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The Crab Pulsar's radio emission is unusual, consisting predominantly of giant pulses, with durations of about a micro-second but structure down to the nano-second level, and brightness temperatures of up to $10^{37}\,$K. It is unclear how giant pulses are produced, but they likely originate near the pulsar's light cylinder, where corotating plasma approaches the speed of light. We report observations in the 400-800 MHz frequency band, where the pulses are broadened by scattering in the surrounding Crab nebula. We find that some pulse frequency spectra show strong bands, which vary during the scattering tail, in one case showing a smooth upward drift. While the banding may simply reflect interference between nano-second scale pulse components, the variation is surprising, as in the scattering tail the only difference is that the source is observed via slightly longer paths, bent by about an arcsecond in the nebula. The corresponding small change in viewing angle could nevertheless reproduce the observed drift by a change in Doppler shift, if the plasma that emitted the giant pulses moved highly relativistically, with a Lorentz factor $\gamma\sim10^4$ (and without much spread in $\gamma$). If so, this would support models that appeal to highly relativistic plasma to transform ambient magnetic structures to coherent GHz radio emission, be it for giant pulses or for potentially related sources, such as fast radio bursts., Comment: 13 pages, 8 figures

The hierarchical triple system PSR J0337+1715 offers an unprecedented laboratory to study secular evolution of interacting systems and to explore the complicated mass-transfer history that forms millisecond pulsars and helium-core white dwarfs. The latter in particular, however, requires knowledge of the properties of the individual components of the system. Here we present precise optical spectroscopy of the inner companion in the PSR J0337+1715 system. We confirm it as a hot, low-gravity DA white dwarf with Teff=15,800+/-100 K and log(g)=5.82+/-0.05. We also measure an inner mass ratio of 0.1364+/-0.0015, entirely consistent with that inferred from pulsar timing, and a systemic radial velocity of 29.7+/-0.3 km/s. Combined with the mass (0.19751 Msun) determined from pulsar timing, our measurement of the surface gravity implies a radius of 0.091+/-0.005 Rsun; combined further with the effective temperature and extinction, the photometry implies a distance of 1300+/-80 pc. The high temperature of the companion is somewhat puzzling: with current models, it likely requires a recent period of unstable hydrogen burning, and suggests a surprisingly short lifetime for objects at this phase in their evolution. We discuss the implications of these measurements in the context of understanding the PSR J0337+1715 system, as well as of low-mass white dwarfs in general., ApJ Letters, in press. 6 pages, two figures. v2 fixes typo

Many physically motivated extensions to general relativity (GR) predict significant deviations in the properties of spacetime surrounding massive neutron stars. We report the measurement of a 2.01 +/- 0.04 solar mass pulsar in a 2.46-hr orbit with a 0.172 +/- 0.003 solar mass white dwarf. The high pulsar mass and the compact orbit make this system a sensitive laboratory of a previously untested strong-field gravity regime. Thus far, the observed orbital decay agrees with GR, supporting its validity even for the extreme conditions present in the system. The resulting constraints on deviations support the use of GR-based templates for ground-based gravitational wave detectors. Additionally, the system strengthens recent constraints on the properties of dense matter and provides insight to binary stellar astrophysics and pulsar recycling., This is the authors' version of the work. The definite version is published in Science Online, 26 April 2013, Vol: 340, Issue: 6131 doi: 10.1126/science.1233232. 54 pages, 18 figures, 4 tables

The merger of two carbon-oxygen white dwarfs can lead either to a spectacular transient, stable nuclear burning or a massive, rapidly rotating white dwarf. Simulations of mergers have shown that the outcome strongly depends on whether the white dwarfs are similar or dissimilar in mass. In the similar-mass case, both white dwarfs merge fully and the remnant is hot throughout, while in the dissimilar case, the more massive, denser white dwarf remains cold and essentially intact, with the disrupted lower mass one wrapped around it in a hot envelope and disk. In order to determine what constitutes "similar in mass" and more generally how the properties of the merger remnant depend on the input masses, we simulated unsynchronized carbon-oxygen white dwarf mergers for a large range of masses using smoothed-particle hydrodynamics. We find that the structure of the merger remnant varies smoothly as a function of the ratio of the central densities of the two white dwarfs. A density ratio of 0.6 approximately separates similar and dissimilar mass mergers. Confirming previous work, we find that the temperatures of most merger remnants are not high enough to immediately ignite carbon fusion. During subsequent viscous evolution, however, the interior will likely be compressed and heated as the disk accretes and the remnant spins down. We find from simple estimates that this evolution can lead to ignition for many remnants. For similar-mass mergers, this would likely occur under sufficiently degenerate conditions that a thermonuclear runaway would ensue., 33 pages, 18 figures, one seven-page table, published in ApJ 8 April 2013

