The efficiency of gamma-ray emission from pulsars

We present a modified scenario of gamma-ray emission from pulsars within the framework of polar cap models. Our model incorporates the possible acceleration of electron–positron pairs created in magnetospheres, and their subsequent contribution to the gamma-ray luminosity L _γ. It also reproduces the empirical trend in L _γ for seven pulsars detected with Compton Gamma-Ray Observatory ( CGRO ) experiments. At the same time it avoids basic difficulties faced by theoretical models when confronted with observational constraints.   We show that the classical and millisecond pulsars form two distinct branches in the L _γ— L _sd diagram (where L _sd is the spin-down luminosity). In particular, we explain why the millisecond pulsar J0437−4715 has not been detected with any of the CGRO instruments despite its very high position in the ranking list of spin-down fluxes (i.e. L _sd/ D ², where D is a distance). The gamma-ray luminosity predicted for this particular object is about one order of magnitude below the upper limit set by EGRET.     

Contribution to diffuse gamma-rays in the Galactic Centre region from unresolved millisecond pulsars

Diffuse gamma-rays in the Galactic Centre region have been studied. We propose that there exists a population of millisecond pulsars in the Galactic Centre, which emit GeV gamma-rays through synchrotron-curvature radiation as predicted by outer gap models. These GeV gamma-rays from unresolved millisecond pulsars probably contribute to the diffuse gamma-ray spectrum detected by EGRET which displays a break at a few GeV. We have used a Monte Carlo method to obtain simulated samples of millisecond pulsars in the Galactic Centre region covered by EGRET  (∼ 15)  according to the different period and magnetic field distributions from observed millisecond pulsars in the Galactic field and globular clusters, and superposed their synchrotron-curvature spectra to derive the total GeV flux. Our simulated results suggest that there probably exist about 6000 unresolved millisecond pulsars in the region of angular resolution of EGRET, the emissions of which could contribute significantly to the observed diffuse gamma-rays in the Galactic Centre.     

Radiation mechanisms and geometry of Cygnus X-1 in the soft state

We present X-ray/ γ -ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA , RXTE and CGRO /OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f ≳0.5 of total available power is dissipated in the corona.
We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA RXTE data yield the most probable black hole mass of M _X≈10 M_⊙ and an accretion rate,     , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.
The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT _e∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲ℓ_h≲7, and a presence of a significant population of electron–positron pairs is ruled out.
We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.     

The micro-glitch in PSR B1821−24: a case for a strange pulsar?

The single glitch observed in PSR B1821−24, a millisecond pulsar in M28, is unusual on two counts. First, the magnitude of this glitch is at least an order of magnitude smaller  (Δν/ν∼ 10⁻¹¹)  than the smallest glitch observed to date. Secondly, all other glitching pulsars have strong magnetic fields with   B ≳ 10¹¹ G  and are young, whereas PSR B1821−24 is an old recycled pulsar with a field strength of  2.25 × 10⁹ G  . We have earlier suggested that some of the recycled pulsars could actually be strange quark stars. In this work, we argue that the crustal properties of such a strange pulsar are just right to give rise to a glitch of this magnitude, explaining the scarcity of larger glitches in millisecond pulsars.     

BeppoSAX observations of 1-Jy BL Lacertae objects – I

We present new BeppoSAX observations of seven BL Lacertae objects selected from the 1-Jy sample plus one additional source. The collected data cover the energy range     (observer's frame), reaching ∼50 keV for one source (BL Lac). All sources characterized by a peak in their multifrequency spectra at infrared/optical energies (i.e., of the low-energy peaked BL Lac type, LBL) display a relatively flat     X-ray spectrum, which we interpret as inverse Compton emission. Four objects (two-thirds of the LBLs) show some evidence for a low-energy steepening, which is probably due to the synchrotron tail merging into the inverse Compton component around ∼     . If this were generally the case with LBLs, it would explain why the     ROSAT spectra of our sources are systematically steeper than the BeppoSAX ones     . The broad-band spectral energy distributions fully confirm this picture, and a synchrotron inverse Compton model allows us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. Combining our results with those obtained by BeppoSAX on BL Lacs covering a wide range of synchrotron peak frequency, ν _peak, we confirm and clarify the dependence of the X-ray spectral index on ν _peak originally found in ROSAT data.     

The contribution of pulsars to diffuse gamma-rays in the Large Magellanic Cloud

We study the contribution of young pulsars, with characteristic ages of less than 10⁶ yr, to the diffuse γ-ray emission from the Large Magellanic Cloud (LMC). Based on the outer gap model for γ-ray emission proposed by Zhang & Cheng and pulsar properties in the LMC given by Hartmann, Brown & Schnepf, we simulate the properties of the young pulsars in the LMC. We show that γ-rays produced by the pulsars in the LMC may make an important contribution to the diffuse γ-rays in the LMC, especially in the high-energy range. We calculate the γ-ray energy spectrum of the pulsars in the LMC and show that the γ-ray component contributed by the pulsars to the diffuse γ-rays in the high-energy range (above ∼1 GeV) becomes dominant. We expect that none of the young pulsars should be detectable as an individual point source of γ-ray emission by EGRET. We also expect that pulsar contribution above ∼1 GeV in the SMC is very important.     

