The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with $μ$s time resolution and at 2 GHz using the Nançay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package. We present the most accurate value for the optical-radio lag of 255 $\pm$ 21 $μ$s and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses.
Simultaneous dual-frequency observations of giant pulses from the Crab pulsar were performed at the frequencies of 61 and 111 MHz. It is shown that scattering of giant pulses from the Crab pulsar occurs at thick, and not at thin screen.
It is argued that the nanosecond giant subpulses detected recently in the Crab pulsar are generated by means of the coherent curvature radiation of charged relativistic solitons associated with sparking discharges of the inner gap potential drop above the polar cap.
We present new data of the Crab pulsar and its environment. Images taken in the infrared with ISAAC allow us to measure the pulsar spectrum and to reveal the red knot close to the pulsar. Complementary optical data from VLT and HST are presented. The full article, with full resolution nice color images, is presented in Messenger, 107, 32
We present the results of a search for pulsed TeV emission from the Crab pulsar using 97 hours of data recorded with the high-resolution GRANITE III camera of the Whipple 10 m gamma-ray telescope.
Photometric data of the Crab pulsar, obtained in stroboscopic mode over a period of more than eight years, are presented here. The applied Fourier analysis reveals a faint 60 second modulation of the pulsar's signal, which we interpret as a free precession of the pulsar.
Variation in the angle $α$ between a pulsar's rotational and magnetic axes would change the torque and spin-down rate. We show that sudden increases in $α$, coincident with glitches, could be responsible for the persistent increases in spin-down rate that follow glitches in the Crab pulsar. Moreover, changes in $α$ at a rate similar to that inferred for the Crab pulsar account naturally for the very low braking index of the Vela pulsar. If $α$ increases with time, all pulsar ages obtained from the conventional braking model are underestimates. Decoupling of the neutron star liquid interior fro…
We show that the relativistic wind in the Crab pulsar, which is commonly thought to be invisible in the region upstream of the termination shock at R < 0.1 pc, in fact could be directly observed through its inverse Compton gamm-ray emission. The search for such specific component of radiation in the gamma-ray spectrum of the Crab can provide unique information about the unshocked pulsar wind that is not accessible at other wavelengths.
The recent Chandra image of the Crab nebula shows a striking, axisymmetric polar jet. It is shown that jets are formed in axisymmetric, magnetized pulsar winds and that the jet luminosity scales relative to the total as (γ_0σ_{eq})^{-4/3}, where σ_{eq} is the ratio of Poynting flux to particle kinetic energy output at the equator at the base of the flow and γ_0 the initial Lorentz factor of the flow. The results are applied to the image of the Crab nebula, and the limit is set for the Crab pulsar of σ_{eq} \leq 100. It is argued that conventional pulsar theory needs to be reexamined in light o…
We present near-infrared observations obtained with ISAAC on the VLT of the Crab pulsar and its environment. Photometry of the pulsar in Js, H and Ks shows the pulsar spectrum to extend fairly smoothly from the UV/optical regime. PSF subtraction of the pulsar allows us to study its immediate neighborhood in some detail. In particular, the knot positioned just 0.6 arcsec from the pulsar has been revealed in the IR. Using also archival HST data for the knot, we have measured its broad band spectrum to rise steeply into the IR, in contrast to the spectrum of the pulsar itself.
We analytically examine the capabilities of rotation-powered pulsars as the sources of gamma-rays and show that their phase-averaged gamma-ray flux is proportional to the product of the spin-down flux and the gap trans-field thickness cubed irrespective of the emission models. Applying the scheme to the Crab pulsar, we demonstrate that the outer-gap model reproduces the observed GeV fluxes and that the slot-gap model reproduces at most twenty per cent of the observed fluxes because of the small trans-field thickness. An implication on the relationship between the gamma-ray and the spin-down fl…
We consider the widespread assumption that coherent pulsar radio emission is based on extended pair production leading to plasma densities highly exceeding the Goldreich-Julian density. We show as an example that the observed low frequency (160 MHz) emission of the Crab pulsar is incompatible to the model of extended pair production. Our results rule out significant pair production if a plasma process is responsible for coherence and the radio emission originates from inside the light cylinder.
Pravdo et al (1997) claimed that the phase resolved x-ray spectrum in Crab pulsar (PSR B0531+21) shows a spectral hardening at the leading edge of the first peak of its integrated profile; this was a new and unexpected result. This article reanalyzes their data, as well as some other related data, and argues that the spectrum is as likely to be unvarying (i.e., neither hardening nor softening).
