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The article examines the causes behind the powerful radio light from dense, magnetic stars dubbed as Lorimer burst in the distant cosmos. Topics include the representation of the Lorimer burst of an unknown cosmic flash, the association of the discovery of perytons and Lorimer burst and the possible effects of astronomical phenomena like supernovae on the burst of radio light.

Einstein’s theory of gravity—the general theory of relativity1—is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field. In contrast to almost all alternative theories of gravity2, the strong equivalence principle of general relativity requires universality of free fall to apply even to bodies with strong self-gravity. Direct tests of this principle using Solar System bodies3,4are limited by the weak self-gravity of the bodies, and tests using pulsar–white-dwarf binaries5,6have been limited by the weak gravitational pull of the Milky Way. PSR J0337+1715 is a hierarchical system of three stars (a stellar triple system) in which a binary consisting of a millisecond radio pulsar and a white dwarf in a 1.6-day orbit is itself in a 327-day orbit with another white dwarf. This system permits a test that compares how the gravitational pull of the outer white dwarf affects the pulsar, which has strong self-gravity, and the inner white dwarf. Here we report that the accelerations of the pulsar and its nearby white-dwarf companion differ fractionally by no more than 2.6 × 10−6. For a rough comparison, our limit on the strong-field Nordtvedt parameter, which measures violation of the universality of free fall, is a factor of ten smaller than that obtained from (weak-field) Solar System tests3,4and a factor of almost a thousand smaller than that obtained from other strong-field tests5,6. The accelerations of a pulsar and a white dwarf in a three-star system differ by at most a few parts per million, providing a much improved constraint on the universality of free fall.

Several letters to the editor are presented in response to articles in previous issues including "Speak Easy" in the August 20, 2016 issue, "Media Maelstrom" by Pattrick Smellie in the August 6, 2016 issue, and "Water War" by Rebecca Macfie in the August 20, 2016 issue.

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.

X-ray pulsations with a 6.85 s period were recently detected in the Small Magellanic Cloud (SMC) and were subsequently identified as originating from the Be/X-ray binary system XTE J0103-728. The recent localization of the source of the X-ray emission has made a targeted search for radio pulsations from this source possible. The detection of pulsed radio emission from XTE J0103-728 would make it only the second system after PSR B1259-63 that is both a Be/X-ray binary and a radio pulsar. We observed XTE J0103-728 in 2008 February with the Parkes 64 m radio telescope soon after the identification of the source of X-ray pulsations was reported in order to search for corresponding radio pulsations. We used a continuous 6.4 hr observation with a 256 MHz bandwidth centered at 1390 MHz using the center beam of the Parkes multibeam receiver. In the subsequent data analysis, which included a folding search, a Fourier search, a fast-folding algorithm search, and a single pulse search, no pulsed signals were found for trial dispersion measures (DMs) between 0 and 800 pc cm-3. This DM range easily encompasses the expected values for sources in the SMC. We place an upper limit of [?]45 mJy kpc2 on the luminosity of periodic radio emission from XTE J0103-728 at the epoch of our observation, and we compare this limit to a range of luminosities measured for PSR B1259-63, the only Be/X-ray binary currently known to emit radio pulses. We also compare our limit to the radio luminosities of neutron stars having similarly long spin periods to XTE J0103-728. Since the radio pulses from PSR B1259-63 are eclipsed and undetectable during the portion of the orbit near periastron, repeated additional radio search observations of XTE J0103-728 may be valuable if it is undergoing similar eclipsing and if such observations are able to sample the orbital phase of this system well.

