Martin Durant

A Strong, Broad Absorption Feature in the X-Ray Spectrum of the Nearby Neutron Star RX J1605.3+3249

We present X-ray spectra taken with XMM-Newton of RX J1605.3+3249, the third brightest in the class of nearby thermally emitting neutron stars. In contrast to what is the case for the brightest object, RX J1856.5-3754, we find that the spectrum of RX J1605.3+3249 cannot be described well by a pure blackbody, but shows a broad absorption feature at 27 ? (0.45 keV). With this, it joins the handful of isolated neutron stars for which spectral features arising from the surface have been detected. We discuss possible mechanisms that might lead to the features, as well as the overall optical to X-ray spectral energy distribution, and compare the spectrum with what is observed for the other nearby thermally emitting neutron stars. We conclude that we may be observing absorption due to the proton cyclotron line, as was suggested for the other sources, but weakened because of the strong-field quantum electrodynamics effect of vacuum resonance mode conversion.

Rapid optical and X-ray timing observations of GX 339−4: multicomponent optical variability in the low/hard state

A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X- ray) observations of the Galactic black hole binary GX 339−4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r � ,gand ufilters show slow (∼20 s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (∼50 ms inrandg � ) and with maximum strengths of more than twice the local mean. Power spectral analysis over ∼0.004-10 Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical-X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ∼0.2 Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ≈150 ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000 A. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ∼10 s and strong coherence in the blue ufilter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is

Extinction Columns and Intrinsic X-Ray Spectra of the Anomalous X-Ray Pulsars

The X-ray spectra of anomalous X-ray pulsars have long been fit by smooth, empirical models such as the sum of a blackbody plus a power law. These reproduce the ~0.5-10 keV range well, but fail at lower and higher energies, grossly overpredicting the optical and underpredicting 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 blackbody 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.

The Broadband Spectrum and Infrared Variability of the Magnetar AXP 1E 1048.1–5937

We present photometry of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 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 anticorrelated. Assuming nominal reddening values, the resultant spectral energy distribution (SED) is found to be inconsistent with the only other AXP SED available (for 4U 0142+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 found for other AXPs. Finally, we discuss the implications of our findings in the context of the magnetar model.

Multiwavelength Variability of the Magnetar 4U 0142+61

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 timescale 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 multicolor 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.

Correlated Infrared and X-Ray Flux Changes Following the 2002 June Outburst of the Anomalous X-Ray Pulsar 1E 2259+586

We present the results of a near-infrared monitoring program of the anomalous X-ray pulsar 1E 2259+586, performed at the Gemini Observatory. This program began 3 days after the pulsars 2002 June outburst and spans ~1.5 yr. We find that after an initial increase associated with the outburst, the near-infrared flux decreased continually and reached the preburst quiescent level after about 1 yr. We compare both the near-infrared flux enhancement and its decay to those of the X-ray afterglow and find them to be remarkably consistent. Fitting simple power laws to the Rossi X-Ray Timing Explorer pulsed flux and near-infrared data for t > 1 day postburst, we find the following decay indices: α = -0.21 ± 0.01 (X-ray) and α = -0.21 ± 0.02 (near-infrared), where flux is a function of time such that F ∝ tα. This suggests that the enhanced infrared and X-ray fluxes have a physical link postoutburst, most likely from the neutron star magnetosphere.

CHANDRA PULSAR SURVEY (ChaPS)

Taking advantage of the high sensitivity of the Chandra X-ray Observatorys (CXO) Advanced CCD Imaging Spectrometer, we have conducted a snapshot survey of pulsars previously undetected in X-rays. We detected 12 pulsars and established deep flux limits for 11 pulsars. Using these new results, we revisit the relationship between the X-ray luminosity, L psr X, and spin-down power, . We find that the obtained limits further increase the extremely large spread in the non-thermal X-ray efficiencies, ηpsr X = L psr X/, with some of them being now below 10–5. Such a spread cannot be explained by poorly known distances or by beaming of pulsar radiation. We also find evidence of a break in the dependence of L psr X on , such that pulsars become more X-ray efficient at -1035 erg s–1. We examine the relationship between the γ-ray luminosity, L psr γ, and , which exhibits a smaller scatter compared to that in X-rays. This confirms that the very large spread in the X-ray efficiencies cannot be explained just by the beaming because the γ-ray emission is generally expected to be beamed stronger than the X-ray emission. Intriguingly, there is also an indication of a break in the L psr γ dependence at erg s–1, with lower- pulsars becoming less γ-ray efficient. We also examine the distance-independent L psr γ/L psr X ratio as a function of for a sample of γ-ray pulsars observed by CXO and find that it peaks at erg s–1, showing that the breaks cannot originate from poorly measured distances. We discuss the implications of our findings for existing models of magnetospheric emission and venues for further exploration.

Multiwavelength Spectroscopy of PSR?B0656+14

Using high-quality Hubble Space Telescope observations, we construct the near-infrared (NIR) to far-ultraviolet (FUV) spectral energy distribution (SED) of PSR B0656+14. The SED is non-monotonic. Fitting it with a simple combination of a Rayleigh-Jeans spectrum (UV) and non-thermal power law (PL; optical/NIR) leaves significant residuals, strongly hinting at one or more spectral features. We consider various models (combination of continuum components, and absorption/emission lines) with possible interpretations and place them in the context of the broader SED. Surprisingly, the extrapolation of the best-fit X-ray spectral model roughly matches the NIR-FUV data, and the PL component is also consistent with the γ-ray fluxes. We compare the multiwavelength SED of B0656+14 with those of other optical-, X-ray-, and γ-ray-detected pulsars, and notice that a simple PL spectrum crudely accounts for most of the non-thermal emission.

High time resolution optical/X-ray cross-correlations for X-ray binaries: anticorrelations and rapid variability

Using simultaneous observations in X-rays and optical, we have performed a homogeneous analysis of the cross-correlation behaviours of four X-ray binaries: SWIFT J1753.5-0127, GX339-4, Sco X-1 and CygX-2. With high-time-resolution observations using ULTRACAM and RXTE, we concentrate on the short time-scale, delta t < 20 s, variability in these sources. Here we present our data base of observations, with three simultaneous energy bands in both the optical and the X-ray, and multiple epochs of observation for each source, all with similar to second or better time resolution. For the first time, we include a dynamical cross-correlation analysis, i.e. an investigation of how the cross-correlation function changes within an observation. We describe a number of trends which emerge. We include the full data set of results, and pick a few striking relationships from among them for further discussion. We find, that the surprising form of X-ray/optical cross-correlation functions, a positive correlation signal preceded by an anticorrelation signal, is seen in all the sources at least some of the time. Such behaviour suggests a mechanism other than reprocessing as being the dominant driver of the short-term variability in the optical emission. This behaviour appears more pronounced when the X-ray spectrum is hard. Furthermore, we find that the cross-correlation relationships themselves are not stable in time, but vary significantly in strength and form. This all hints at dynamic interactions between the emitting components which could be modelled through non-linear or differential relationships.

The Helical Jet of the Vela Pulsar

We have studied the fascinating dynamics of the nearby Vela pulsars nebula in a campaign comprising eleven 40ks observations with Chandra X-ray Observatory (CXO). The deepest yet images revealed the shape, structure, and motion of the 2-arcminute-long pulsar jet. We find that the jets shape and dynamics are remarkably consistent with that of a steadily turning helix projected on the sky. We discuss possible implications of our results, including free precession of the neutron star and MHD instability scenarios.