We discuss our recent discovery of the giant radio emission from the Crab pulsar at its high frequency components (HFCs) phases and show the polarization characteristic of these pulses. This leads us to a suggestion that there is no difference in the emission mechanism of the main pulse (MP), interpulse (IP) and HFCs. We briefly review the size distributions of the Crab giant radio pulses (GRPs) and discuss general characteristics of the GRP phenomenon in the Crab and other pulsars.
Optical photometry for the pulsar PSR0531+21 has been extended with new observations that strengthen evidence for a previously observed 60 seconds periodicity. This period is found to be increasing with time at approximately the same rate as the rotational period of the pulsar. The observed period and its time dependence fit a simple free precession model.
It has been suggested that the Crab pulsar's proper motion is well aligned with the symmetry axis of the pulsar wind nebula. We have re-visited this question, examining over 6 years of F547M WFPC2 chip 3 images to obtain a best-fit value of $μ_\ast = 14.9 \pm 0.8$mas/yr at PA $278\arcdeg \pm 3\arcdeg$. At $26\arcdeg\pm 3\arcdeg$ to the nebula axis, this substantially relaxes constraints on the birth kick of this pulsar. Such misalignment allows the momentum to be imparted over $\sim$1s timescales.
The Crab nebula and its pulsar have been observed for about 3 hours with the high-speed photo-polarimeter OPTIMA in January 2002 at the Calar Alto 3.5m telescope. The Crab pulsar intensity and polarization are determined at all phases of rotation with higher statistical accuracy than ever. Therefore, we were able to separate the so-called 'off-pulse' phase emission (with an intensity of about 1.2% compared to the main peak, assumed to be present at all phases) from the pulsed emission and show the 'net' polarization of the pulsed structures. Recent theoretical results indicate that the measure…
We investigate the consequences of the acceleration of heavy nuclei (e.g. iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in collisions with soft photons produced in the pulsar's outer gap, injecting energetic neutrons which decay either inside or outside the Crab Nebula. The protons from neutron decay inside the nebula are trapped by the Crab Nebula magnetic field, and accumulate inside the nebula producing gamma-rays and neutrinos in collisions with the matter in the nebula. Neutrons decaying outside the Crab Nebula contribute to the Galactic cosmic rays. We compute…
We searched for pulsed gamma-ray emissions from the Crab pulsar using data of the Tibet-III air shower array from November 1999 through November 2005. No evidence for the pulsed emissions was found in our analysis. Upper limits at different energies were calculated for a $3 σ$ confidence level in the energy range of multi-TeV to several hundred TeV.
Using the high speed 2-d TRIFFID photometer, we have obtained phase resolved photometry of the Crab pulsar in UBV that allows us to flux the unpulsed light curve component. Following de-extinction, weighted least-square fitting indicates a power-law exponent of alpha = -0.62 +/- 0.49. This is steeper than that reported for the peak components and its origin remains unclear with respect to contemporary magnetospheric theory.
Observations of the Crab Nebula have proven to be the best tool to calibrate and to characterize the performance of a Cherenkov telescope. Scientifically, it is interesting to measure the energy spectrum of the Crab Nebula close to the inverse-Compton peak where a deviation is expected from the power law seen at energies above 300 GeV. Additionally, it is important to search for pulsed emission from the Crab Pulsar at energies beyond the 10 GeV upper limit of the EGRET pulsar detection. Since current models predict a cut-off in pulsed emission between 10 and 100 GeV, measurements at energies c…
The poor angular resolution of imaging gamma-ray telescopes is offset by the large reflector areas of next generation telescopes such as MAGIC (17~m diameter), which makes the study of optical emission associated with some gamma-ray sources feasible. Furthermore, the extremely fast time response of photomultipliers (PMs) makes them ideal detectors for fast (subsecond) optical transients and periodic sources like pulsars. The optical pulse of the Crab pulsar was detected with the HEGRA CT1 central pixel using a modified PM, similar to the future MAGIC camera PMs. The purpose of these periodic o…
We present the time-averaged characteristics of the Crab pulsar in the 0.75-30 MeV energy window using data from the imaging Compton Telescope COMPTEL aboard the Compton Gamma-Ray Observatory (CGRO) collected over its 9 year mission. Exploiting the exceptionally long COMPTEL exposure on the Crab allowed us to derive significantly improved COMPTEL spectra for the Crab nebula and pulsar emissions, and for the first time to accurately determine at low-energy gamma-rays the pulse profile as a function of energy. These timing data, showing the well-known main pulse and second pulse at a phase separ…
The supernova remnant N157B (30 Dor B, SNR 0539-69.1, NGC 2060), located in the Tarantula Nebula of the Large Magellanic Cloud, has long been considered a possible Crab-like remnant. This hypothesis has been confirmed, quite spectacularly, with the discovery of PSR J0537-6910, the remarkable 16 ms X-ray pulsar in N157B. PSR J0537-6910 is the most rapidly spinning pulsar found to be associated with a supernova remnant. Here we report our discovery and summarize the properties of this pulsar and its supernova remnant.
