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Fast radio bursts (FRBs) are microsecond-to-millisecond-duration radio transients1that originate mostly from extragalactic distances. The FRB emission mechanism remains debated, with two main competing classes of models: physical processes that occur within close proximity to a central engine2, 3–4; and relativistic shocks that propagate out to large radial distances5, 6, 7–8. The expected emission-region sizes are notably different between these two types of models9. Here we present the measurement of two mutually coherent scintillation scales in the frequency spectrum of FRB 20221022A10: one originating from a scattering screen located within the Milky Way, and the second originating from its host galaxy or local environment. We use the scattering media as an astrophysical lens to constrain the size of the observed FRB lateral emission region9to ≲3 × 104kilometres. This emission size is inconsistent with the expectation for the large-radial-distance models5, 6, 7–8, and is more naturally explained by an emission process that operates within or just beyond the magnetosphere of a central compact object. Recently, FRB 20221022A was found to exhibit an S-shaped polarization angle swing10, most likely originating from a magnetospheric emission process. The scintillation results presented in this work independently support this conclusion, while highlighting scintillation as a useful tool in our understanding of FRB emission physics and progenitors.

Fast radio bursts (FRBs) last for milliseconds and arrive at Earth from cosmological distances. Although their origins and emission mechanisms are unknown, their signals bear similarities with the much less luminous radio emission generated by pulsars within our Miky Way Galaxy1, with properties suggesting neutron star origins2,3. However, unlike pulsars, FRBs typically show minimal variability in their linear polarization position angle (PA) curves4. Even when marked PA evolution is present, their curves deviate significantly from the canonical shape predicted by the rotating vector model (RVM) of pulsars5. Here we report on FRB 20221022A, detected by the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst project (CHIME/FRB) and localized to a nearby host galaxy (about 65 Mpc), MCG+14-02-011. This FRB shows a notable approximately 130° PA rotation over its about 2.5 ms burst duration, resembling the characteristic S-shaped evolution seen in many pulsars and some radio magnetars. The observed PA evolution supports magnetospheric origins6, 7–8over models involving distant shocks9, 10–11, echoing similar conclusions drawn from tempo-polarimetric studies of some repeating FRBs12,13. The PA evolution is well described by the RVM and, although we cannot determine the inclination and magnetic obliquity because of the unknown period or duty cycle of the source, we exclude very short-period pulsars (for example, recycled millisecond pulsars) as the progenitor.

Fast radio bursts (FRBs) are millisecond-duration flashes of radio waves that are visible at distances of billions of light years1. The nature of their progenitors and their emission mechanism remain open astrophysical questions2. Here we report the detection of the multicomponent FRB 20191221A and the identification of a periodic separation of 216.8(1) ms between its components, with a significance of 6.5σ. The long (roughly 3 s) duration and nine or more components forming the pulse profile make this source an outlier in the FRB population. Such short periodicity provides strong evidence for a neutron-star origin of the event. Moreover, our detection favours emission arising from the neutron-star magnetosphere3,4, as opposed to emission regions located further away from the star, as predicted by some models5.

Astronomers have uncovered a source of radio waves that pulsate more slowly than expected. Meticulous records reveal that the emission has been detected for decades, highlighting the remarkable foresight of scientists in bygone years.

We report on observations of the unusual neutron-star binary system FIRST J102347.6+003841 carried out using the XMM-Newton satellite. This system consists of a radio millisecond pulsar (PSR J1023+0038) in a 0.198 day orbit with a [?]0.2 M Roche-lobe-filling companion and appears to have had an accretion disk in 2001. We observe a hard power-law spectrum (G = 1.26(4)) with a possible thermal component, and orbital variability in X-ray flux and possibly hardness of the X-rays. We also detect probable pulsations at the pulsar period (single-trial significance [?]4.5s from an 11(2)% modulation), which would make this the first system in which both orbital and rotational X-ray pulsations are detected. We interpret the emission as a combination of X-rays from the pulsar itself and from a shock where material overflowing the companion meets the pulsar wind. The similarity of this X-ray emission to that seen from other millisecond pulsar binary systems, in particular 47 Tuc W (PSR J0024 - 7204W) and PSR J1740 - 5340, suggests that they may also undergo disk episodes similar to that seen in J1023 in 2001.

Anomalous X-ray pulsars (AXPs) are now established to exhibit significant X-ray variability and be prolific glitchers, with some glitches being accompanied by large radiative changes. An open issue is whether AXP glitches are generically accompanied by radiative changes, relevant for understanding magnetar physical properties. Here we report on an analysis of archival X-ray data from the AXP 1E 1841-045, obtained between 1993 and 2007. This AXP, located in the center of the supernova remnant Kes 73, has exhibited three glitches between 2002 and 2007, as determined by RXTE monitoring since 1999. We have searched for evidence of phase-averaged flux variability that could be present if glitches in AXPs are usually accompanied by radiative changes. We find no evidence for glitch-correlated flux changes from this source after 1999, supporting the existence of radiatively silent glitches in AXPs.

We report a large spin-up glitch in PSR J1846-0258 which coincided with the onset of magnetar-like behavior on 2006 May 31. We show that the pulsar experienced an unusually large glitch recovery, with a recovery fraction of Q = 8.7 +- 2.5, resulting in a net decrease of the pulse frequency. Such a glitch recovery has never before been observed in a rotation-powered pulsar (RPP); however, similar but smaller glitch over-recovery has been recently reported in the magnetar AXP 4U 0142+61 and may have occurred in SGR 1900+14. We also report a large increase in the timing noise of the source. We discuss the implications of the unusual timing behavior in PSR J1846-0258 on its status as the first identified magnetically active RPP.

