Gamma-ray Pulsars Research Articles

Gamma-ray emission in near critical density plasmas at laser intensities of 1021 W/cm2

We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 1021 W∕cm2 using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW∕50 fs laser pulse at the optimal plasma density (∼1nc), about 108 photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.

Blind search for radio-quiet and radio-loud gamma-ray pulsars with Fermi-LAT data

The Fermi Large Area Telescope (LAT) has observed more than a hundred of gamma-ray pulsars, about one third of which are radio-quiet, i.e., not detected at radio frequencies. The most of radio-loud pulsars are detected by Fermi LAT by using the radio timing models, while the radio-quiet ones are discovered in a blind search. The difference in the techniques introduces an observational selection bias and, consequently, the direct comparison of populations is complicated. In order to produce an unbiased sample, we perform a blind search of gamma-ray pulsations using Fermi-LAT data alone. No radio data or observations at optical or X-ray frequencies are involved in the search process. We produce a gamma-ray selected catalog of 25 non-recycled gamma-ray pulsars found in a blind search, including 16 radio-quiet and 9 radio-loud pulsars. This results in the direct measurement of the fraction of radio-quiet pulsars ɛRQ = 64 ± 10%, which is in agreement with the existing estimates from the population modeling in the outer magnetosphere model. The Polar cap models are disfavored due to a lower expected fraction and the prediction of age dependence. The age, gamma-ray energy flux, spin-down luminosity and sky location distributions of the radio-loud and radio-quiet pulsars from the catalog do not demonstrate any statistically significant difference. The results indicate that the radio-quiet and radio-loud pulsars belong to one and the same population. The catalog shows no evidence for the radio beam evolution.

Effect of geodetic precession on the evolution of pulsar high-energy pulse profiles as derived with the striped-wind model

Geodetic precession has been observed directly in the double-pulsar system PSR J0737-3039. Its rate has even been measured and agrees with predictions of general relativity. Very recently, the double pulsar has been detected in X-rays and gamma-rays. This fuels the hope observing geodetic precession in the high-energy pulse profile of this system. Unfortunately, the geometric configuration of the binary renders any detection of such an effect unlikely. Nevertheless, this precession is probably present in other relativistic binaries or double neutron star systems containing at least one X-ray or gamma-ray pulsar.}{We compute the variation of the high-energy pulse profile expected from this geodetic motion according to the striped-wind model. We compare our results with two-pole caustic and outer gap emission patterns.}{For a sufficient misalignment between the orbital angular momentum and the spin angular momentum, a significant change in the pulse profile as a result of geodetic precession is expected in the X-ray and gamma-ray energy band.}{The essential features of the striped wind are indicated in several plots showing the evolution of the maximum of the pulsed intensity, the separation of both peaks, if present, and the variation in the width of each peak. We highlight the main differences with other competing high-energy models.}{We make some predictions about possible future detection of high-energy emission from double neutron star systems with the highest spin precession rate. Such observations will definitely favour some pulsed high-energy emission scenarios.

Studying the shielding properties of lead glass composites using neutrons and gamma rays

The present work deals with the shielding properties of lead glass composites to find out its integrity for practical shielding applications and radiological safety. Composites of different lead oxide ratios (x=0, 5, 10, 15 and 25wt.%) have been prepared by the Nasser Glass and Crystal Company (Egypt). Attenuation measurements have been carried out using a collimated emitted beam from a fission 252Cf (100μg) neutron source, and the neutron–gamma spectrometer with stilbene scintillator. The pulse shape discriminating (P.S.D.) technique based on the zero cross-over method was used to discriminate between neutron and gamma-ray pulses. Thermal neutron fluxes were measured using the BF3 detector and thermal neutron detection system. The attenuation relations were used to evaluate fast neutron macroscopic effective removal cross-section ΣR-Meas (cm−1), gamma rays total attenuation coefficient μ (cm−1) and thermal neutron macroscopic cross-section ΣMeas (cm−1). Theoretical calculations have been achieved using MCNP5 code to calculate the same two parameters. Also, MERCSF-N program was used to calculate fast neutron macroscopic removal cross-section ΣR-MER (cm−1). Measured and MCNP5 calculated results have been compared and were found to be in reasonable agreement.

Comparative study of neutron and gamma-ray pulse shape discrimination of anthracene, stilbene, and p-terphenyl

Solid state organic scintillators, such as anthracene, stilbene, and p-terphenyl were investigated on their basic scintillation properties and neutron–gamma discrimination capabilities. Scintillation wavelengths under X-ray irradiation of anthracene, stilbene, and p-terphenyl were 445–525, 400–500, and 350–450nm, respectively. Scintillation light yields of anthracene, stilbene, and p-terphenyl under 137Cs gamma-ray irradiation were 20100, 16000, and 19400ph/MeV, respectively. Neutron and gamma-ray events discrimination capabilities were examined and anthracene exhibited the best figure of merit among three organic scintillators.