We present the results of recent Arecibo and Green Bank observations of the globular cluster NGC 6760. Using Arecibo, a phase-coherent timing solution has been obtained for the previously known binary pulsar in this cluster, PSR J1911+0102A. We have also discovered a new millisecond pulsar in NGC 6760, PSR J1911+0101B, an isolated object with a rotational period of 5.38 ms and a dispersion measure DM = 196.7 cm-3 pc. Both pulsars are located within 1.3 core radii of the cluster center and have negative period derivatives. The resulting lower limits for the accelerations of the pulsars are within the range expected given a simple model of the cluster. A search for eclipses in the PSR J1911+0102A binary system using both telescopes yielded negative results. The corresponding limits on the extra gas column density at superior conjunction are consistent with the hypothesis that the observational properties of ultra-low-mass binary pulsars like PSR J1911+0102A are strongly affected by the inclination of the orbital plane of the system. Among globular cluster pulsar populations, that of NGC 6760 exhibits one of the largest known spreads in DM. This quantity seems to be roughly proportional to a cluster's central DM; this suggests that the observed spread is caused by a turbulent interstellar medium at spatial scales of 1 pc.

We consider the statistics of pulsar binaries with white dwarf companions (NS-WD). Using the statistical analysis method developed by Kim and coworkers, we calculate the Galactic coalescence rate of NS-WD binaries due to gravitational wave emission. We find that the most likely values for the total Galactic coalescence rate (tot) of NS-WD binaries lie in the range 0.2-10 Myr-1, depending on different assumed pulsar population models. For our reference model, we obtain tot = 4.11img1.gif Myr-1 at a 68% statistical confidence level. These rate estimates are not corrected for pulsar beaming, and as such they are found to be about a factor of 20 smaller than the Galactic coalescence rate estimates for double neutron star systems. Based on our rate estimates, we calculate the gravitational wave background due to coalescing NS-WD binaries out to extragalactic distances within the frequency band of the Laser Interferometer Space Antenna. We find the contribution from NS-WD binaries to the gravitational wave background to be negligible.

In search of the counterpart to the brightest unidentified gamma-ray source, 3EG J2020+4017 (2CG 078+2), we report on new X-ray and radio observations of the g Cygni field with the Chandra X-Ray Observatory and with the Green Bank Telescope (GBT). We also report on reanalysis of archival ROSAT data. With Chandra it became possible for the first time to measure the position of the putative gamma-ray counterpart, RX J2020.2+4026, with subarcsecond accuracy and to deduce its X-ray spectral characteristics. These observations demonstrate that RX J2020.2+4026 is associated with a K field star and therefore is unlikely to be the counterpart of the bright gamma-ray source 2CG 078+2 in the SNR G78.2+2.1 as had been previously suggested. The Chandra observation detected 37 additional X-ray sources, which were correlated with catalogs of optical and infrared data. Subsequent GBT radio observations covered the complete 99% EGRET likelihood contour of 3EG J2020+4017 with a sensitivity limit of L820 [?] 0.1 mJy kpc2, which is lower than most of the recent deep radio search limits. If there is a pulsar operating in 3EG J2020+4017, this sensitivity limit suggests that either the pulsar does not produce significant amounts of radio emission or that its geometry is such that the radio beam does not intersect with the line of sight. Finally, reanalysis of archival ROSAT data leads to a flux upper limit of fX(0.1 - 2.4 keV) < 1.8 x 10-13 ergs s-1 cm-2 for a putative pointlike X-ray source located within the 68% confidence contour of 3EG J2020+4017. Adopting the supernova remnant age of 5400 yr and assuming a spin-down to X-ray energy conversion factor of 10-4, this upper limit constrains the parameters of a putative neutron star as a counterpart for 3EG J2020+4017 to be P [?] 160 (d 1.5 kpc)-1 ms, img1.gif [?] 5 x 10-13 (d 1.5 kpc-1)-1 s s-1, and B[?] [?] 9 x 1012 (d 1.5 kpc)-1 G.

