Stellar Forensics II: Millisecond Pulsar Binaries

We use the grid of models described in paper~I to analyse those millisecond pulsar binaries whose secondaries have been studied optically. In particular, we find cooling ages for these binary systems that range from $< 1 \rm Gyr$ to $\sim \rm 15 Gyr$. Comparison of cooling ages and characteristic spin down ages allows us to constrain the initial spin periods and spin-up histories for individual systems, showing that at least some millisecond pulsars had sub-Eddington accretion rates and long magnetic field decay times.Comment: Latex, 14 pages, and 15 postscript figures. Accepted by Monthly Notice

The Pulsar Kick Velocity Distribution

We analyse the sample of pulsar proper motions, taking detailed account of the selection effects of the original surveys. We treat censored data using survival statistics. From a comparison of our results with Monte Carlo simulations, we find that the mean birth speed of a pulsar is 250-300 km/s, rather than the 450 km/s foundby Lyne & Lorimer (1994). The resultant distribution is consistent with a maxwellian with dispersion $\sigma_v = 190 km/s$. Despite the large birth velocities, we find that the pulsars with long characteristic ages show the asymmetric drift, indicating that they are dynamically old. These pulsars may result from the low velocity tail of the younger population, although modified by their origin in binaries and by evolution in the galactic potential.Comment: Latex, 10 pages, and 11 postscript figures. Accepted by Monthly Notice

La Freccia Rossa: An IR-dark cloud hosting the Milky Way intermediate-mass black hole candidate

The dynamics of the high-velocity compact molecular cloud CO-0.40-0.22 have been interpreted as evidence for a $\sim10^{5}M_{\odot}$ black hole within 60 pc of Sgr A*. Recently, Oka et al. have identified a compact millimetre-continuum source, CO-0.40-0.22*, with this candidate black hole. Here we present a collation of radio and infrared data at this location. ATCA constraints on the radio spectrum, and the detection of a mid-infrared counterpart, are in tension with an Sgr A*-like model for CO-0.40-0.22* despite the comparable bolometric to Eddington luminosity ratios under the IMBH interpretation. A protostellar-disk scenario is, however, tenable. CO-0.40-0.22(*) is associated with an arrowhead-shaped infrared-dark cloud (which we call the Freccia Rossa). Radio-continuum observations reveal a candidate HII region associated with the system. If the $V_{\rm LSR}\approx70$ km s$^{-1}$ systemic velocity of CO-0.40-0.22 is common to the entire Freccia Rossa system, we hypothesise that it is the remnant of a high-velocity cloud that has plunged into the Milky Way from the Galactic halo.Comment: 6 pages, 3 figures, submitted to MNRAS Letter

Imprint of local environment on fast radio burst observations

When fast radio burst (FRB) waves propagate through the local (⁠≲1pc⁠) environment of the FRB source, electrons in the plasma undergo large-amplitude oscillations. The finite-amplitude effects cause the effective plasma frequency and cyclotron frequency to be dependent on the wave strength. The dispersion measure and rotation measure should therefore vary slightly from burst to burst for a repeating source, depending on the luminosity and frequency of the individual burst. Furthermore, free–free absorption of strong waves is suppressed due to the accelerated electrons’ reduced energy exchange in Coulomb collisions. This allows bright low-frequency bursts to propagate through an environment that would be optically thick to low-amplitude waves. Given a large sample of bursts from a repeating source, it would be possible to use the deficit of low-frequency and low-luminosity bursts to infer the emission measure of the local intervening plasma and its distance from the source. Information about the local environment will shed light on the nature of FRB sources

Imprint of local environment on fast radio burst observations

When fast radio burst (FRB) waves propagate through the local (⁠≲1pc⁠) environment of the FRB source, electrons in the plasma undergo large-amplitude oscillations. The finite-amplitude effects cause the effective plasma frequency and cyclotron frequency to be dependent on the wave strength. The dispersion measure and rotation measure should therefore vary slightly from burst to burst for a repeating source, depending on the luminosity and frequency of the individual burst. Furthermore, free–free absorption of strong waves is suppressed due to the accelerated electrons’ reduced energy exchange in Coulomb collisions. This allows bright low-frequency bursts to propagate through an environment that would be optically thick to low-amplitude waves. Given a large sample of bursts from a repeating source, it would be possible to use the deficit of low-frequency and low-luminosity bursts to infer the emission measure of the local intervening plasma and its distance from the source. Information about the local environment will shed light on the nature of FRB sources

Stellar Forensics I: Cooling Curves

The presence of low mass, degenerate secondaries in millisecond pulsar binaries offers the opportunity to determine an age for the binary system independent of the rotational properties of the pulsar. To this end, we present here a detailed calculation of the evolution of a grid of low mass ($<0.5 M_{\odot}$) helium core white dwarfs. We investigate the effects of different Hydrogen layer masses and provide results for well-known optical band-passes. We supplement the OPAL opacity calculations with our own calculations for low effective temperatures ($T_{eff} < 6000 K$) and also provide fitting formulae for the gravity as a function of mass and effective temperature. In paper II we shall apply these results to individual cases.Comment: Latex, 10 pages, and 16 postscript figures. Accepted by Monthly Notice

Laser interferometry for the Big Bang Observer

The Big Bang Observer is a proposed space-based gravitational-wave detector intended as a follow on mission to the Laser Interferometer Space Antenna (LISA). It is designed to detect the stochastic background of gravitational waves from the early universe. We discuss how the interferometry can be arranged between three spacecraft for this mission and what research and development on key technologies are necessary to realize this scheme

A cool accretion disk around the Galactic Centre black hole

There is a supermassive black hole of mass 4 × 10^6 solar masses at the centre of the Milky Way. A large reservoir of hot (10^7 kelvin) and cooler (10^2 to 10^4 kelvin) gas surrounds it within a few parsecs. Although constraints on the amount of hot gas in the accretion zone of the black hole—that is, within 10^5 Schwarzschild radii (0.04 parsecs)—have been provided by X-ray observations, the mass in cooler gas has been unconstrained. One possible way this cooler gas could be detected is by its emission in hydrogen recombination spectral lines. Here we report imaging of a 10^4-kelvin ionized gas disk within 2 × 10^4 Schwarzschild radii, using the 1.3-millimetre recombination line H30α. This emission line is double-peaked, with full velocity linewidth of about 2,200 kilometres per second. The emission is centred on the radio source Sagittarius A*, but the redshifted side is displaced 0.11 arcsec (0.004 parsecs at a distance of 8 kiloparsecs) to the northeast and the blueshifted side is displaced a similar distance to the southwest. We interpret these observations in terms of a rotating disk of mass 10^(−5) to 10^(−4) solar masses and mean hydrogen density of about 10^5 to 10^6 per cubic centimetre, with the values being sensitive to the assumed geometry. The emission is stronger than expected, given the upper limit on the strength of the Brγ spectral line of hydrogen. We suggest that the H30α transition is enhanced by maser emission