V. M. Kaspi

Changes in the X-ray Emission from the Magnetar Candidate 1E 2259+586 during its 2002 Outburst

An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the anomalous X-ray pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile, and spin-down of the underlying X-ray source. We present Rossi X-Ray Timing Explorer and X-Ray Multi-Mirror Mission observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode and identify recovery timescales for each. Specifically, we observe an X-ray flux increase (pulsed and phase-averaged) by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within ~2 days, during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 ? 0.01. The pulsed fraction decreased initially to ~15% rms but recovered rapidly to the preoutburst level of ~23% within the first 3 days. The pulse profile changed significantly during the outburst and recovered almost fully within 2 months of the outburst. A glitch of size ??max/? = (4.24 ? 0.11) ? 10-6 was observed in 1E 2259+586, which preceded the observed burst activity. The glitch could not be well fitted with a simple partial exponential recovery. An exponential rise of ~20% of the frequency jump with a timescale of ~14 days results in a significantly better fit to the data; however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. A fraction of the glitch (~19%) was recovered in a quasi-exponential manner having a recovery timescale of ~16 days. The long-term postglitch spin-down rate decreased in magnitude relative to the preglitch value. The changes in the source properties of 1E 2259+586 during its 2002 outburst are shown to be qualitatively similar to changes seen during or following burst activity in two SGRs, thus further solidifying the common nature of SGRs and AXPs as magnetars. The changes in persistent emission properties of 1E 2259+586 suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior (crustal and possibly core superfluid) and the magnetosphere. Finally, the changes in persistent emission properties coincident with burst activity in 1E 2259+586 enabled us to infer previous burst-active episodes from this and other AXPs. The nondetection of these outbursts by all-sky gamma-ray instruments suggests that the number of active magnetar candidates in our Galaxy is larger than previously thought.

CHANDRA IMAGING OF THE X-RAY NEBULA POWERED BY PULSAR B1509- 58

We present observations with the Chandra X-Ray Observatory of the pulsar wind nebula (PWN) powered by the energetic young pulsar B1509-58. These data confirm the complicated morphology of the system indicated by previous observations, and in addition reveal several new components to the nebula. The overall PWN shows a clear symmetry axis oriented at a position angle 150? ? 5? (north through east), which we argue corresponds to the pulsar spin axis. We show that a previously identified radio feature matches well with the overall extent of the X-ray PWN, and propose the former as the long-sought radio nebula powered by the pulsar. We further identify a bright collimated feature, at least 4 long, lying along the nebulas main symmetry axis; we interpret this feature as a physical outflow from the pulsar, and infer a velocity for this jet of greater than 0.2c. The lack of any observed counterjet implies that the pulsar spin axis is inclined at ~30? to the line of sight, contrary to previous estimates made from lower resolution data. We also identify a variety of compact features close to the pulsar. A pair of semicircular X-ray arcs lie 17 and 30 to the north of the pulsar; the latter arc shows a highly polarized radio counterpart. We show that these features can be interpreted as ion-compression wisps in a particle-dominated equatorial flow, and use their properties to infer a ratio of electromagnetic to particle energy in pairs at the wind shock ? ~ 0.005, similar to that seen in the Crab Nebula. We further identify several compact knots seen very close to the pulsar; we use these to infer ? < 0.003 at a separation from the pulsar of 0.1 pc.

The Mouse that Soared: High-Resolution X-Ray Imaging of the Pulsar-powered Bow Shock G359.23–0.82

We present an observation with the Chandra X-Ray Observatory of the unusual radio source G359.23-0.82 (the Mouse), along with updated radio timing data from the Parkes radio telescope on the coincident young pulsar J1747-2958. We find that G359.23-0.82 is a very luminous X-ray source [LX(0.5-8.0 keV) = 5 × 1034 ergs s-1 for a distance of 5 kpc] whose morphology consists of a bright head coincident with PSR J1747-2958 plus a 45 long narrow tail whose power-law spectrum steepens with distance from the pulsar. We thus confirm that G359.23-0.82 is a bow shock pulsar wind nebula powered by PSR J1747-2958; the nebular standoff distance implies that the pulsar is moving with a Mach number of ~60, suggesting a space velocity ≈600 km s-1 through gas of density ≈0.3 cm-3. We combine the theory of ion-dominated pulsar winds with hydrodynamic simulations of pulsar bow shocks to show that a bright elongated X-ray and radio feature extending 10 behind the pulsar represents the surface of the wind termination shock. The X-ray and radio trails seen in other pulsar bow shocks may similarly represent the surface of the termination shock, rather than particles in the postshock flow as is usually argued. The tail of the Mouse contains two components: a relatively broad region seen only at radio wavelengths, and a narrow region seen in both radio and X-rays. We propose that the former represents material flowing from the wind shock ahead of the pulsars motion, while the latter corresponds to more weakly magnetized material streaming from the backward termination shock. This study represents the first consistent attempt to apply our understanding of Crab-like nebulae to the growing group of bow shocks around high-velocity pulsars.