We present timing data spanning 6.4 yr for the young and energetic PSR J0205+6449, in the supernova remnant 3C 58. Data were obtained with the Rossi X-ray Timing Explorer, the Jodrell Bank Observatory, and the Robert C. Byrd Green Bank Telescope. We present phase-coherent timing analyses showing timing noise and two spin-up glitches with fractional frequency increases of [?]3.4 x 10-7 near MJD 52555 and [?]3.8 x 10-6 between MJDs 52777 and 53062. These glitches are unusually large if the pulsar was created in the historical supernova in 1181, as has been suggested. For the X-ray timing, we developed a new unbinned maximum likelihood method for determining pulse arrival times, which performs significantly better than the traditional binned techniques. In addition, we present an X-ray pulse profile analysis of four years of RXTE data showing that the pulsar is detected up to [?]40 keV. We also present the first measurement of the phase offset between the radio and X-ray pulse for this source, showing that the radio pulse leads the X-ray pulse by ph = 0.10 +- 0.01 in phase. We compile all known measurements of the phase offsets between radio and X-ray and radio and g-ray pulses for X-ray and g-ray pulsars. We show that there is no relationship between pulse period and phase offset, supported by our measurement of the phase offset for PSR J0205+6449.

Using observations made with the XMM-Newton Observatory, we report the first X-ray detection of the high-magnetic-field radio pulsar PSR B1916+14. We show that the X-ray spectrum of the pulsar can be well fitted with an absorbed blackbody with temperature in the range of 0.08-0.23 keV, or a neutron-star hydrogen atmosphere model with best-fit effective temperature of [?]0.10 keV, higher than expected from fast cooling models. The origin of the likely thermal emission is not well constrained by our short observation and is consistent with initial cooling or return-current heating. We found no pulsations in these data and set a 1s upper limit on the pulsed fraction in the 0.1-2 keV band of [?]0.7. Implications of these results for our understanding of the different observational properties of isolated neutron stars are discussed.

The Sloan Digital Sky Survey (SDSS) source J102347.6+003841 was recently revealed to be a binary 1.69 ms radio pulsar with a 4.75 hr orbital period and a [?]0.2 M companion. Here, we analyze the SDSS spectrum of the source in detail. The spectrum was taken on 2001 February 1, when the source was in a bright state and showed broad, double-peaked hydrogen and helium lines--dramatically different from the G-type absorption spectrum seen from 2002 May onward. The lines are consistent with emission from a disk around the compact primary. We derive properties of the disk by fitting the SDSS continuum with a simple disk model, and find a temperature range of 2000-34,000 K from the outer to inner edge of the disk. The disk inner and outer radii were approximately 109 and 5.7x1010 cm, respectively. These results further emphasize the unique feature of the source: it is a system likely at the end of its transition from an X-ray binary to a recycled radio pulsar. The disk mass is estimated to have been [?]1023 g, most of which would have been lost due to pulsar wind ablation (or due to the propeller effect if the disk had extended inside the light cylinder of the pulsar) before the final disk disruption event. The system could undergo repeated episodes of disk formation. Close monitoring of the source is needed to catch the system in its bright state again, so that this unusual example of a pulsar-disk interaction can be studied in much finer detail.

After three years of no unusual activity, Anomalous X-ray Pulsar 1E 1048.1 - 5937 reactivated in 2007 March. We report on the detection of a large glitch (Dn/n = 1.63(2) x 10-5) on 2007 March 26 (MJD 54185.9), contemporaneous with the onset of a pulsed-flux flare, the third flare observed from this source in 10 years of monitoring with the Rossi X-ray Timing Explorer (RXTE). Additionally, we report on a detailed study of the evolution of the timing properties, the pulsed flux, and the pulse profile of this source as measured by RXTE from 1996 July to 2008 January. In our timing study, we attempted phase-coherent timing of all available observations. We show that in 2001 a timing anomaly of uncertain nature occurred near the rise of the first pulsed flux flare; we show that a likely glitch (Dn/n = 2.91(9) x 10-6) occurred in 2002, near the rise of the second flare, and we present a detailed description of the variations in the spin down. In our pulsed flux study, we compare the decays of the three flares and discuss changes in the hardness ratio. In our pulse profile study, we show that the profile exhibited large variations near the peak of the first two flares, and several small short-term profile variations during the most recent flare. Finally, we report on the discovery of a small burst 27 days after the peak of the last flare, the fourth burst discovered from this source. We discuss the relationships between the observed properties in the framework of the magnetar model.

We report on optical imaging of the X-ray binary SAX J1808.4 - 3658 with the 8 m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved r'-band light curves (LCs) of the source over five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P = 7251.9 s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8 s and 0.008 (90% confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris. By considering this consistency and the sinusoidal shape of the LCs, we rule out the possibility of the modulation arising from the accretion disk. Our study supports the previous suggestion that the X-ray pulsar becomes rotationally powered in quiescence, with its energy output irradiating the companion star, causing the optical modulation. While it has also been suggested that the accretion disk would be evaporated by the pulsar, we argue that the disk exists and gives rise to the persistent optical emission. The existence of the disk can be verified by long-term, multiwavelength optical monitoring of the source in quiescence, as an increasing flux and spectral changes from the source would be expected based on the standard disk-instability model.

