image and catalog at 1.4 GHz. The survey is designed to detect rare, very bright transients as well as to verify the capabilities of the ATA to form large mosaics. The combined image using data from all 12 ATATS epochs has rms noise sigma = 3.94 mJy beam(-1) and dynamic range 180, with a circular beam of 150 FWHM. It contains 4408 sources to a limiting sensitivity of 5 sigma = 20 mJy beam(-1). We compare the catalog generated from this 12 epoch combined image to the NRAO VLA Sky Survey (NVSS), a legacy survey at the same frequency, and find that we can measure source positions to better than similar to 20 . For sources above the ATATS completeness limit, the median flux density is 97% of the median value for matched NVSS sources, indicative of an accurate overall flux calibration. We examine the effects of source confusion due to the effects of differing resolution between ATATS and NVSS on our ability to compare flux densities. We detect no transients at flux densities greater than 40 mJy in comparison with NVSS and place a 2 sigma upper limit of 0.004 deg(-2) on the transient rate for such sources. These results suggest that the greater than or similar to 1 Jy transients reported by Matsumara et al. may not be true transients, but rather variable sources at their flux density threshold.

A mid-infrared survey of the inner 2 × 1.5 degrees of the Galaxy with Spitzer/IRAC

We present a survey of Spitzer Space Telescope/IRAC observations of the central 2 × 1.5 degrees (265 × 200 pc) of the Galaxy at 3-8 μm. These data represent the highest spatial resolution and sensitivity large-scale map made to date of the Galactic Center (GC) at mid-infrared wavelengths. The IRAC data provide a census of the optically obscured stellar sources as well as a detailed map of the highly filamentary structure in the interstellar medium. The diffuse emission is dominated by PAH emission from small grains in star-forming regions. Dark clouds displaying a large variety of sizes and morphologies are imaged, many of which remain opaque at IRAC wavelengths. Using a multiwavelength comparison, we determine which objects are likely to be in the foreground and which are located at the GC. We find no counterparts at IRAC wavelengths to the unique system of linear, nonthermal radio filaments present at the GC.

The Repeating Fast Radio Burst FRB 121102 as Seen on Milliarcsecond Angular Scales

The millisecond-duration radio flashes known as fast radio bursts (FRBs) represent an enigmatic astrophysical phenomenon. Recently, the sub-arcsecond localization (∼100 mas precision) of FRB 121102 using the Very Large Array has led to its unambiguous association with persistent radio and optical counterparts, and to the identification of its host galaxy. However, an even more precise localization is needed in order to probe the direct physical relationship between the millisecond bursts themselves and the associated persistent emission. Here, wereport very-long-baseline radio interferometric observations using the European VLBI Network and the 305 m Arecibo telescope, which simultaneously detect both the bursts and the persistent radio emission at milliarcsecond angular scales and show that they are co-located to within a projected linear separation of ≲40 pc (≲12 mas angular separation, at 95% confidence). We detect consistent angular broadening of the bursts and persistent radio source (∼2–4 mas at 1.7 GHz), which are both similar to the expected Milky Way scattering contribution. The persistent radio source has a projected size constrained to be ≲ 0.7 pc (≲0.2 mas angular extent at 5.0 GHz) and a lower limit for the brightness temperature of Tb ≳ 5× 107 K. Together, these observations provide strong evidence for a direct physical link between FRB 121102 and the compact persistent radio source. We argue that a burst source associated with a low-luminosity active galactic nucleus or a young neutron star energizing a supernova remnant are the two scenarios for FRB 121102 that best match the observed data.