Our high time resolution observations of individual pulses from the Crab pulsar show that the main pulse and interpulse differ in temporal behavior, spectral behavior, polarization and dispersion. The main pulse properties are consistent with one current model of pulsar radio emission, namely, soliton collapse in strong plasma turbulence. The high-frequency interpulse is quite another story. Its dynamic spectrum cannot easily be explained by any current emission model; its excess dispersion must come from propagation through the star's magnetosphere. We suspect the high-frequency interpulse do…
We present a modified outer gap model to study the phase-resolved spectra of the Crab pulsar. A theoretical double peak profile of the light curve containing the whole phase is shown to be consistent with the observed light curve of the Crab pulsar by shifting the inner boundary of the outer gap inwardly to $\sim 10$ stellar radii above the neutron star surface. In this model, the radial distances of the photons corresponding to different phases can be determined in the numerical calculation. Also the local electrodynamics, such as the accelerating electric field, the curvature radius of the m…
Using the first three and a half years of observations from the Energetic Gamma Ray Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO), phase-resolved analyses are performed on the emission from the three brightest high-energy gamma-ray pulsars, Crab, Geminga, and Vela. For each pulsar, it is found that there is detectable high-energy gamma-ray emission above the galactic diffuse background throughout much of the pulsar rotation cycle. A hardness ratio is introduced to characterize the evolution of the spectral index as a function of pulsar phase. While the hardest emission fr…
Our high time resolution observations of individual pulses from the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Main pulses can occasionally be resolved into short-lived, relatively narrow-band nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Interpulses at centimeter wavelengths are very different. Their dynamic spectrum contains regular, microsecond-long emission bands. We have detected these bands, proportionately spaced in frequency, from 4.5 to 10.5 …
We have detected occasional, short-lived ``echoes'' of giant pulses from the Crab pulsar. These echo events remind us of previously reported echoes from this pulsar, but they differ significantly in detail. Our echo events last at most only a few days; the echo emission lags the primary emission by only 40-100 musec. The echoes are consistently weaker and broader than the primary emission, and appear only at the lower of our two simultaneous observing frequencies. We suggest that these echoes are created by refraction in small plasma structures -- plasma clouds or magnetic flux ropes -- deep w…
We report the first observations of an astronomical object using a superconducting tunnel junction (STJ) device, a pixel detector with intrinsic energy resolution in the optical wavelength range. The Crab pulsar was observed using a 6 times 6 array of Tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. Each array element provides photon counting capability recording up to ~ 10^3 photons per pix per s with an arrival time accuracy of about 5 microsecond, and providing a wavelength resolution of about 100 nm. We derive a photometrically resolved light curve which, however, shows n…
In this paper we present some preliminary result on the spectral and timing analysis of the X-ray pulsed emission from the 16 ms pulsar PSR J0537-6910 in the energy range 0.1--30 keV, based on archival BeppoSAX and RossiXTE observations. This pulsar, discovered by Marshall et al.(1998) in the LMC field with RXTE, is the fastest spinning pulsar associated with a supernova remnant. It is characterized by strong glitch activity with the highest rate of all known Crab-like system.