The rotation-powered pulsar PSR J1846-0258 in the supernova remnant Kes 75 was recently shown to have exhibited magnetar-like X-ray bursts in mid-2006. Radio emission has not yet been observed from this source, but other magnetar-like sources have exhibited transient radio emission following X-ray bursts. We report on a deep 1.9 GHz radio observation of PSR J1846-0258 with the 100 m Green Bank Telescope in late 2007, designed to search for radio pulsations or bursts from this target. We have also analyzed three shorter serendipitous 1.4 GHz radio observations of the source taken with the 64 m Parkes telescope during the 2006 bursting period. We detected no radio emission from PSR J1846-0258 in either the Green Bank or Parkes data sets. We place an upper limit of 4.9 mJy on coherent pulsed emission from PSR J1846-0258 based on the 2007 November 2 observation, and an upper limit of 27 mJy around the time of the X-ray bursts. Serendipitously, we observed radio pulses from the nearby RRAT J1846-02, and place a 3 s confidence level upper limit on its period derivative of 1.7 x 10[?]13, implying its surface dipole magnetic field is less than 2.6 x 1013 G.

We present an analysis of five XMM-Newton observations of the anomalous X-ray pulsar (AXP) 1E 2259+586 taken in 2004 and 2005 during its relaxation following its 2002 outburst. We compare these data with those of five previous XMM-Newton observations taken in 2002 and 2003, and find the observed flux decay is well described by a power law of index - 0.69 +- 0.03. As of mid-2005, the source may still have been brighter than preoutburst, and was certainly hotter. We find a strong correlation between hardness and flux, as seen in other AXPs. We discuss the implications of these results for the magnetar model.

We report on optical and infrared observations of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 made during its ongoing X-ray flare that started in 2007 March. We detected the source in the optical I and near-infrared Ks bands in two ground-based observations and obtained deep flux upper limits from four observations, including one with the Spitzer Space Telescope at 4.5 and 8.0 mm. The detections indicate that the source was approximately 1.3-1.6 mag brighter than in 2003-2006, when it was at the tail of a previous similar X-ray flare. Similar related flux variations have been seen in two other AXPs during their X-ray outbursts, suggesting common behavior for large X-ray flux variation events in AXPs. The Spitzer flux limits are sufficiently deep that we can exclude mid-infrared emission similar to that from the AXP 4U 0142+61, which has been interpreted as arising from a dust disk around the AXP. The optical/near-infrared emission from 1E 1048.1-5937 probably has a magnetospheric origin. The similarity in the flux spectra of 1E 1048.1-5937 and 4U 0142+61 challenges the dust disk model proposed for the latter.

We report on new and archival X-ray and near-IR observations of the anomalous X-ray pulsar 1E 1048.1-5937 performed between 2001 and 2007 with the Rossi X-Ray Telescope Explorer (RXTE), the Chandra X-Ray Observatory, the Swift Gamma-Ray Burst Explorer, the Hubble Space Telescope (HST), and the Very Large Telescope. Monitoring with RXTE revealed that following its ~2001-2004 active period, 1E 1048.1-5937 entered a phase of timing stability; at the same time, simultaneous observations with Chandra and HST in 2006 showed that its X-ray and near-IR radiative properties, all variable prior to 2005, stabilized. Specifically, the 2006 X-ray spectrum is consistent with a two-component blackbody plus power law, with an average kt = 0.52 keV and G = 2.8, at a mean flux level of ~ 6.5 x 10[?]12 erg cm[?]2 s[?]1 ( 2-10 keV). The near-IR counterpart in 2005-2006 was detected at H [?] 22.7 mag and Ks [?] 21.0 mag, considerably fainter than previously measured. In 2007 March, this newfound quiescence was interrupted by sudden X-ray flux, spectral, and pulse morphology changes, simultaneous with a large glitch and near-IR enhancement. Our RXTE observations revealed a factor of ~3 increase in pulsed flux (2-10 keV), while observations with Chandra and Swift saw the total X-ray flux increase much more than the pulsed flux, reaching a peak value of >7 times the quiescent value (2-10 keV). We find a strong anticorrelation between X-ray flux and pulsed fraction, and a correlation between X-ray spectral hardness and flux. Simultaneously with the radiative and timing changes, we observed the X-ray pulse profile change significantly from nearly sinusoidal to having multiple peaks. We compare these remarkable events with other magnetar outbursts and discuss implications in the context of AXP emission models.

We report on 8.7 and 7.6 yr of Rossi X-Ray Timing Explorer (RXTE) observations of the anomalous X-ray pulsars (AXPs) RXS J170849.0-400910 and 1E 1841-045, respectively. These observations have allowed us to study the long-term timing, pulsed flux, and pulse profile evolution of these objects. We report on four new glitches, one from RXS J170849.0-400910 and three from 1E 1841-045. With nearly all known persistent AXPs now seen to glitch, such behavior is clearly generic to this source class. We show that in terms of fractional frequency change, AXPs are among the most actively glitching neutron stars, with glitch amplitudes in general larger than in radio pulsars. However, in terms of absolute glitch amplitude, AXP glitches are unremarkable. Unlike radio pulsar glitches, AXP glitches can sometimes, though not always, be accompanied by radiative events. We show that the largest observed AXP glitches have recoveries that are unusual among those of radio pulsar glitches, with the combination of recovery time scale and fraction yielding changes in spin-down rates following the glitch similar to, or larger than, the long-term average. We also observed a large long-term fractional increase in the magnitude of the spin-down rate of 1E 1841-045, following its largest glitch, with \Delta \dot{\nu }/\dot{\nu }=0.1 $Δ ˙{ν }/˙{ν } = 0.1$ . These observations are challenging to interpret in standard glitch models, as is the frequent occurence of large glitches given AXPs' high measured temperatures. We speculate that the stellar core may be involved in the largest AXP glitches.

