Radio Pulsars: The Neutron Star Population & Fundamental Physics

Radio pulsars are unique laboratories for a wide range of physics and astrophysics. Understanding how they are created, how they evolve and where we find them in the Galaxy, with or without binary companions, is highly constraining of theories of stellar and binary evolution. Pulsars' relationship with a recently discovered variety of apparently different classes of neutron stars is an interesting modern astrophysical puzzle which we consider in Part I of this review. Radio pulsars are also famous for allowing us to probe the laws of nature at a fundamental level. They act as precise cosmic clocks and, when in a binary system with a companion star, provide indispensable venues for precision tests of gravity. The different applications of radio pulsars for fundamental physics will be discussed in Part II. We finish by making mention of the newly discovered class of astrophysical objects, the Fast Radio Bursts, which may or may not be related to radio pulsars or neutron stars, but which were discovered in observations of the latter.Comment: Rapporteur talks in the Proceedings of the 26th Solvay Conference on Physics on Astrophysics and Cosmology, pp 22-61, R. Blandford and A. Sevrin eds., World Scientific (2015

Magnetars

Magnetars are young and highly magnetized neutron stars which display a wide array of X-ray activity including short bursts, large outbursts, giant flares and quasi-periodic oscillations, often coupled with interesting timing behavior including enhanced spin-down, glitches and anti-glitches. The bulk of this activity is explained by the evolution and decay of an ultrastrong magnetic field, stressing and breaking the neutron star crust, which in turn drives twists of the external magnetosphere and powerful magnetospheric currents. The population of detected magnetars has grown to about 30 objects and shows unambiguous phenomenological connection with very highly magnetized radio pulsars. Recent progress in magnetar theory includes explanation of the hard X-ray component in the magnetar spectrum and development of surface heating models, explaining the sources' remarkable radiative output.Comment: 40 pages, Annual Review of Astronomy and Astrophysics, in pres

Limits on the Number of Galactic Young Supernova Remnants Emitting in the Decay Lines of 44Ti

We revise the assumptions of the parameters involved in predicting the number of supernova remnants detectable in the nuclear lines of the decay chain of 44Ti. Specifically, we consider the distribution of the supernova progenitors, the supernova rate in the Galaxy, the ratios of supernova types, the Galactic production of 44Ti, and the 44Ti yield from supernovae of different types, to derive credible bounds on the expected number of detectable remnants. We find that, within 1 sigma uncertainty, the Galaxy should contain an average of 5.1+2.4-2.0 remnants detectable to a survey with a 44Ti decay line flux limit of 10E-5 photons/cm2/s, with a probability of detecting a single remnant of (2.7+10.0-2.4)%, and an expected number of detections between 2 and 9 remnants, making the single detection of Cas A unlikely but consistent with our models. Our results show that the probability of detecting the brightest 44Ti flux source at the high absolute Galactic longitude of Cas A or above is ~10%. Using the detected flux of Cas A, we attempt to constrain the Galactic supernova rate and Galactic production of 44Ti, but find the detection to be only weakly informative. We conclude that even future surveys having 200 times more sensitivity than state-of-the art surveys can be guaranteed to detect only a few new remnants, with an expected number of detections between 8 and 21 at a limiting 44Ti decay flux of 10E-7 photons/cm2/s.Comment: ApJ in press, 21 pages, 9 figure

16 Years of RXTE Monitoring of Five Anomalous X-ray Pulsars

We present a summary of the long-term evolution of various properties of the five non-transient Anomalous X-ray Pulsars (AXPs) 1E 1841-045, RXS J170849.0-400910, 1E 2259+586, 4U 0142+61, and 1E 1048.1-5937, regularly monitored with RXTE from 1996 to 2012. We focus on three properties of these sources: the evolution of the timing, pulsed flux, and pulse profile. We report several new timing anomalies and radiative events, including a putative anti-glitch seen in 1E 2259+586 in 2009, and a second epoch of very large spin-down rate fluctuations in 1E 1048.1-5937 following a large flux outburst. We compile the properties of the 11 glitches and 4 glitch candidates observed from these 5 AXPs between 1996 and 2012. Overall, these monitoring observations reveal several apparent patterns in the behavior of this sample of AXPs: large radiative changes in AXPs (including long-lived flux enhancements, short bursts, and pulse profile changes) are rare, occurring typically only every few years per source; large radiative changes are almost always accompanied by some form of timing anomaly, usually a spin-up glitch; only 20-30% of timing anomalies are accompanied by any form of radiative change. We find that AXP radiative behavior at the times of radiatively loud glitches is sufficiently similar to suggest common physical origins. The similarity in glitch properties when comparing radiatively loud and radiatively silent glitches in AXPs suggests a common physical origin in the stellar interior. Finally, the overall similarity of AXP and radio pulsar glitches suggests a common physical origin for both phenomena.Comment: 36 pages, 7 figures, 9 tables, ApJ Supplements, in pres

Searching for X-ray Variability in the Glitching Anomalous X-ray Pulsar 1E 1841-045 in Kes 73

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 SNR Kes~73, has exhibited three glitches between 2002 and 2007, as determined by {\it 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, arguing that such behavior is not generic to AXPs.Comment: 16 pags, 3 tables, 2 figures, submitted to ApJ