AGILE Highlights after Six Years in Orbit

AGILE is an ASI space mission in collaboration with INAF, INFN and CIFS, dedicated to the observation of the gamma-ray Universe in the 30 MeV - 50 GeV energy range, with simultaneous X-ray imaging capability in the 18-60 keV band. The AGILE satellite was launched on April 23rd, 2007, and produced several important scientic results, among which the unexpected discovery of strong <br />ares from the Crab Nebula. This discovery won to the AGILE PI and the AGILE Team the Bruno Rossi Prize for 2012 by the High Energy Astrophysics division of the American Astronomical Society. Thanks to its sky monitoring capability and fast ground segment alert system, AGILE detected many Galactic and extragalactic sources: among other results AGILE discovered gamma-ray emission from the microquasar Cygnus X-3, detected many bright blazars, discovered several new gamma-ray pulsars, and discovered emission up to 100 MeV from Terrestrial Gamma-Ray Flashes. We present an overview of the main AGILE Data Center activities and the AGILE scientic highlights after 6 years of operations.

Deep searches for decametre-wavelength pulsed emission from radio-quiet gamma-ray pulsars

We report the results of (a) extensive follow-up observations of the gamma-ray pulsar J1732-3131 that has been recently detected at decameter wavelengths, and (b) deep searches for counterparts of 9 other radio-quiet gamma-ray pulsars at 34 MHz, using the Gauribidanur radio telescope. No periodic signal from J1732-3131 could be detected above a detection threshold of $8\sigma$, even with an effective integration time of more than 40 hours. However, the average profile obtained by combining data from several epochs, at a dispersion measure of 15.44 pc/cc, is found to be consistent with that from the earlier detection of this pulsar at a confidence level of 99.2 %. We present this consistency between the two profiles as an evidence that J1732-3131 is a faint radio pulsar with an average flux density of 200--400 mJy at 34 MHz. Detection sensitivity of our deep searches, despite the extremely bright sky background at such low frequencies, is generally comparable to that of higher frequency searches for these pulsars, when scaled using reasonable assumptions about the underlying pulsar spectrum. We provide details of our deep searches, and put stringent upper limits on the decameter wavelength flux densities of several radio-quiet gamma-ray pulsars.

ELECTRODYNAMICS OF AXISYMMETRIC PULSAR MAGNETOSPHERE WITH ELECTRON-POSITRON DISCHARGE: A NUMERICAL EXPERIMENT

We present the first self-consistent global simulations of pulsar magnetospheres with operating $e^\pm$ discharge. We focus on the simple configuration of an aligned or anti-aligned rotator. The star is spun up from zero (vacuum) state to a high angular velocity, and we follow the coupled evolution of its external electromagnetic field and plasma particles using the "particle-in-cell" method. A plasma magnetosphere begins to form through the extraction of particles from the star; these particles are accelerated by the rotation-induced electric field, producing curvature radiation and igniting $e^\pm$ discharge. We follow the system evolution for several revolution periods, longer than required to reach a quasi-steady state. Our numerical experiment puts to test previous ideas for the plasma flow and gaps in the pulsar magnetosphere. We first consider rotators capable of producing pairs out to the light cylinder through photon-photon collisions. We find that their magnetospheres are similar to the previously obtained force-free solutions with a current sheet and the Y-point near the light cylinder. The magnetosphere continually ejects $e^\pm$ pairs and ions. Pair creation is sustained by a strong electric field along the current sheet. We observe powerful curvature and synchrotron emission from the current sheet, consistent with Fermi observations of gamma-ray pulsars. We then study pulsars that can only create pairs in the strong-field region near the neutron star, well inside the light cylinder. We find that both aligned and anti-aligned rotators relax to the "dead" state with suppressed pair creation and electric currents, regardless of the discharge voltage.

OPTIMIZED BLIND GAMMA-RAY PULSAR SEARCHES AT FIXED COMPUTING BUDGET

The sensitivity of blind gamma-ray pulsar searches in multiple years worth of photon data, as from the Fermi LAT, is primarily limited by the finite computational resources available. Addressing this "needle in a haystack" problem, we here present methods for optimizing blind searches to achieve the highest sensitivity at fixed computing cost. For both coherent and semicoherent methods, we consider their statistical properties and study their search sensitivity under computational constraints. The results validate a multistage strategy, where the first stage scans the entire parameter space using an efficient semicoherent method and promising candidates are then refined through a fully coherent analysis. We also find that for the first stage of a blind search incoherent harmonic summing of powers is not worthwhile at fixed computing cost for typical gamma-ray pulsars. Further enhancing sensitivity, we present efficiency-improved interpolation techniques for the semicoherent search stage. Via realistic simulations we demonstrate that overall these optimizations can significantly lower the minimum detectable pulsed fraction by almost 50% at the same computational expense.