We report on a Chandra observation of the Crab Nebula that gives the first clear view of the faint boundary of the Crab's X-ray-emitting Pulsar Wind Nebula, or PWN. There is structure in all directions. Fingers, loops, bays, and the South Pulsar Jet all indicate that either filamentary material or the magnetic field are controlling the relativistic electrons. In general, spectra soften as distance from the pulsar increases but do not change rapidly along linear features. This is particularly true for the Pulsar Jet. The termination of the Jet is abrupt; the E side is close to an [O {\small III…
We have used a set of archived HST/WFPC2 exposures of the inner regions of the Crab Nebula to obtain a new measurement of the pulsar proper motion, the first after the original work of Wyckoff & Murray, more than 20 years ago. Our measurement of the pulsar displacement, mu = 18 +/- 3 mas/yr, agrees well with the value obtained previously. This is noteworthy, since the data we have used span less than 2 years, as opposed to the 77 years required in the previous work. With a position angle of 292 +/- 10 deg, the proper motion vector appears aligned with the axis of symmetry of the inner Crab neb…
The conversion of the Crab pulsar wind from one dominated by Poynting flux close to the star to one dominated by particle-born energy at the termination shock is considered. The idea put forward by Coroniti (1990) and criticised by Lyubarsky & Kirk (2001) that reconnection in a striped wind is responsible, is generalised to include faster prescriptions for the a priori unknown dissipation rate. Strong acceleration of the wind is confirmed, and the higher dissipation rates imply complete conversion of Poynting flux into particle-born flux within the unshocked wind.
We have observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with $μ$s time resolution. Our aim was to study the delay between the radio and optical pulse. The Crab Pulsar was observed three times over a time span of almost 7 years, on two different locations, using three different versions of the instrument, and using two different GPS units. We consistently find that the optical peak leads the radio peak by 49$\pm$90, 254$\pm$170, and 291$\pm$100 $μ$s. On assumption of a constant optical lead, the weighted-average value is $\si…
We present evidence of wavelength-dependent variations within the infrared, optical, and ultraviolet pulse profile peaks of the Crab Nebula pulsar. The leading and trailing edge half-width half-maxima of the peaks display clear differences in their wavelength dependences. In addition, phase-resolved infrared-to-ultraviolet color spectra show significant variations from the leading to trailing edges of the peaks. The color variations between the leading and trailing edges remain significant over phase differences smaller than 0.0054, corresponding to timescales of $<180 μ$s. These results are n…
The deeper and more extended survey of the central parts of the Galactic Plane by H.E.S.S. during 2005-2007 has revealed a number of new point-like, as well as, extended sources. Two point-like sources can be associated to two remarkable objects around "Crab-like" young and energetic pulsars in our Galaxy: G21.5-0.9 and Kes 75. The characteristics of each of the sources are presented and possible interpretations are briefly discussed.
One fundamental question about pulsars concerns the mechanism of their pulsed electromagnetic emission. Measuring the high-end region of a pulsar's spectrum would shed light on this question. By developing a new electronic trigger, we lowered the threshold of the Major Atmospheric gamma-ray Imaging Cherenkov (MAGIC) telescope to 25 GeV. In this configuration, we detected pulsed gamma-rays from the Crab pulsar that were greater than 25 GeV, revealing a relatively high cutoff energy in the phase-averaged spectrum. This indicates that the emission occurs far out in the magnetosphere, hence exclud…
New polarimetric observations of the Crab pulsar at frequencies between 1.4 and 8.4 GHz are presented. Additional pulse components discovered in earlier observations (Moffett & Hankins 1996, astro-ph/9604163) are found to have high levels of linear polarization, even at 8.4 GHz. No abrupt sweeps in position angle are found within pulse components; however, the position angle and rotational phase of the interpulse do change dramatically between 1.4 and 4.9 GHz. The multi-frequency profile morphology and polarization properties indicate a non-standard origin of the emission. Several emission geo…
We present the results of a search for pulsed TeV emission from the Crab pulsar using the Whipple Observatory's 10 m gamma-ray telescope. The direction of the Crab pulsar was observed for a total of 73.4 hours between 1994 November and 1997 March. During this period the Whipple 10 m telescope was operated at its lowest energy threshold to date. Spectral analysis techniques were applied to search for the presence of a gamma-ray signal from the Crab pulsar over the energy band 250 GeV to 4 TeV. We do not see any evidence of the 33 ms pulsations present in other energy bands from the Crab pulsar.…