We present optical spectroscopic measurements of the eclipsing high-mass X-ray binary (HMXB) XMMU J013236.7+303228 in M 33. Based on spectra taken at multiple epochs of the 1.73 day binary orbital period we determine physical as well as orbital parameters for the donor star. We find the donor to be a B1.5IV subgiant with effective temperature T = 22, 000-23, 000 K. From the luminosity, temperature, and known distance to M 33 we derive a radius of R = 8.9 ± 0.5 R_☉. From the radial-velocity measurements, we determine a velocity semi-amplitude of K_opt = 63 ± 12 km s^(–1). Using the physical properties of the B star determined from the optical spectrum, we estimate the star's mass to be M_opt = 11 ± 1 M_☉. Based on the X-ray spectrum, the compact companion is likely a neutron star, although no pulsations have yet been detected. Using the spectroscopically derived B star mass we find the neutron star companion mass to be M_X = 2.0 ± 0.4 M_☉, consistent with the neutron star mass in the HMXB Vela X-1, but heavier than the canonical value of 1.4 M_☉ found for many millisecond pulsars. We attempt to use as an additional constraint that the B star radius inferred from temperature, flux, and distance should equate to the Roche radius, since the system accretes by Roche lobe overflow. This leads to substantially larger masses, but by trying to apply the technique to known systems, we find that the masses are consistently overestimated. Attempting to account for that in our uncertainties, we derive M_X = 2.2^(+0.8)_(–0.6) M_☉ and M opt = 13 ± 4 M_☉. We conclude that precise constraints require detailed modeling of the shape of the Roche surface.

We present the results of a multiplicity survey of 212 T Tauri stars in the Chamaeleon I and Taurus-Auriga star-forming regions, based on high-resolution spectra from the Magellan Clay 6.5 m telescope. From these data, we achieved a typical radial velocity precision of ~80 m/s with slower rotators yielding better precision, in general. For 174 of these stars, we obtained multi-epoch data with sufficient time baselines to identify binaries based on radial velocity variations. We identified eight close binaries and four close triples, of which three and two, respectively, are new discoveries. The spectroscopic multiplicity fractions we find for Cha I (7%) and Tau-Aur (6%) are similar to each other, and to the results of field star surveys in the same mass and period regime. However, unlike the results from imaging surveys, the frequency of systems with close companions in our sample is not seen to depend on primary mass. Additionally, we do not find a strong correlation between accretion and close multiplicity. This implies that close companions are not likely the main source of the accretion shut down observed in weak-lined T Tauri stars. Our results also suggest that sufficient radial velocity precision can be achieved for at least a subset of slowly rotating young stars to search for hot Jupiter planets., Accepted for publication in ApJ. 55 pages, 37 figures, 7 tables. Formatted with emulateapj. Full version with 221 figures will be available on ApJ or upon request. Revised Tables 4, 5 & 6 and individual source figures to correct MJD

Among the initial results from Kepler were two striking lightcurves, for KOI 74 and KOI 81, in which the relative depths of the primary and secondary eclipses showed that the more compact, less luminous object was hotter than its stellar host. That result became particularly intriguing because a substellar mass had been derived for the secondary in KOI 74, which would make the high temperature challenging to explain; in KOI 81, the mass range for the companion was also reported to be consistent with a substellar object. We re-analyze the Kepler data and demonstrate that both companions are likely to be white dwarfs. We also find that the photometric data for KOI 74 show a modulation in brightness as the more luminous star orbits, due to Doppler boosting. The magnitude of the effect is sufficiently large that we can use it to infer a radial velocity amplitude accurate to 1 km/s. As far as we are aware, this is the first time a radial-velocity curve has been measured photometrically. Combining our velocity amplitude with the inclination and primary mass derived from the eclipses and primary spectral type, we infer a secondary mass of 0.22+/-0.03 Msun. We use our estimates to consider the likely evolutionary paths and mass-transfer episodes of these binary systems., 8 pages, 4 figures, ApJ 715, 51 (v4 is updated to match the published version, including a note added in proof with measured projected rotational velocities).