We have made observations of 98 low Galactic latitude pulsars to measure pulse broadening caused by multipath propagation through the interstellar medium. Data were collected with the 305 m Arecibo telescope at four radio frequencies between 430 and 2380 MHz. We used a CLEAN-based algorithm to deconvolve interstellar pulse broadening from the measured pulse shapes. We employed two distinct pulse-broadening functions (PBFs): PBF1 is appropriate for a thin screen of scattering material between the Earth and a pulsar, while PBF2 is appropriate for scattering material uniformly distributed along the line of sight from the Earth to a pulsar. We found that some observations were better fitted by PBF1 and some by PBF2. Pulse-broadening times (td) are derived from fits of PBFs to the data and are compared with the predictions of a smoothed model of the Galactic electron distribution. Several lines of sight show excess broadening, which we model as clumps of high-density scattering material. A global analysis of all available data finds that the pulse broadening scales with frequency, n, as td [?] n-a, where a ~ 3.9 +- 0.2. This is somewhat shallower than the value a = 4.4 expected from a Kolmogorov medium but could arise if the spectrum of turbulence has an inner cutoff at ~300-800 km. A few objects follow particularly shallow scaling laws (the mean scaling index [?]a[?] ~ 3.1 +- 0.1 and ~3.8 +- 0.2, respectively, for PBF1 and PBF2), which may arise from large-scale refraction or from the truncation of scattering screens transverse to the Earth-pulsar line of sight.

Radio astronomical measurements of extended emission require knowledge of the beam shape and response because the measurements need correction for quantities such as beam efficiency and beamwidth. We describe a scheme that characterizes the main beam and sidelobe in all Stokes parameters employing parameters that allow reconstruction of the complete beam patterns and, also, afford an easy way to see how the beam changes with azimuth, zenith angle, and time. For the main beam in Stokes I, the parameters include the beamwidth, ellipticity and its orientation, coma and its orientation, the point-source gain, and the integrated gain (or, equivalently, the main-beam efficiency); for the other Stokes parameters, the beam parameters include beam squint and beam squash. For the first sidelobe ring in Stokes I, the parameters include an eight-term Fourier series describing the height, radius, and radial width; for the other Stokes parameters they include only the sidelobe's fractional polarization.

Modern digital cross-correlators permit the simultaneous measurement of all four Stokes parameters. However, the results must be calibrated to correct for the polarization transfer function of the receiving system. The transfer function for any device can be expressed by its Mueller matrix. We express the matrix elements in terms of fundamental system parameters that describe the voltage transfer functions (known as the Jones matrix) of the various system devices in physical terms and thus provide a means for comparing with engineering calculations and investigating the effects of design changes. We describe how to determine these parameters with astronomical observations. We illustrate the method by applying it to some of the receivers at the Arecibo Observatory.

We have extensively searched for periodic signals from the soft gamma repeater SGR 1900+14 at 430 and 1410 MHz with the Arecibo Telescope. Our observations did not reveal the 5.16 s periodicity discovered at X-ray wavelengths by Hurley and coworkers. We place pulsed flux-density upper limits of 150 and 30 mJy at 430 and 1410 MHz, respectively. In the course of the 1410 MHz search we discovered a 226 ms radio pulsar, PSR J1907+0918. Its period derivative implies that the age of J1907+0918 is only 38 kyr, making it one of the youngest members of the known pulsar population. Independent lines of evidence in support of this apparent youth are the unusually high degree of circular polarization and a relatively flat radio spectrum. The close proximity of this young radio pulsar to the supernova remnant G42.8+0.6 poses a problem for the proposed association between the G42.8+0.6 and SGR 1900+14.

In the context of assessing the detectability of the coalescence of two neutron stars (NS) by currently built gravitational wave experiments, we present a way of obtaining an upper limit to the coalescence rate in the Galaxy. We consider the NS/NS progenitors just before the second supernova explosion. We combine the theoretical understanding of orbital dynamics at NS formation with methods of empirically estimating pulsar birth rates and derive an upper limit of a few mergers every 105 yr. Such a Galactic rate implies a possible detection by the "enhanced" LIGO of up to a few to 10 mergers per year.