The Parkes Multibeam Pulsar Survey – II. Discovery and timing of 120 pulsars

The Parkes Multibeam Pulsar Survey is a sensitive survey of a strip of the Galactic plane with |b| < 5 ◦ and 260 ◦ < l < 50 ◦ at 1374 MHz. Here we report the discovery of 120 new pulsars and subsequent timing observations, primarily using the 76-m Lovell radio telescope at Jodrell Bank. The main features of the sample of 370 published pulsars discovered during the multibeam survey are described. Furthermore, we highlight two pulsars: PSR J1734−3333, a young pulsar with the second highest surface magnetic field strength among the known radio pulsars, Bs = 5.4 × 10 13 G, and PSR J1830−1135, the second slowest radio pulsar known,

Chandra observation of the globular cluster NGC 6440 and the nature of cluster X-ray luminosity functions

As part of our campaign to determine the nature of the various source populations of the low-luminosity globular cluster X-ray sources, we have obtained a Chandra X-Ray Observatory ACIS-S3 image of the globular cluster NGC 6440. We detect 24 sources to a limiting luminosity of ~2 × 1031 ergs s-1 (0.5-2.5 keV) inside the clusters half-mass radius, all of which lie within ~2 core radii of the cluster center. We also find excess emission in and around the core that could be due to unresolved point sources. Based on X-ray luminosities and colors, we conclude that there are 4-5 likely quiescent low-mass X-ray binaries and that most of the other sources are cataclysmic variables. We compare these results to Chandra results from other globular clusters and find the X-ray luminosity functions differ among the clusters.

Rossi X-Ray Timing Explorer Absolute Timing Results for the Pulsars B1821–24 and B1509–58

Observations with the Rossi X-Ray Timing Explorer and the Jodrell Bank, Parkes, and Green Bank telescopes have enabled us to determine the time delay between radio and X-ray pulses in the two isolated pulsars B1821-24 and B1509-58. For the former we find that the narrow X-ray and radio pulse components are close to being coincident in time, with the radio peak leading by 0.02 period (60 ± 20 μs), while the wide X-ray pulse component lags the last of the two wider radio components by about 0.08 period. For the latter pulsar we find, using the standard value for the dispersion measure, that the X-ray pulse lags the radio by about 0.27 period, with no evidence for any energy dependence in the range 2-100 keV. However, uncertainties in the history of the dispersion measure for this pulsar make a comparison to previous results difficult. It is clear that there are no perceptible variations in either the lag or the dispersion measure at timescales of a year or less.

A Radio Supernova Remnant Associated with the Young Pulsar J1119?6127

We report on Australia Telescope Compact Array observations in the direction of the young high-magnetic-field radio pulsar PSR J1119-6127. In the resulting images we identify a non-thermal radio shell of diameter 15 arcminutes, which we classify as a previously uncataloged young supernova remnant (SNR), G292.2-0.5. This SNR is positionally coincident with PSR J1119-6127, and we conclude that the two objects are physically associated. No radio emission is detected from any pulsar wind nebula (PWN) associated with the pulsar; our observed upper limits are consistent with the expectation that high-magnetic-field pulsars produce radio nebulae which fade rapidly. This system suggests a possible explanation for the lack of associated radio pulsars and/or PWNe in many SNRs.

X-Ray Detection of Pulsar PSR B1757–24 and Its Nebular Tail

We report the first X-ray detection of the radio pulsar PSR B1757-24 using the Chandra X-Ray Observatory. We detect point-source emission at the pulsar position plus a faint tail extending nearly 20 east of the pulsar, in the same direction and with comparable morphology to the radio tail. Assuming the point-source X-ray emission is magnetospheric, the observed X-ray tail represents only ~0.01% of the pulsars spin-down luminosity. This is significantly lower than the analogous efficiencies of most known X-ray nebulae surrounding rotation-powered pulsars. Assuming a nonthermal spectrum for the tail photons, we show that the tail is unlikely to be emission left behind following the passage of the pulsar but rather is probably from synchrotron-emitting pulsar wind particles having a flow velocity of ~7000 km s-1. We also show that there must be a significant break in the tail synchrotron spectrum between the radio and X-ray bands that is intrinsic to the particle spectrum. No emission is detected from the shell supernova remnant G5.4-1.2. The upper limits on remnant emission are unconstraining.

Discovery of Five Binary Radio Pulsars

We report on five binary pulsars discovered in the Parkes multibeam Galactic plane survey. All of the pulsars are old, with characteristic ages (1-11) × 109 yr, and have relatively small inferred magnetic fields, (5-90) × 108 G. The orbital periods range from 1.3 to 15 days. As a group these objects differ from the usual low-mass binary pulsars (LMBPs): their spin periods of 9-88 ms are relatively long; their companion masses, 0.2-1.1 M☉, are, in at least some cases, suggestive of CO or more massive white dwarfs; and some of the orbital eccentricities, 10-5 e 0.002, are unexpectedly large. We argue that these observed characteristics reflect binary evolution that is significantly different from that of LMBPs. We also note that intermediate-mass binary pulsars apparently have a smaller scale height than LMBPs.

X-Ray/Gamma-Ray Observations of the PSR B1259–63/SS 2883 System near Apastron

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with ASCA and the Compton Gamma-Ray Observatory (CGRO), performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of LX = (9 ± 3) × 1032 (d/2 kpc)2 ergs s-1, while CGRO/OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA are well fitted by a single power-law spectrum with a photon index of 1.6 ± 0.3. No pulsations were detected in either the ASCA or the OSSE data. We combine all existing X-ray and hard X-ray observations and present the orbital modulation in the luminosity and photon index for the entire orbit. The results are in agreement with predictions based on a synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.