We present a comprehensive study of rotation, disk and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of v sin i. The rotational velocities in both regions show a clear mass dependence, with F--K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M^0.5 for our sample which spans a mass range of ~0.16 to ~3 M_sun. This power law complements previous studies in Orion which estimated j is proportional to M^0.25 for < ~2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M_sun. For a subsample of 67 objects with mid-IR photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks., Accepted to ApJ. 10 pages, 7 figures, 2 tables. Formatted with emulateapj

We present Gemini near-infrared adaptive optics imaging and spectroscopy of a planetary mass candidate companion to 1RXS J160929.1-210524, a roughly solar-mass member of the 5 Myr-old Upper Scorpius association. The object, separated by 2.22" or 330 AU at ~150 pc, has infrared colors and spectra suggesting a temperature of 1800(-100/+200) K, and spectral type of L4(-2/+1). The H- and K-band spectra provide clear evidence of low surface gravity, and thus youth. Based on the widely used DUSTY models, we infer a mass of 8(-2/+4)Mjupiter. If gravitationally bound, this would be the lowest mass companion imaged around a normal star thus far, and its existence at such a large separation would pose a serious challenge to theories of star and planet formation., Revised accepted version, ApJL, in press

We estimate cluster ages from lithium depletion in five pre-main-sequence groups found within 100 pc of the Sun: TW Hydrae Association, Eta Chamaeleontis Cluster, Beta Pictoris Moving Group, Tucanae-Horologium Association and AB Doradus Moving Group. We determine surface gravities, effective temperatures and lithium abundances for over 900 spectra through least squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model dependent ages. We find that the Eta Chamaelontis Cluster and the TW Hydrae Association are the youngest, with ages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the Beta Pictoris Moving Group at 21+/-9 Myr, the Tucanae-Horologium Association at 27+/-11 Myr, and the AB Doradus Moving Group at an age of at least 45 Myr (where we can only set a lower limit since the models -- unlike real stars -- do not show much lithium depletion beyond this age). Here, the ordering is robust, but the precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more precise. Our observations do show that with improved models, much stronger constraints should be feasible: the intrinsic uncertainties, as measured from the scatter between measurements from different spectra of the same star, are very low: around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex in lithium abundance., Accepted to ApJ, 15 pages, 8 colour figures

We present the results of a multiplicity survey of 126 stars spanning ~0.1-3 solar masses in the ~2-Myr-old Chamaeleon I star-forming region, based on adaptive optics imaging with the ESO Very Large Telescope. Our observations have revealed 30 binaries and 6 triples, of which 19 and 4, respectively, are new discoveries. The overall multiplicity fraction we find for Cha I (~30%) is similar to those reported for other dispersed young associations, but significantly higher than seen in denser clusters and the field, for comparable samples. Both the frequency and the maximum separation of Cha I binaries decline with decreasing mass, while the mass ratios approach unity; conversely, tighter pairs are more likely to be equal mass. We confirm that brown dwarf companions to stars are rare, even at young ages at wide separations. Based on follow-up spectroscopy of two low-mass substellar companion candidates, we conclude that both are likely background stars. The overall multiplicity fraction in Cha I is in rough agreement with numerical simulations of cloud collapse and fragmentation, but its observed mass dependence is less steep than predicted. The paucity of higher-order multiples, in particular, provides a stringent constraint on the simulations, and seems to indicate a low level of turbulence in the prestellar cores in Cha I., Accepted for publication in ApJ

Characterizing multiplicity in the very low mass (VLM) domain is a topic of much current interest and fundamental importance. Here we report on a near-IR AO imaging survey of 31 young brown dwarfs and VLM stars, 28 of which are in Chamaeleon I, using the ESO VLT. Our survey is sensitive enough to detect equal mass binaries down to separations of 0.04-0.07" (6-10 AU at 160 pc) and, typically, companions with mass ratios as low as 0.2 outside of 0.2" (~30 AU). We resolve the suspected 0.16" (~26 AU) binary Cha_Halpha 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13" (~20 AU) and 0.30" (~50 AU); the latter is one of the widest VLM systems discovered to date. We do not find companions around the majority of our targets, giving a binary frequency of 11% [+9,-6], thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLMOs (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLMOs in the field. In addition we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few Myr to several Gyr. Instead, the observations to date suggest that VLM objects are either less likely to be born in fragile multiple systems than solar mass stars or such systems are disrupted very early., accepted by The Astrophysical Journal

We have collected data spanning seven years of observations of the magnetar 4U 0142+61 in the infrared, optical and soft X-rays. These combine our own observations and analysis of archival data. We find that the source is variable in the optical, in contrast to what had been previously reported, that the K-band flux can vary by over a magnitude on the time-scale of days, and that the X-ray pulsed flux is not obviously correlated with either the total X-ray flux or infrared and optical fluxes. Furthermore, from multi-color photometry of the source within single nights, we conclude that there are two separate components to the infrared emission. The overall picture is unclear, and prompts the need for further, more frequent observations., 20 pages, published in ApJ