We report on 10 years of monitoring of the 8.7 s anomalous X-ray pulsar 4U 0142+61 using the Rossi X-Ray Timing Explorer (RXTE). This pulsar exhibited stable rotation from 2000 March until 2006 February; the rms phase residual for a spin-down model, which includes n, img1.gif, and img2.gif, is 2.3%. We report a possible phase-coherent timing solution valid over a 10 yr span extending back to 1996 March. A glitch may have occurred between 1998 and 2000, but is not required by the existing timing data. The pulse profile has been evolving since 2000. In particular, the dip of emission between its two peaks got shallower between 2002 and 2006, as if the profile were evolving back to its pre-2000 morphology, following an earlier event, which possibly also included the glitch suggested by the timing data. These profile variations are seen in the 2-4 keV band, but not in 6-8 keV. We also detect a slow increase in the pulsed flux between 2002 May and 2004 December, such that it has risen by 36% +- 3% over 2.6 yr in the 2-10 keV band. The pulsed flux variability and the narrowband pulse profile changes present interesting challenges to aspects of the magnetar model.

We report on mid-infrared imaging observations of the anomalous X-ray pulsars (AXPs) 1E 1048.1-5937, 1RXS J170849.0-400910, and XTE J1810-197. The observations were carried out at 4.5 and 8.0 mm with the Infrared Array Camera and at 24 mm with the Multiband Imaging Photometer on the Spitzer Space Telescope. No mid-infrared counterparts were detected. As infrared emission from AXPs may be related to their X-ray emission either via the magnetosphere or via a dust disk, we compare the derived upper limits on the infrared/X-ray flux ratios of the AXPs to the same ratio for 4U 0142+61, an AXP previously detected in the mid-infrared range. The upper limits are above the flux ratio for 4U 0142+61, indicating that if AXPs have similar infrared/X-ray flux ratios, our observations were not sufficiently deep to detect our AXP targets. For XTE J1810-197, the upper limits set a constraint on its rising radio energy spectrum, suggesting a spectral break between 4.2 x 1010 and 6 x 1013 Hz.

We have searched for persistent radio pulsations, bright single pulses, and bursts from four southern anomalous X-ray pulsars (AXPs). Deep observations were conducted at 1.4 GHz in 1999 July and August with the Parkes 64 m telescope. For all of the target AXPs, the upper limits on integrated pulsed emission are ~0.02 mJy, while the limits on the flux of single pulses are ~1 Jy or better. The corresponding radio luminosity limits are significantly below the observed luminosities of the majority of the observed radio pulsars, and they are significantly below the radio luminosity of XTE J1810-197, the only AXP known thus far to emit radio pulsations, at the epoch when radio pulses were first detected from this source. Our null results support the hypothesis that pulsed radio emission from AXPs is only present in conjunction with X-ray outburst activity. However, we cannot rule out the possibility that pulsed radio emission is present, but that it is too weak to be detected in our observations, or that unfavorable viewing geometries prevent this emission from being seen by terrestrial observers. Given the possible association between the radio emission and transient X-ray behavior of XTE J1810-197, continued radio searches of these and other AXPs at different epochs are warranted, particularly after periods of X-ray burst activity.

We have conducted a radio pulsar survey of 56 unidentified g-ray sources from the third EGRET catalog that are at intermediate Galactic latitudes (5deg < |b| < 73deg). For each source, four interleaved 35 minute pointings were made with the 13 beam, 1400 MHz multibeam receiver on the Parkes 64 m radio telescope. This covered the 95% error box of each source at a limiting sensitivity of ~0.2 mJy to pulsed radio emission for periods P [?] 10 ms and dispersion measures [?]50 pc cm-3. Roughly half of the unidentified g-ray sources at |b| > 5deg with no proposed active galactic nucleus counterpart were covered in this survey. We detected nine isolated pulsars and four recycled binary pulsars, with three from each class being new discoveries. Timing observations suggest that only one of the pulsars has a spin-down luminosity that is even marginally consistent with the inferred luminosity of its coincident EGRET source. Our results suggest that population models, which include the Gould Belt as a component, overestimate the number of isolated pulsars among the midlatitude Galactic g-ray sources, and that it is unlikely that Gould Belt pulsars make up the majority of these sources. However, the possibility of steep pulsar radio spectra and the confusion of terrestrial radio interference with long-period pulsars (P [?] 200 ms) having very low dispersion measures ([?]10 pc cm-3, expected for sources at a distance of less than about 1 kpc) prevent us from strongly ruling out this hypothesis. Our results also do not support the hypothesis that millisecond pulsars make up the majority of these sources. Nonpulsar source classes should therefore be further investigated as possible counterparts to the unidentified EGRET sources at intermediate Galactic latitudes.