DISCOVERY OF AN ULTRACOMPACT GAMMA-RAY MILLISECOND PULSAR BINARY CANDIDATE

We report multi-wavelength observations of the unidentified Fermi object 2FGL J1653.6-0159. With the help of high-resolution X-ray observations, we have identified an X-ray and optical counterpart to 2FGL J1653.6-0159. The source exhibits a periodic modulation of 75 minutes in the optical and possibly also in the X-ray. We suggest that 2FGL J1653.6-0159 is a compact binary system with an orbital period of 75 minutes. Combining the gamma-ray and X-ray properties, 2FGL J1653.6-0159 is potentially a black-widow-/redback-type gamma-ray millisecond pulsar (MSP). The optical and X-ray light curve profiles show that the companion is mildly heated by the high-energy emission and that the X-rays are from intrabinary shock. Although no radio pulsation has yet been detected, we estimated that the spin period of the MSP is ∼ 2 ms based on a theoretical model. If pulsation can be confirmed in the future, 2FGL J1653.6-0159 will become the first ultracompact rotation-powered MSP.

Six faint gamma-ray pulsars seen with theFermiLarge Area Telescope

Context: GeV gamma-ray pulsations from over 140 pulsars have been characterized using the Fermi Large Area Telescope, enabling improved understanding of the emission regions within the neutron star magnetospheres, and the contributions of pulsars to high energy electrons and diffuse gamma rays in the Milky Way. The first gamma-ray pulsars to be detected were the most intense and/or those with narrow pulses. Aims: As the Fermi mission progresses, progressively fainter objects can be studied. In addition to more distant pulsars (thus probing a larger volume of the Galaxy), or ones in high background regions (thus improving the sampling uniformity across the Galactic plane), we detect pulsars with broader pulses or lower luminosity. Adding pulsars to our catalog with inclination angles that are rare in the observed sample, and/or with lower spindown power, will reduce the bias in the currently known gamma-ray pulsar population. Methods: We use rotation ephemerides derived from radio observations to phase-fold gamma rays recorded by the Fermi Large Area Telescope, to then determine the pulse profile properties. Spectral analysis provides the luminosities and, when the signal-to-noise ratio allows, the cutoff energies. We constrain the pulsar distances by different means in order to minimize the luminosity uncertainties. Results: We present six new gamma-ray pulsars with an eclectic mix of properties. Three are young, and three are recycled. They include the farthest, the lowest power, two of the highest duty-cycle pulsars seen, and only the fourth young gamma-ray pulsar with a radio interpulse. We discuss the biases existing in the current gamma-ray pulsar catalog, and steps to be taken to mitigate the bias.

Emission mechanism of GeV-quiet soft gamma-ray pulsars: a case for peculiar geometry?

There is a growing new class of young spin-down powered pulsars called GeV-quiet soft gamma-ray pulsar; (1) spectral turnover appears around~10MeV, (2) the X-ray spectra of below 20 keV can be described by power law with photon index around 1.2 and (3) the light curve in X-ray/soft gamma-ray bands shows single broad pulse. Their emission properties are distinct from the normal gamma-ray pulsars, for which the spectral peak in $\nu F_{\nu}$ appears in GeV energy bands and the X-ray/gamma-ray light curves show sharp and double (or more) peaks. In this paper, we discuss that X-ray/soft gamma-ray emissions of the GeV-quiet soft gamma-ray pulsars are caused bythe synchrotron radiation of the electron/positron pairs, which are created by the magnetic pair-creation process near the stellar surface. In our model, the viewing geometry is crucial factor to discriminate between the normal gamma-ray pulsars and soft gamma-ray pulsars. Our model suggests that the difference between the magnetic inclination angle ($\alpha$) and the Earth viewing angle ($\beta$) of the soft gamma-ray pulsars is small, so that the synchrotron emissions from the high magnetic field region around the polar cap region dominates in the observed emissions. Furthermore, the inclination angle of the soft gamma-ray pulsar is relatively small, $\alpha\leq 30$~degree, and our line of sight is out of the gamma-ray beam emitted via the curvature radiation process in the outer gap. We also analysis the six year $Fermi$ data for four soft gamma-ray pulsars to determine the upper limit of the GeV flux.