The X-ray spectra of Anomalous X-ray Pulsars have long been fit by smooth, empirical models such as the sum of a black-body plus a power law. These reproduce the ~0.5 to 10 keV range well, but fail at lower and higher energies, grossly over-predicting the optical and under-predicting the hard X-ray emission. A poorly constrained source of uncertainty in determining the true, intrinsic spectra, in particular at lower energies, is the amount of interstellar extinction. In previous studies, extinction column densities with small statistical errors were derived as part of the fits of the spectra to simple continuum models. Different choices of model, however, each produced statistically acceptable fits, but a wide range of columns. Here, we attempt to measure the interstellar extinction in a model-independent way, using individual absorption edges of the elements O, Fe, Ne, Mg and Si in X-ray grating spectra taken with XMM-Newton. We find that our inferred equivalent hydrogen column density NH for 4U 0142+61 is a factor of 1.4 lower than the typically quoted value from black-body plus power-law fits, and is now consistent with estimates based on the dust scattering halo and visual extinction. For three other sources, we find column densities consistent with earlier estimates. We use our measurements to recover the intrinsic spectra of the AXPs empirically, without making assumptions on what the intrinsic spectral shapes ought to be. We find that the power-law components that dominate at higher energies do not extend below the thermal peak., 24 pages, 4 figures. Accepted for publication in ApJ

Archival Hubble Space Telescope Wide Field/Planetary Camera 2 observations of the Small Magellanic Cloud serendipitously reveal a possible counterpart to the Anomalous X-ray Pulsar CXOU J010043.1-721134. The candidate is faint, but its location and strange colours make it an interesting object. We estimate, that the probability of such a detection being due to a non-physical source is less than 1.5%. We have tried to confirm the identification with Gemini-South and Magellan, but the conditions were insufficiently favourable. If confirmed, the object will allow the first detailed studies of the optical and ultraviolet emission of magnetars., 9 pages LaTeX, 2 figures. Accepted for publication in ApJL

We present photometry of the Anomalous X-ray pulsar 1E1048.1-5937 in the infrared and optical, taken at Magellan and the VLT. The object is detected in the I, J and Ks bands under excellent conditions. We find that the source has varied greatly in its infrared brightness and present these new magnitudes. No correlation is found between the infrared flux and spin-down rate, but the infrared flux and X-ray flux may be anti-correlated. Assuming nominal reddening values, the resultant spectral energy distribution is found to be inconsistent with the only other AXP SED available (for 4U0142+61). We consider the effect of the uncertainty in the reddening to the source on its SED. We find that although both the X-ray and infrared fluxes have varied greatly for this source, the most recent flux ratio is remarkably consistent with what is is found for other AXPs. Finally, we discuss the implications of our findings in the context of the magnetar model., 21 pages, 5 eps figures. Submitted to ApJ

We present time-resolved optical spectrophotometry of the pulsating hydrogen atmosphere (DA) white dwarf G 117-B15A. We find three periodicities in the pulsation spectrum (215s, 272s, and 304s) all of which have been found in earlier studies. By comparing the fractional wavelength dependence of the pulsation amplitudes (chromatic amplitudes) with models, we confirm a previous report that the strongest mode, at 215s, has l=1. The chromatic amplitude for the 272s mode is very puzzling, showing an increase in fractional amplitude with wavelength that cannot be reproduced by the models for any l at optical wavelengths. Based on archival HST data, we show that while the behaviour of the 215s mode at ultra-violet wavelengths is as expected from models, the weird behaviour of the 272s periodicity is not restricted to optical wavelengths in that it fails to show the expected increase in fractional amplitude towards shorter wavelengths. We discuss possible causes for the discrepancies found for the 272s variation, but find that all are lacking, and conclude that the nature of this periodicity remains unclear., 9 pages, 9 figures; accepted for publication in A&A

We present a detailed analysis of time-resolved optical spectra of the ZZ Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed mode amplitudes, we deduce the spherical degree, l, of the modes, most of which have l=1. The presence of a large number of combination frequencies (linear sums or differences of the real modes) enabled us not only to test theoretical predictions but also to indirectly infer spherical and azimuthal degrees of real modes that had no observed splittings. In addition to the above, we measure line-of-sight velocities from our spectra. We find only marginal evidence for periodic modulation associated with the pulsation modes: at the frequency of the strongest mode in the lightcurve, we measure an amplitude of 2.6+/-1.0 km/s, which has a probability of 2% of being due to chance; for the other modes, we find lower values. Our velocity amplitudes and upper limits are smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find that this is consistent with expectations based on the position of HS 0507+0434B in the instability strip. Combining all the available information from data such as ours is a first step towards constraining atmospheric properties in a convectionally unstable environment from an observational perspective., Comment: 16 pages, 12 figs.; accepted for publication in A&A