We present the first phase-coherent measurement of a braking index for the young, energetic rotation-powered pulsar PSR J1846-0258. This 324 ms pulsar is located at the center of the supernova remnant Kesteven 75 and has a characteristic age of tc = 723 yr, a spin-down energy of E = 8.3 x 1036 ergs s-1, and an inferred magnetic field of 4.9 x 1013 G. Two independent phase-coherent timing solutions are derived, which together span 5.5 yr of data obtained with the Rossi X-Ray Timing Explorer. In addition, a partially phase-coherent timing analysis confirms the fully phase-coherent result. The measured value of the braking index, n = 2.65 +- 0.01, is significantly less than 3, the value expected from magnetic dipole radiation, implying another physical process must contribute to the pulsar's rotational evolution. Assuming the braking index has been constant since birth, we place an upper limit on the spin-down age of PSR J1846-0258 of 884 yr, the smallest age estimate of any rotation-powered pulsar.

We investigate the birth and evolution of Galactic isolated radio pulsars. We begin by estimating their birth space velocity distribution from proper-motion measurements of Brisken and coworkers. We find no evidence for multimodality of the distribution and favor one in which the absolute one-dimensional velocity components are exponentially distributed and with a three-dimensional mean velocity of 380img1.gif km s-1. We then proceed with a Monte Carlo-based population synthesis, modeling the birth properties of the pulsars, their time evolution, and their detection in the Parkes and Swinburne Multibeam surveys. We present a population model that appears generally consistent with the observations. Our results suggest that pulsars are born in the spiral arms, with a galactocentric radial distribution that is well described by the functional form proposed by Yusifov & Kucuk, in which the pulsar surface density peaks at radius ~3 kpc. The birth spin period distribution extends to several hundred milliseconds, with no evidence of multimodality. Models that assume the radio luminosities of pulsars to be independent of the spin periods and period derivatives are inadequate, as they lead to the detection of too many old simulated pulsars in our simulations. Dithered radio luminosities proportional to the square root of the spin-down luminosity accommodate the observations well and provide a natural mechanism for the pulsars to dim uniformly as they approach the death line, avoiding an observed pileup on the latter. There is no evidence for significant torque decay (due to magnetic field decay or otherwise) over the lifetime of the pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate and total number of pulsars in the Galaxy.

We report on the 2004 June 29 X-ray burst detected from the direction of the AXP 1E 1048.1-5937 using the RXTE. We find a simultaneous increase of ~3.5 times the quiescent value in the 2-10 keV pulsed flux of 1E 1048.1-5937 during the tail of the burst, which identifies the AXP as the burst's origin. The burst was overall very similar to the two others reported from the direction of this source in 2001. The unambiguous identification of 1E 1048.1-5937 as the burster here confirms that it was the origin of the 2001 bursts as well. The epoch of the burst peak was very close to the arrival time of 1E 1048.1-5937's pulse peak. The burst exhibited significant spectral evolution, with the trend going from hard to soft. Although the average spectrum of the burst was comparable in hardness (G ~ 1.6) to those of the 2001 bursts, the peak of this burst was much harder (G ~ 0.3). During the 11 days following the burst, the AXP was observed further with RXTE, XMM-Newton, and Chandra. Pre- and postburst observations revealed no change in the total flux or spectrum of the quiescent emission. Comparing all three bursts detected thus far from this source, we find that this event was the most fluent (>3.3 x 10-8 ergs cm-2 in the 2-20 keV band), had the highest peak flux (59 +- 9 x 10-10 ergs s-1 cm-2 in the 2-20 keV band), and had the longest duration (>699 s). The long duration of the burst differentiates it from SGR bursts, which have typical durations of ~0.1 s. Bursts that occur preferentially at pulse maximum, have fast rises, and long X-tails containing the majority of the total burst energy have been seen uniquely from AXPs. The marked differences between AXP and SGRs bursts may provide new clues to help understand the physical differences between these objects.

We present a new phase-coherent timing analysis for the young, energetic pulsar PSR B0540-69 using 7.6 yr of data from the Rossi X-Ray Timing Explorer. We measure the braking index, n = 2.140 +- 0.009, and discuss our measurement in the context of other discordant values reported in the literature. We present an improved source position from the phase-coherent timing of the pulsar, to our knowledge, the first of its kind from X-ray pulsar timing. In addition, we detect evidence for a glitch that had been previously reported but later disputed. The glitch occurred at MJD 51,335 +- 12 with Dn/n = (1.4 +- 0.2) x 10-9 and Dimg1.gif/img1.gif = (1.33 +- 0.02) x 10-4. We calculate that the glitch activity parameter for PSR B0540-69 is 2 orders of magnitude smaller than that of the Crab pulsar, which otherwise has very similar properties. This suggests that neutron stars of similar apparent ages, rotation properties, and inferred dipolar B fields can have significantly different internal properties.