DISCOVERY OF X-RAY PULSATIONS FROM THE NEXT GEMINGA, PSR J1836+5925

We report the X-ray pulsation of ~173.3 ms for the "next Geminga", PSR J1836+5925, with recent XMM-Newton investigations. The X-ray periodicity is consistent wtih the gamma-ray ephemeris at the same epoch. The X-ray folded light curve has a sinusoidal structure which is different from the double-peaked gamma-ray pulse profile. We have also analysed the X-ray phase-averaged spectra which shows the X-ray emission from PSR J1836+5925 is thermal dominant. This suggests the X-ray pulsation mainly originates from the modulated hot spot on the stellar surface.

Nonlinear vacuum electrodynamics birefringence effect in a pulsar’s strong magnetic field

The motion of x-ray and gamma-ray pulses in the magnetic field of the pulsar, taking into account the nonlinearity of electrodynamics in a vacuum, is calculated. It is shown that in these conditions, each pulse propagates in the form of two orthogonally polarized normal waves with different speeds. The propagation time from the common source in the pulsars' magnetosphere to a remote observer is calculated for each mode. An experiment to measure the delay time of one pulse with respect to the other is proposed.

MAGNETIC PAIR CREATION TRANSPARENCY IN GAMMA-RAY PULSARS

Magnetic pair creation $\gamma \to e^+e^-$ has been at the core of radio pulsar paradigms and central to polar cap models of gamma-ray pulsars for over three decades. The Fermi gamma-ray pulsar population now exceeds 140 sources and has defined an important part of Fermi's science legacy. Among the population characteristics well established is the common occurrence of exponential turnovers in their spectra in the 1--10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres. By demanding insignificant photon attenuation precipitated by such single-photon pair creation, the energies of these turnovers for Fermi pulsars can be used to compute lower bounds for the typical altitude of GeV band emission. This paper explores such pair transparency constraints below the turnover energy, and updates earlier altitude bound determinations of that have been deployed in various Fermi pulsar papers. For low altitude emission locales, general relativistic influences are found to be important. Rotational aberration influences are also explored, and are found to be small at low altitudes, except near the magnetic pole. The altitude bounds are typically in the range of 2-7 stellar radii for the young Fermi pulsar population, and provide key information on the emission altitude in radio quiet pulsars that do not possess double-peaked pulse profiles. The bound for the Crab pulsar is at a much higher altitude, with the putative detection by MAGIC out to 350-400 GeV implying a lower bound of 310km to the emission region, i.e., approximately 20% of the light cylinder radius. These results are also extended to the super-critical field domain, where it is found that emission in magnetars originating below around 10 stellar radii will not appear in the Fermi-LAT band.

Testing pulse density distribution for terrestrial gamma ray flashes

AbstractMaximum likelihood fits for the time profiles of 51 terrestrial gamma ray flashes (from Compton Gamma Ray Observatory/Burst and Transient Source Experiment and Fermi Gamma‐Ray Space Telescope/Gamma‐Ray Burst Monitor) were calculated for five proposed probability densities. A lognormal distribution, which had been used by other researchers, was compared with piecewise Gaussian, piecewise exponential, inverse Gaussian, and Ornstein‐Uhlenbeck probability density functions. The piecewise Gaussian and piecewise exponential distributions are justified physically through assuming exponential growth and decay of the electron avalanches which result in the gamma ray bursts and are therefore highly relevant in this context. However, identifying the electron avalanche phenomenon as a form of stochastic exit time process, the inverse Gaussian and Ornstein‐Uhlenbeck are reasonable alternatives. Results of the maximum likelihood calculations indicate that the five probability densities fit the gamma ray pulse data equally well. By this comparison, our aim is to indicate to terrestrial gamma ray flash researchers these other at least equally valid distribution functions, which may give insights into the physical processes that the electrons (and the positrons) undergo in the gamma ray flashes.

Pulse properties of terrestrial gamma‐ray flashes detected by the Fermi Gamma‐Ray Burst Monitor