We present evidence that the integrated profiles of some millisecond pulsars exhibit severe changes that are inconsistent with the moding phenomenon as known from slowly rotating pulsars. We study these profile instabilities in particular for PSR J1022+1001 and show that they occur smoothly, exhibiting longer time constants than those associated with moding. In addition, the profile changes of this pulsar seem to be associated with a relatively narrowband variation of the pulse shape. Only parts of the integrated profile participate in this process, which suggests that the origin of this phenomenon is intrinsic to the pulsar magnetosphere and unrelated to the interstellar medium. A polarization study rules out profile changes due to geometrical effects produced by any sort of precession. However, changes are observed in the circularly polarized radiation component. In total we identify four recycled pulsars that also exhibit instabilities in the total power or polarization profiles due to an unknown phenomenon (PSRs J1022+1001, J1730-2304, B1821-24, and J2145-0750). The consequences for high-precision pulsar timing are discussed in view of the standard assumption that the integrated profiles of millisecond pulsars are stable. As a result we present a new method to determine pulse times of arrival that involves an adjustment of relative component amplitudes of the template profile. Applying this method to PSR J1022+1001, we obtain an improved timing solution with a proper-motion measurement of -17 +- 2 mas yr-1 in ecliptic longitude. Assuming a distance to the pulsar as inferred from the dispersion measure, this corresponds to a one-dimensional space velocity of 50 km s-1.

In this paper we present the first observations of a large sample of millisecond pulsars at frequencies of 2.7 GHz (11 cm) and 4.9 GHz (6 cm). For almost all sources, these represent the first 11 cm observations ever. The new measurements more than double the number of millisecond pulsars studied at 6 cm. Our new flux measurements extend the known spectra for millisecond pulsars to the highest frequencies to date. The coverage of more than a decade of the radio spectrum allows us for the first time to search for spectral breaks, as so often observed in normal pulsars around 1 GHz. The results suggest that, unlike normal pulsars, millisecond pulsar spectra can be largely described by a single power law. We align the observed millisecond pulsar profiles with data from lower frequencies to search for indications of disturbed magnetic fields, and attempt to resolve questions that have been raised in recent literature. Deviations from a dipolar magnetic field structure are not evident, and absolute timing across the wide frequency range with a single dispersion measure is possible. We seem to observe mainly unfilled emission beams, which must originate from a very compact region. The existence of nondipolar field components therefore cannot be excluded. A compact emission region is also suggested by a remarkably constant profile width or component separation over a very wide frequency range. This observed difference from the emission properties of normal pulsars is highly significant. For a few sources, polarization data at 2.7 and 4.9 GHz could also be obtained that indicate that despite the typically larger degree of polarization at lower frequencies, millisecond pulsars are weakly polarized or even unpolarized at frequencies above 3 GHz. The simultaneous decrease in degree of polarization and the constant profile width thus question proposals that link depolarization and decreasing profile width for normal pulsars to the same propagation effect (i.e., birefringence). Comparing the properties of core and conal-like profile components to those of normal pulsars, we find less significant patterns in their spectral evolution for the population of millisecond pulsars. Hence, we suggest that core and conal emission may be created by the same emission process. Given the small change in profile width, the indicated depolarization of the radiation, and the possible simple flux density spectra, MSP emission properties tend to resemble those of normal pulsars, only shifted toward higher frequencies.