We have discovered four X-ray bursts, recorded with the Rossi X-ray Timing Explorer Proportional Counter Array between 2003 September and 2004 April, that we show to originate from the transient magnetar candidate XTE J1810-197. The burst morphologies consist of a short spike or multiple spikes lasting ~1 s each, followed by extended tails of emission where the pulsed flux from XTE J1810-197 is significantly higher. The burst spikes are likely correlated with the pulse maxima, having a chance probability of a random phase distribution of 0.4%. The burst spectra are best fitted to a blackbody with temperatures 4-8 keV, considerably harder than the persistent X-ray emission. During the X-ray tails following these bursts, the temperature rapidly cools as the flux declines, maintaining a constant emitting radius after the initial burst peak. During the brightest X-ray tail, we detect a narrow emission line at 12.6 keV, with an equivalent width of 1.4 keV and a probability of chance occurrence of less than 4 x 10-6. The temporal and spectral characteristics of these bursts closely resemble the bursts seen from 1E 1048.1-5937 and a subset of the bursts detected from 1E 2259+586, thus establishing XTE J1810-197 as a magnetar candidate. The bursts detected from these three objects are sufficiently similar to one another, yet significantly different from those seen from soft gamma repeaters, that they likely represent a new class of bursts from magnetar candidates exclusive (thus far) to the anomalous X-ray pulsar-like sources.

As part of a large-scale search for coherent pulsations from low-mass X-ray binaries (LMXBs) in the Rossi X-Ray Timing Explorer (RXTE) archive, we have completed a detailed series of searches for coherent pulsations of 4U 1820-30, an ultracompact LMXB with a binary period of 11.4 minutes, located in the globular cluster NGC 6624. This binary's short period implies that any coherent signal would be highly accelerated, so we used phase modulation searches, orbital-parameter-fitting coherent searches, and standard acceleration searches to give significant sensitivity to millisecond pulsations. At a range of luminosities and in four energy bands, we searched a total of 34 archival RXTE observations, 32 of which had on-source integration times longer than 10 ks and some of which were made consecutively, which allowed us to combine them. We found no pulsations. Using our phase modulation search technique, which we ran on all 34 observations, we have been able to place the first stringent (95% confidence) pulsed fraction limits of [?]0.8% on all realistic spin frequencies (i.e., [?]2 kHz) and likely companion masses (0.02 M [?] Mc [?] 0.3 M). Using our orbital-parameter-fitting coherent search, which we ran on only 11 selected observations, we have placed a pulsed fraction limit of [?]0.3% on spin frequencies [?]1.25 kHz and companion masses Mc [?] 0.106 M. By contrast, all five LMXBs known to emit coherent pulsations have intrinsic pulsed fractions in the range 3%-7% when pulsations are observed. Our searches therefore rule out even pulsations with significantly lower pulsed fractions than those already observed.

We present an updated timing solution for the young, energetic pulsar PSR B1509-58 based on 21.3 yr of radio timing data and 7.6 yr of X-ray timing data. No glitches have occurred in this time span, in contrast to other well-studied young pulsars, which show frequent glitches. We report a measurement of the third frequency derivative of img1.gif = (-1.28 +- 0.21) x 10-31 s-4. This value is 1.65 s from (i.e., consistent with) that predicted by the simple constant magnetic dipole model of pulsar spin-down. We measured the braking index to be n = 2.839 +- 0.003 and show that it varies by 1.5% over 21.3 yr owing to contamination from timing noise. Results of a low-resolution power spectral analysis of the significant noise apparent in the data yield a spectral index of a = -4.6 +- 1.0 for the red-noise component.

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is LX [?] 1029 ergs s-1 for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify six X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar, and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

We present a statistical analysis of the X-ray bursts observed from the 2002 June 18 outburst of the anomalous X-ray pulsar (AXP) 1E 2259+586, observed with the Proportional Counter Array (PCA) aboard the Rossi X-Ray Timing Explorer. We show that the properties of these bursts are similar to those of soft gamma repeaters (SGRs). We find the following similarities: the burst durations follow a lognormal distribution that peaks at 99 ms, the differential burst fluence distribution is well described by a power law of index -1.7, the burst fluences are positively correlated with the burst durations, the distribution of waiting times is well described by a log normal distribution of mean 47 s, and the bursts are generally asymmetric, with shorter rise than fall times. However, we find several quantitative differences between the AXP and SGR bursts. Specifically, the AXP bursts we observed exhibit a wider range of durations, the correlation between burst fluence and duration is flatter than for SGRs, the observed AXP bursts are on average less energetic than observed SGR bursts, and the more energetic AXP bursts have the hardest spectra--the opposite of what is seen for SGRs. Unlike the case of SGRs, we find a correlation of burst phase with pulsed intensity. We conclude that the bursts are sufficiently similar that AXPs and SGRs can be considered united as a source class, yet there are some interesting differences that may help determine what physically differentiates the two closely related manifestations of neutron stars.

We report the discovery of two binary millisecond pulsars in the core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20 cm. PSR J2140-2310A (M30A) is an eclipsing 11 ms pulsar in a 4 hr circular orbit, and PSR J2140-23B (M30B) is a 13 ms pulsar in an as yet undetermined but most likely highly eccentric (e > 0.5) and relativistic orbit. Timing observations of M30A with a 20 month baseline have provided precise determinations of the pulsar's position (within 4'' of the optical centroid of the cluster) and spin and orbital parameters, which constrain the mass of the companion star to be m2 [?] 0.1 M. The position of M30A is coincident with a possible thermal X-ray point source found in archival Chandra data, which is most likely caused by emission from hot polar caps on the neutron star. In addition, there is a faint (V555 ~ 23.8) star visible in archival Hubble Space Telescope (HST) F555W data that may be the companion to the pulsar. Eclipses of the pulsed radio emission from M30A by the ionized wind from the compact companion star show a frequency-dependent duration ([?]n-a with a ~ 0.4-0.5) and delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms. Future observations of M30 may allow both the measurement of post-Keplerian orbital parameters from M30B and the detection of new pulsars through the effects of strong diffractive scintillation.