AbstractThe Gamma‐ray Burst Monitor (GBM) on board the Fermi Gamma‐ray Space Telescope has triggered on over 300 terrestrial gamma‐ray flashes (TGFs) since its launch in June 2008. With 14 detectors, GBM collects on average ∼100 counts per triggered TGF, enabling unprecedented studies of the time profiles of TGFs. Here we present the first rigorous analysis of the temporal properties of a large sample of TGFs (278), including the distributions of the rise and fall times of the individual pulses and their durations. A variety of time profiles are observed with 19% of TGFs having multiple pulses separated in time and 31 clear cases of partially overlapping pulses. The effect of instrumental dead time and pulse pileup on the temporal properties are also presented. As the observed gamma ray pulse structure is representative of the electron flux at the source, TGF pulse parameters are critical to distinguish between relativistic feedback discharge and lightning leader models. We show that at least 67% of TGFs at satellite altitudes are significantly asymmetric. For the asymmetric pulses, the rise times are almost always shorter than the fall times. Those which are not are consistent with statistical fluctuations. The median rise time for asymmetric pulses is ∼3 times shorter than for symmetric pulses while their fall times are comparable. The asymmetric shapes observed are consistent with the relativistic feedback discharge model when Compton scattering of photons between the source and Fermi is included, and instrumental effects are taken into account.

CONSTRAINTS ON THE EMISSION GEOMETRIES AND SPIN EVOLUTION OF GAMMA-RAY MILLISECOND PULSARS

Millisecond pulsars (MSPs) are a growing class of gamma-ray emitters. Pulsed gamma-ray signals have been detected from more than 40 MSPs with the Fermi Large Area Telescope (LAT). The wider radio beams and more compact magnetospheres of MSPs enable studies of emission geometries over a broader range of phase space than non-recycled radio-loud gamma-ray pulsars. We have modeled the gamma-ray light curves of 40 LAT-detected MSPs using geometric emission models assuming a vacuum retarded-dipole magnetic field. We modeled the radio profiles using a single-altitude hollow-cone beam, with a core component when indicated by polarimetry; however, for MSPs with gamma-ray and radio light curve peaks occurring at nearly the same rotational phase we assume that the radio emission is co-located with the gamma rays and caustic in nature. The best-fit parameters and confidence intervals are determined using a maximum likelihood technique. We divide the light curves into three model classes, with gamma-ray peaks trailing (Class I), aligned (Class II) or leading (Class III) the radio peaks. Outer gap and slot gap (two-pole caustic) models best fit roughly equal numbers of Class I and II, while Class III are exclusively fit with pair-starved polar cap models. Distinguishing between the model classes based on typical derived parameters is difficult. We explore the evolution of magnetic inclination angle with period and spin-down power, finding possible correlations. While the presence of significant off-peak emission can often be used as a discriminator between outer gap and slot gap models, a hybrid model may be needed.

NuSTAR DISCOVERY OF A YOUNG, ENERGETIC PULSAR ASSOCIATED WITH THE LUMINOUS GAMMA-RAY SOURCE HESS J1640–465

We report the discovery of a 206 ms pulsar associated with the TeV gamma-ray source HESS J1640-465 using the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory. PSR J1640-4631 lies within the shell-type supernova remnant (SNR) G338.3-0.0, and coincides with an X-ray point source and putative pulsar wind nebula (PWN) previously identified in XMM-Newton and Chandra images. It is spinning down rapidly with period derivative Pdot = 9.758(44)E-13, yielding a spin-down luminosity Edot = 4.4E36 erg s-1, characteristic age = P/2Pdot = 3350 yr, and surface dipole magnetic field strength Bs = 1.4E13 G. For the measured distance of 12 kpc to G338.3-0.0, the 0.2 - 10 TeV luminosity of HESS J1640-465 is 6% of the pulsar's present Edot. The Fermi source 1FHL J1640.5-4634 is marginally coincident with PSR J1640-4631, but we find no gamma-ray pulsations in a search using 5 years of Fermi Large Area Telescope (LAT) data. The pulsar energetics support an evolutionary PWN model for the broad-band spectrum of HESS J1640-465, provided that the pulsar's braking index is approximately 2, and that its initial spin period was Po ~ 15 ms.

HIGH-ENERGY EMISSION OF THE FIRST MILLISECOND PULSAR

We report on X-ray and gamma-ray observations of the millisecond pulsar B1937+21 taken with the Chandra X-ray Observatory, XMM-Newton, and the Fermi Large Area Telescope. The pulsar X-ray emission shows a purely non-thermal spectrum with a hard photon index of 0.9+/-0.1, and is nearly 100% pulsed. We found no evidence of varying pulse profile with energy as previously claimed. We also analyzed 5.5 yr of Fermi survey data and obtained much improved constraints on the pulsar's timing and spectral properties in gamma-rays. The pulsed spectrum is adequately fitted by a simple power-law with a photon index of 2.38+/-0.07. Both the gamma-ray and X-ray pulse profiles show similar two-peak structure and generally align with the radio peaks. We found that the aligned profiles and the hard spectrum in X-rays seem to be common properties among millisecond pulsars with high magnetic fields at the light cylinder. We discuss a possible physical scenario that could give rise to these features.