A Search for Optical Pulsations from Two Young Southern Pulsars

We report on high-speed optical photometry of the radio positions of two young rotation-powered pulsars. No pulsations were detected from the optical counterpart proposed by Caraveo et al. (1994) for PSR B1509-58, with a 2 sigma upper limit on the pulsed fraction of 12 percent, significantly lower than that measured in the five known optical pulsars. Given its low pulsed fraction, high optical luminosity, and significant (8 percent) chance coincidence probability, we suggest that this candidate is not associated with the pulsar. We also find that the still-unidentified optical counterpart of PSR B1706-44 has R>18 and lies within 3 arcsec of an R=16.6 star.Comment: 12 pages including 1 PS figure (revised). To appear in ApJ Letter

Long-Term X-ray Monitoring of the Young Pulsar PSR B1509-58

It has long been thought that the pulsed X-ray properties of rotation-powered pulsars are stable on long time scales. However, long-term, systematic studies of individual sources have been lacking. Furthermore, dramatic X-ray variability has now been observed from two pulsars having inferred sub-critical dipole magnetic fields. Here we present an analysis of the long-term pulsed X-ray properties of the young, energetic pulsar PSR B1509-58 using data from the Rossi X-ray Timing Explorer. We measured the 2-50 keV pulsed flux for 14.7 yr of X-ray observations and found that it is consistent with being constant on all relevant time scales, and place a 3 sigma upper limit on day-to-week variability of <28%. In addition, we searched for magnetar-like X-ray bursts in all observations and found none, which we use to constrain the measurable burst rate to less than one per 750 ks of observations. We also searched for variability in the pulse profile and found that it is consistent with being stable on time scales of days to decades. This supports the hypothesis that X-ray properties of rotation-powered X-ray pulsars can be stable on decade-long time scales. In addition, we extend the existing timing solution by 7.1 yr to a total of 28.4 yr and report updated values of the braking index, n=2.832+/-0.003 and the second braking index, m=17.6+/-1.9.Comment: 8 pages, 5 figures. Accepted for publication in the Astrophysical Journa

Flux Relaxation after two outbursts of the magnetar SGR 1627−-41 and possible hard X-ray emission

We report on the long-term flux relaxation of the magnetar SGR 1627-41 after its 2008 outburst, and evidence for hard X-ray excess measured with NuSTAR. We use new observations made with Chandra and XMM-Newton, and an archival NuSTAR observation which add flux measurements at ~2000 days into quiescence after the 2008 outburst. We find that the source flux has further declined since the last measurement made in 2011, ~1000 days after the outburst in 2008. This trend is similar to the relaxation after the source's 1998 outburst. We use crustal cooling models to reproduce the flux relaxation; if the whole surface of the star is heated in the outbursts, the modeling suggests that the 2008 outburst of SGR 1627-41 deposited energy into the inner crust and that the core temperature of SGR 1627-41 is low (T_c<10^8 K) as previously suggested. On the other hand, if only a small fraction of the surface is heated or the temperature in the crust reached the melting temperature, relaxation at early times requires another emission mechanism. Finally, we report on evidence for hard X-ray emission in SGR 1627-41 which follows the observational correlation suggested by Kaspi & Boydstun (2010) in magnetars.Comment: 9 pages, 4 figures. Accepted for publication in Ap

Radio Non-Detection of the SGR 1806-20 Giant Flare and Implications for Fast Radio Bursts

We analyse archival data from the Parkes radio telescope which was observing a location 35.6$^\circ$ away from SGR 1806-20 during its giant $\gamma$-ray flare of 2004 December 27. We show that no FRB-like burst counterpart was detected, and set a radio limit of 110\,MJy at 1.4 GHz, including the estimated 70\,dB suppression of the signal due to its location in the far side lobe of Parkes and the predicted scattering from the interstellar medium. The upper limit for the magnetar giant flare radio to $\gamma$-ray fluence ratio is $\eta_\mathrm{SGR} \lesssim 10^{7}\,\mathrm{Jy\,ms\,erg^{-1}\,cm^{2}}$. Based on the non-detection of a short and prompt $\gamma$-ray counterpart of fifteen FRBs in $\gamma$-ray transient monitors, we set a lower limit on the fluence ratios of FRBs to be $\eta_\mathrm{FRB} \gtrsim 10^{7-9}\,\mathrm{Jy\,ms\,erg^{-1}\,cm^{2}}$. The fluence ratio limit for \sgr\ is inconsistent with all but one of the fifteen FRBs. We discuss possible variations in the magnetar-FRB emission mechanism and observational caveats that may reconcile the theory with observations.Comment: 9 pages, 4 figures. Accepted in ApJ. Added a discussion on FRB rates compared to magnetar giant flare rates. Results unchange

The X-ray Bursts from the Magnetar Candidate 1E 2259+586

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 aboard the Rossi X-ray Timing Explorer. We show that the properties of these bursts are similar to those of Soft Gamma-Repeaters (SGRs). The similarities we find are: the burst durations follow a log-normal distribution which 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 faster rise than fall times. However, we find several quantitative differences between the AXP and SGR bursts. Specifically, there is a correlation of burst phase with pulsed intensity, 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. 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.Comment: 4 pages, 4 figures, 2003: Rossi and Beyond, ed. P. Kaaret, F.K. Lamb, & J.H. Swank held in Cambridge, MA, Nov. 3-5, 200