The extreme physical conditions in millisecond pulsar magnetospheres, as well as an evolutionary history that differs from that of normal pulsars, raise the question whether these objects also differ in their radio emission properties. We have monitored a large sample of millisecond pulsars for a period of 3 yr using the 100 m Effelsberg radio telescope in order to compare the radio emission properties of these two pulsar populations. Our sample comprises a homogeneous data set of very high quality. With some notable exceptions, our findings suggest that the two groups of objects share many common properties. A comparison of the spectral indices between samples of normal and millisecond pulsars demonstrates that millisecond pulsar spectra are not significantly different from those of normal pulsars. This is contrary to what has previously been thought. There is evidence, however, that millisecond pulsars are slightly less luminous and less efficient radio emitters than normal pulsars. We confirm recent suggestions that a diversity exists among the luminosities of millisecond pulsars, with the isolated millisecond pulsars being less luminous than the binary millisecond pulsars, implying that the different evolutionary history has an influence on the emission properties. There are indications that old millisecond pulsars exhibit somewhat flatter spectra than the presumably younger ones. We present evidence that, contrary to common belief, the millisecond pulsar profiles are only marginally more complex than those found among the normal pulsar population. Moreover, the development of the profiles with frequency is rather slow, suggesting very compact magnetospheres. The profile development seems to anticorrelate with the companion mass and the spin period, again suggesting that the amount of mass transfer in a binary system might directly influence the emission properties. The angular radius of radio beams of millisecond pulsars does not follow the scaling predicted from a canonical pulsar model applicable for normal pulsars. Instead, they are systematically smaller, supporting the concept of a critical rotational period below which such a scaling ceases to exist. The smaller inferred luminosity and narrower emission beams will need to be considered in future calculations of the birthrate of the Galactic population.

We have made polarimetric monitoring observations of most of the millisecond pulsars visible from the northern hemisphere at 1410 MHz over a period of 3 yr. Their emission properties are presented here and compared with those of normal pulsars. Although we demonstrated in Paper I that millisecond pulsars exhibit the same flux density spectra and a similar profile complexity, the results presented here suggest that millisecond pulsar profiles do not comply with the predictions of classification schemes based on normal pulsars. The frequency development of a large number of millisecond pulsar profiles is abnormal when compared with the development seen in normal pulsars. Moreover, the polarization characteristics suggest that millisecond-pulsar magnetospheres might not simply represent scaled versions of the magnetospheres of normal pulsars, supporting the results of Paper I. However, phenomena such as mode-changing activity in both intensity and polarization are recognized here for the first time (e.g., for J1730-2304). This suggests that while the basic emission mechanism remains insensitive to rotational period, the conditions that regulate the radio emission in the canonical pulsar model might be satisfied at different regions in millisecond pulsar magnetospheres. At least three types of model have been proposed to describe the millisecond pulsar magnetospheres, ranging from distorted magnetic field configurations resulting from the recycled nature of these sources to traditional polar-cap emission and emission from outer gaps. A comparison of the predictions of these models with observations suggests that individual cases are better explained by different processes. However, we show that millisecond pulsars can be grouped according to common emission properties, a grouping that awaits verification with future multifrequency observations.

We review the main properties, demographics and applications of binary and millisecond radio pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1800. There are now 83 binary and millisecond pulsars associated with the disk of our Galaxy, and a further 140 pulsars in 26 of the Galactic globular clusters. Recent highlights include the discovery of the young relativistic binary system PSR J1906+0746, a rejuvination in globular cluster pulsar research including growing numbers of pulsars with masses in excess of 1.5 M⊙, a precise measurement of relativistic spin precession in the double pulsar system and a Galactic millisecond pulsar in an eccentric (e= 0.44) orbit around an unevolved companion.

We review the main properties, demographics and applications of binary and millisecond radio pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1700. There are now 80 binary and millisecond pulsars associated with the disk of our Galaxy, and a further 103 pulsars in 24 of the Galactic globular clusters. Recent highlights have been the discovery of the first ever double pulsar system and a recent flurry of discoveries in globular clusters, in particular Terzan 5.

We review the properties and applications of binary and millisecond pulsars. Our knowledge of these exciting objects has greatly increased in recent years, mainly due to successful surveys which have brought the known pulsar population to over 1300. There are now 56 binary and millisecond pulsars in the Galactic disk and a further 47 in globular clusters. This review is concerned primarily with the results and spin-offs from these surveys which are of particular interest to the relativity community.