We present an X-ray and radio study of the wind nebula surrounding the central pulsar PSR J1811-1925 in the supernova remnant G11.2-0.3. Using high-resolution data obtained with the Chandra X-Ray Observatory and with the VLA radio telescope, we show the X-ray and radio emission is asymmetric around the pulsar, despite the latter's central position in the very circular shell. The new X-ray data allow us to separate the synchrotron emission of the pulsar wind nebula from the surrounding thermal emission and that from the pulsar itself. On the basis of X-ray data from two epochs, we observe temporal variation of the location of X-ray hot spots near the pulsar, indicating relativistic motion. We compare thermal emission observed within the shell, which may be associated with the forward shock of the pulsar wind nebula, to thermal emission from a nearby portion of the remnant shell, the temperature of which implies an expansion velocity consistent with the identification of the remnant with the historical event of 386 A.D. The measured X-ray and radio spectral indices of the nebula synchrotron emission are found to be consistent with a single synchrotron cooling break. The magnetic field implied by the break frequency is anomalously large, given the apparent size and age of the nebula, if a spherical morphology is assumed but is consistent with a bipolar morphology.

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 x 1031 ergs s-1 (0.5-2.5 keV) inside the cluster's 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.

We present a high-resolution radio study of the supernova remnant (SNR) G11.2-0.3 using archival Very Large Array data. Spectral tomography is performed to determine the properties of this composite-type SNR's individual components, which have only recently been distinguished through X-ray observations. Our results indicate that the spectral index of the pulsar wind nebula (PWN), or plerion, is aP = 0.25img1.gif. We observe a spectral index of aS = 0.56 +- 0.02 throughout most of the SNR shell region but also detect a gradient in a in the southeastern component. We compare the spectral index and flux density with recent single-dish radio data of the source. Also, the radio efficiency and morphological properties of this PWN are found to be consistent with results for other known PWN systems.

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect six X-ray sources within the 10.''5 core radius and 13 more within the 115'' half-mass radius down to a limiting luminosity of LX [?] 1030 ergs s-1 for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), one to three likely RS CVn or BY Dra systems, and one or two possible background objects. Of the seven sources for which no optical identifications were made, we expect that approximately two to four are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

We report on long-term monitoring of three anomalous X-ray pulsars (AXPs) using the Rossi X-Ray Timing Explorer (RXTE). We present a phase-coherent timing ephemeris for 4U 0142+61 and show that it has rotated with high stability over 4.4 yr, with rms phase deviations of 7% of the pulse period from a simple fit including only n andimg1.gif . We report on the continued timing stability of 1E 2259.1+586, for which phase coherence has now been maintained over 4.5 yr, as well as on the detection of a significantimg2.gif in 1.4 yr of monitoring of 1RXS J170849.0-400910, consistent with recovery following a glitch. We note a correlation in which timing stability in AXPs decreases with increasingimg1.gif . The timing stability of soft gamma repeaters in quiescence is consistent with this trend, given their large spin-down rates. This trend is similar to one seen in radio pulsars, suggesting a connection between the three populations. We find no large variability in pulse morphology as a function of time. We present high signal-to-noise ratio average pulse profiles for each AXP and consider them as a function of energy. We find a variety of different behaviors and consider possible trends in the data. We also find no large variations in pulsed flux and set 1 s upper limits of ~20%-30% of the mean.

Thermal emission from the surface of an ultramagnetic neutron star is believed to contribute significantly to the soft X-ray flux of the anomalous X-ray pulsars (AXPs). We compare the detailed predictions of models of the surface emission from a magnetar to the observed spectral and variability properties of AXPs. In particular, we focus on the combination of their luminosities and energy-dependent pulsed fractions. We use the results of recent calculations for strongly magnetized atmospheres in radiative equilibrium to obtain the angle- and energy-dependence of the surface emission. We also include in our calculations the significant effects of general relativistic photon transport to an observer at infinity as well as the effects of interstellar extinction. We find that the combination of the large pulsed fractions and the high inferred luminosities of AXPs cannot be accounted for by surface emission from a magnetar with two antipodal hot regions or a temperature distribution characteristic of a magnetic dipole. This result is robust for reasonable neutron star radii, for the range of magnetic field strengths inferred from the observed spin down rates, and for surface temperatures consistent with the spectral properties of AXPs. Models with a single hot emitting region can reproduce the observations, provided that the distance to one of the sources is ~30% less than the current best estimate, and allowing for systematic uncertainties in the spectral fit of a second source. Finally, the thermal emission models with antipodal emission geometry predict a characteristic strong increase of the pulsed fraction with photon energy, which is apparently inconsistent with the current data. The energy-dependence of the pulsed fraction in the models with one hot region shows a wider range of behavior and can be consistent with the existing data. Upcoming high-resolution observations with Chandra and XMM-Newton will provide a conclusive test.