The origin of relativistic electrons in the Crab nebula which are producing the broad-band flat radio spectrum of this prototype plerion has proved difficult to understand. Here I show that these electrons can be naturally explained as a relic population of the pulsar wind electrons that have lost most of their energy in the expanding nebula because of intensive radiative and adiabatic cooling in the past. The observed radio spectrum suggests that the initial slowing-down time of the pulsar was about 30 yrs (or less), which implies that it has been born with a spin period P_0 ~ 3-5 ms, several…
We present the results of a search for pulsed TeV emission from the Crab pulsar using the Whipple Observatory's 10m gamma-ray telescope. The direction of the Crab pulsar was observed for a total of 73.4 hours between 1994 November and 1997 March. Spectral analysis techniques were applied to search for the presence of a gamma-ray signal from the Crab pulsar over the energy band 250GeV to 4TeV. At these energies we do not see any evidence of the 33ms pulsations present at lower energies from the Crab pulsar. The 99.9% confidence level upper limit for pulsed emission above 250GeV is derived to be…
We investigate polarization of high-energy emissions from the Crab pulsar in the frame work of the outer gap accelerator. The recent version of the outer gap, which extends from inside the null charge surface to the light cylinder, is used for examining the light curve, the spectrum and the polarization characteristics, simultaneously. The polarization position angle curve and the polarization degree are calculated to compare with the Crab optical data. We show that the outer gap model explains the general features of the observed light curve, the spectrum and the polarization by taking into a…
Our high time resolution observations of individual giant pulses in the Crab pulsar show that both the time and frequency signatures of the interpulse are distinctly different from those of the main pulse. Giant main pulses can occasionally be resolved into short-lived, relatively narrow-band nanoshots. We believe these nanoshots are produced by soliton collapse in strong plasma turbulence. Giant interpulses are very different. Their dynamic spectrum contains narrow, microsecond-long emission bands. We have detected these proportionately spaced bands from 4.5 to 10.5 GHz. The bands cannot easi…
We discuss non-thermal emission mechanism of the Crab-like pulsars with both a two-dimensional electrodynamical study and a three-dimensional model. We investigate the emission process in the outer gap accelerator. In the two-dimensional electrodynamical study, we solve the Poisson equation of the accelerating electric field in the outer gap and the equation of motion of the primary particles with the synchrotron and the curvature radiation process and the pair-creation process. We show a solved gap structure which produces a consistent gamma-ray spectrum with EGRET observation. Based on the t…
Recent Chandra observations of nearby galaxies have revealed a number of ultraluminous X-ray sources (ULXs) with super-Eddington luminosities, away from the central regions of non-active galaxies. The nature of these sources is still debated. We argue that a fraction of them could be young, Crab-like pulsars, the X-ray luminosity of which is powered by rotation. We use the pulsar birth parameters estimated from radio pulsar data to compute the steady-state pulsar X-ray luminosity distribution as a function of the star formation rate (SFR) in the galaxy. We find that ~10% of optically dull gala…
We investigate polarization of high-energy emissions from the Crab pulsar in the frame work of the outer gap accelerator, following previous works of Cheng and coworkers. The recent version of the outer gap, which extends from inside the null charge surface to the light cylinder, is used for examining the synchrotron radiations from the secondary and the tertiary pairs, which are produced outside the gap. We calculate the light curve, the spectrum and the polarization characteristics, simultaneously, by taking into account gyration motion of the particles. The polarization position angle curve…
The Crab-like SNR G21.5-0.9 was observed in the X-ray band (0.5-10 keV) by the XMM-Newton satellite for over 100 ks. The large effective area of the EPIC instrument has allowed us to to perform a deep search for pulsations from the central core of G21.5-0.9. No pulsations were found with upper limits on the pulsed fraction between 7.5 % and 40 % (depending on frequency and energy range).
We present a review of observed parameters of giant radio pulses, based on the observations conducted by our group during recent years. The observations cover a broad frequency range of about 3 octaves, concentrating between 600 and 4850 MHz. Giant pulses of both the Crab pulsar and the millisecond pulsar B1937+21 were studied with the 70-m Tidbinbilla, the 100-m GBT, 64-m Kalyazin and Westerbork radio telescopes. We discuss pulse energy distribution, dependence of peak flux density from the pulse width, peculiarities of radio spectra, and polarization properties of giant radio pulses.