We report on long-term monitoring of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-Ray Timing Explorer (RXTE). The timing behavior of this pulsar is different from that of other AXPs being monitored with RXTE. In particular, we show that the pulsar shows significant deviations from simple spin-down such that phase-coherent timing has not been possible over time spans longer than a few months. We find that the deviations from simple spin-down are not consistent with single "glitch" type events nor are they consistent with radiative precession. We show that in spite of the rotational irregularities, the pulsar exhibits neither pulse profile changes nor large pulsed flux variations. We discuss the implications of our results for AXP models. We suggest that 1E 1048.1-5937 may be a transition object between the soft gamma-ray repeater and AXP populations and may be the AXP most likely to one day undergo an outburst.

We report the results of a survey of the Small Magellanic Cloud (SMC) for radio pulsars conducted with the 20 cm multibeam receiver of the Parkes 64 m telescope. This survey targeted a more complete region of the SMC than a previous pulsar search and had an improvement in sensitivity by a factor of about 2 for most pulsar periods. This survey is much more sensitive to fast young pulsars (with P [?] 100 ms) and is the first survey of the SMC with any sensitivity to millisecond pulsars. Two new pulsars were discovered in the survey, one of which is located within the SMC. The number of pulsars found in the survey is consistent with the expected number derived using several methods. We also report the serendipitous discovery of a new pulsar in the 30 Doradus region of the Large Magellanic Cloud (LMC). These discoveries bring the total number of rotation-powered pulsars currently known in the Magellanic Clouds to eight. We have also made refined timing measurements for the new discoveries as well as for three previously known LMC pulsars. The age distribution of luminous Magellanic Cloud pulsars supports the conjecture that pulsars younger than about 5 Myr are more luminous on average than older pulsars.

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (b > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 [?] b [?] 3.7, which brackets the Kolmogorov value of b = 3.67. The flux density variations are greater than predicted by this model for six pulsars--including the Crab and Vela--but this group is consistent with a Kolmogorov spectrum and an inner scale of [?]1010 cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of [?]1013 cm.

We report on the results of a post-periastron ASCA observation of the PSR B1259–63 system, containing a 48 ms pulsar orbiting a Be star in an eccentric 3.4 yr orbit. The ASCA observation was carried out on 1994 February 28 when the radio pulsar was again visible after a prolonged 40-day radio eclipse near to periastron. The source was clearly detected with an X-ray luminosity (1–10 keV) LXof (1.03 ± 0.09) × 1034(d/2 kpc)2erg s−1, where dis the pulsar's distance. The X-ray spectrum is well-represented by a single power-law model of the photon index, α = 1.61 ± 0.06, and a small photoelectric absorption, NH= (5.6 ± 0.6) × 1021cm−2. No significant X-ray pulsations were detected, and the upper limit for the pulsed component was estimated to be 15% of the total X-ray flux, assuming sinusoidal modulation. The characteristics of the X-ray emission detected on 1994 February 28 are similar to those detected by ASCA near periastron when the pulsed radio emission from PSR B1259-63 was eclipsed. Our results strongly favor a non-thermal model of X-ray emission driven by synchrotron radiation from relativistic shocked particles in the pulsar wind interacting with the outflow from the Be star companion.

We report on our Spitzer observations of the anomalous X-ray pulsar 4U 0142+61, made following a large X-ray burst that occurred on 2007 February 7. To search for mid-infrared flux variations, four imaging observations were carried out at 4.5 and 8.0 mm with the Infrared Array Camera from February 14 to 21. No significant flux variations were detected, and the average fluxes were 32.1 +- 2.0 mJy at 4.5 mm and 59.8 +- 8.5 mJy at 8.0 mm, consistent with those obtained in 2005. The nondetection of variability is interesting in light of reported rapid variability from this source in the near-infrared but consistent with the fact that the source already went back to its quiescent state before our observations began, as indicated by contemporaneous X-ray observations. In order to understand the origin of the near-infrared variability, frequent, simultaneous multiwavelength observations are needed.

The [?]800 year old pulsar J1846-0258 is a unique transition object between rotation-powered pulsars and magnetars: though behaving like a rotation-powered pulsar most of the time, in 2006 it exhibited a distinctly magnetar-like outburst accompanied by a large glitch. Here, we present X-ray timing observations taken with the Rossi X-ray Timing Explorer over a 2.2 year period after the X-ray outburst and glitch had recovered. We observe that the braking index of the pulsar, previously measured to be n = 2.65 +- 0.01, is now n = 2.16 +- 0.13, a decrease of 18% +- 5%. We also note a persistent increase in the timing noise relative to the pre-outburst level. Despite the timing changes, a 2009 Chandra X-ray Observatory observation shows that the X-ray flux and spectrum of the pulsar and its wind nebula are consistent with the quiescent levels observed in 2000.

We report on data obtained with the Chandra, XMM-Newton, Suzaku, and Swift X-ray observatories, following the 2006 outburst of the Anomalous X-ray Pulsar CXOU J164710.2-455216. Using a more complete and higher signal-to-noise data set, we find no evidence for the very large glitch and rapid exponential decay as was reported previously for this source. We set a 3s upper limit on any fractional frequency increase at the time of the outburst of Dn/n < 1.5 x 10-5. Our timing analysis, based on the longest time baseline yet, yields a spin-down rate for the pulsar that implies a surface dipolar magnetic field of [?]9 x 1013 G, although this could be biased high by possible recovery from an undetected glitch. We also present an analysis of the source flux and spectral evolution, and find no evidence for long-term spectral relaxation post-outburst as was previously reported.