We construct for the first time, the sequences of stable neutron star (NS) models capable of explaining simultaneously, the glitch healing parameters, $Q$, of both the pulsars, the Crab ($Q \geq 0.7$) and the Vela ($Q \leq 0.2$), on the basis of starquake mechanism of glitch generation, whereas the conventional NS models cannot give such consistent explanation. Furthermore, our models also yield an upper bound on NS masses similar to those obtained in the literature for a variety of modern equations of state (EOSs) compatible with causality and dynamical stability. The results obtained in this…
The stability of the optical pulse of the Crab pulsar is analyzed based on the 1 $μ$s resolution observations with the Russian 6-meter and William Hershel telescopes equipped with different photon-counting detectors. The search for the variations of the pulse shape along with its arrival time stability is performed. Upper limits on the possible short time scale free precession of the pulsar are placed. The evidence of pulse time of arrival (TOA) variations on 1.5-2 hours time scale is presented, along with evidence of small light curve (shape and separation of main and secondary peaks) changes…
The traditional view of radio pulsar wind nebulae (PWN), encouraged by the Crab nebula's X-ray and radio morphologies, is that they are a result of the integrated history of their pulsars' wind. The radio emission should therefore be largely unaffected by recent pulsar activity, and hence minimally correlated with structures in the X-ray nebulae. Observations of several PWN, both stationary (sPWN) and rapidly moving (rPWN), now show clear morphological relationships between structures in the radio and X-ray with radio intensity variations on the order of unity. We present high-resolution X-ray…
We are undertaking an extensive X-ray monitoring campaign of the two Crab-like pulsars in the Large Magellanic Clouds, PSR B0540-69 and PSR J0537-6910. We present our current phase-connected timing analysis derived from a set of 50 pointed X-ray observations spanning several years. From our initial 1.2 yr monitoring program of the young 50 ms pulsar PSR B0540-69, we find the first compelling evidence for a glitch in its rotation. This glitch is characterized by dv / v = 1.90 +/- 0.05 X 10E-9 and $dvdot/ vdot = (8.5 +/- 0.5 X 10E-5. Taking into account the glitch event, we derive a braking inde…
From 1995 to 1999, daily monitoring of the radio emission from the Crab pulsar at the Green Bank and Jodrell Bank observatories revealed a series of six sudden rotational spinups or glitches, doubling the number of glitches observed for this pulsar since 1969. With these observations, the range of time intervals between significant Crab glitches has widened considerably, indicating that the occurrence of Crab glitches may be more random than previously thought. The new glitch amplitudes ($Δν/ν$) span an order of magnitude from 2e-9 to 3e-8. Except in one case, which we suggest may represent an…
A dedicated stroboscopic device was used to obtain optical spectra of the Crab main-pulse and inter-pulse as well as the spectrum of the underlying nebula when the pulsar is turned off. As the nebular emission is very inhomogeneous, our ability to effectively subtract the nebular background signal is crucial. No spectral lines intrinsic to the pulsar are detected. The main-pulse and the inter-pulse behave as power laws, both with the same de-reddened index Alpha = +0.2 +- 0.1. This value was obtained by subtracting the nebular spectrum at the exact position of the pulsar. The underlying nebula…
We present the first phase-resolved study of the X-ray spectral properties of the Crab Pulsar that covers all pulse phases. The superb angular resolution of the Chandra X-ray Observatory enables distinguishing the pulsar from the surrounding nebulosity, even at pulse minimum. Analysis of the pulse-averaged spectrum measures interstellar X-ray extinction due primarily to photoelectric absorption and secondarily to scattering by dust grains in the direction of the Crab Nebula. We confirm previous findings that the line-of-sight to the Crab is underabundant in oxygen, although more-so than recent…
The main inference from the experiments of the '80s that the time-averaged energy spectra of pulsars had to steepen in the GeV-TeV energy region has been reinforced in the '90s from experiments with higher sensitivities. However, results from several experiments from the past and the more sensitive experiments at present can be reconciled by invoking a possibly different component arising in the TeV region. The results of the preliminary analysis of the data being collected with the PACT array will be presented.
In separate series of observations of the Crab pulsar, pulse broadening due to scattering was measured at 111 MHz, and variations of dispersion due to pulse delay were measured at higher radio frequencies. In a remarkable event lasting 200 days a large increase occurred in both parameters and with similar time signatures. The increases in scattering and dispersion measure observed over the 200 days MJD 53950 - 54150 are attributable to the effects of an ionised cloud or filament crossing the line of sight. The cloud would be $10^11 - 10^12 m across, with electron density 10^3 - 10^4 cm^-3. The…
Recent estimates of the properties of the Crab nebula are used to derive constraints on the moment of inertia, mass and radius of the pulsar. To this purpose, we employ an approximate formula combining these three parameters. Our "empirical formula" I = a(x)MR^2, where x = (M/M_sun)(km/R), is based on numerical results obtained for thirty theoretical equations of state of dense matter. The functions a(x) for neutron stars and strange stars are qualitatively different. For neutron stars a_NS(x) = x/(0.1+2x) for x <= 0.1 (valid for M > 0.2 M_sun) and a_NS(x) = (2/9)(1+5x) for x > 0.1. For strang…