The Parkes Southern Pulsar Survey — II. Final results and population analysis

A survey of the entire southern sky for millisecond and low-luminosity pulsars using the ATNF Parkes radio telescope has now been completed. The survey detected 298 pulsars, of which 101 were previously unknown. The new pulsars include 17 millisecond pulsars. This is the largest sample of both normal and millisecond pulsars detected in any survey. Combining our sample with other recent surveys in the Northern Hemisphere, we present a statistical study of the populations of both normal and millisecond pulsars. We find that the improved statistics allow us to estimate the number and birth-rate of both types of pulsar down to a 400-MHz luminosity limit of 1 mJy kpc². The local surface densities of potentially observable normal pulsars and millisecond pulsars are both about 30 kpc⁻², corresponding to ∼ 30000 potentially observable pulsars of each type in the Galaxy. Once beaming effects are taken into consideration we estimate that the active population of normal pulsars is ∼ 160000. Although there is evidence for flattening of the luminosity function of normal pulsars, this is not evident for millisecond pulsars which probably have a substantial population with luminosities below 1 mJy kpc². After correcting for beaming effects, we estimate that a normal pulsar is born with a luminosity greater than 1 mJy kpc² between once every 60 and 330 yr in the Galaxy. The birth-rate of millisecond pulsars is at least 3 × 10⁻⁶ yr⁻¹ above the same luminosity limit. Modelling the observed transverse speeds of millisecond pulsars using a dynamical simulation, we find their mean birth velocity to be 130 ± 30 km s⁻¹, significantly lower than that of the normal pulsars.     

High-energy afterglow emission from giant flares of soft gamma-ray repeaters: the case of the 2004 December 27 event from SGR 1806–20

We discuss the high-energy afterglow emission (including high-energy photons, neutrinos and cosmic rays) following the 2004 December 27 giant flare from the soft gamma-ray repeater (SGR) 1806−20. If the initial outflow is relativistic with a bulk Lorentz factor  Γ₀∼  tens, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the initial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentially detectable by the Gamma-Ray Large Area Telescope ( GLAST ) if a similar giant flare occurs in the GLAST era. With the available 10-MeV data, we constrain that  Γ₀ < 50  if the electron distribution is a single power law. For a broken power-law distribution of electrons, a higher Γ₀ is allowed. At energies higher than 1 GeV, the flux is lower because of a high-energy cut-off of the synchrotron emission component. The synchrotron self-Compton emission component and the inverse Compton scattering component off the photons in the giant flare oscillation tail are also considered, but they are found not significant given a moderate Γ₀ (e.g. ≤ 10). The forward shock also accelerates cosmic rays to the maximum energy 10¹⁷ eV, and generates neutrinos with a typical energy 10¹⁴ eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.     

The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars

We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory . Furthermore, the contribution to the Galactic diffuse γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ∼5 to ∼10 per cent to the measured Galactic diffuse γ-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ∼50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the γ-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (∼60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray–matter interaction only.     

Spectral lags caused by the curvature effect of fireballs

Recently, Shen et al. have studied the contributions of the curvature effect of fireballs to the spectral lag and have shown that the observed lags can be accounted for by the effect. Here, we check their results by performing a more precise calculation with both formulae presented by Shen et al. and Qin et al. Several other aspects which were not considered by Shen et al. are investigated. We find that in the case of ultrarelativistic motions, both formulae are identical as long as the whole fireball surface is concerned. In our analysis, the previous conclusion that the detected spectral lags can be accounted for by the curvature effect is confirmed, while the conclusion that the lag has no dependence on the radius of fireballs is not true. We find that introducing extreme physical parameters is not the only outlet to explain these observed large lags. Even for the larger lags (∼5 s), a wider local pulse  (Δ t _θ,FWHM= 10⁷ s)  can account for it. Some conclusions not presented in Shen et al. or those modified in our analysis are listed below: (i)  lag ∝Γ^−ε  with  ε > 2  ; (ii) lag is proportional to the local pulse width and the full width at half-maximum of the observed light curves; (iii) a large lag requires a large α₀ and a small β₀ as well as a large   E _0,p  ; (iv) when the rest-frame spectrum varies with time, the lag would become larger; (v) lag decreases with the increase of   R_c   ; (vi) lag ∝ E within the certain energy range for a given Lorentz factor; (vii) lag is proportional to the opening angle of uniform jets when  θ_j < 0.6Γ⁻¹  .     

Hot accretion discs with thermal Comptonization and advection in luminous black hole sources

We solve for the structure of a hot accretion disc with unsaturated thermal Comptonization of soft photons and with advection, generalizing the classical model of Shapiro et al. The upper limit on the accretion rate due to advection constrains the luminosity to ≲ 0.15 y^3/5 α^7/5 of the Eddington limit, where y and α are the Compton and viscosity parameters, respectively. The characteristic electron temperature and Thomson optical depth of the inner flow at accretion rates within an order of magnitude of that upper limit are ∼ 10⁹ K and ∼ 1, respectively. The resulting spectra are then in close agreement with the X-ray and soft γ-ray spectra from black hole binaries in the hard state and Seyferts. At low accretion rates, bremsstrahlung becomes the dominant radiative process.     

Prompt optical emission and synchrotron self-absorption constraints on emission site of GRBs

We constrain the distance of the gamma-ray burst (GRB) prompt emission site from the explosion centre R , by determining the location of the electron's self-absorption frequency in the GRB prompt optical-to-X/γ-ray spectral energy distribution, assuming that the optical and the γ-ray emissions are among the same synchrotron radiation continuum of a group of hot electrons. All possible spectral regimes are considered in our analysis. The method has only two assumed parameters, namely the bulk Lorentz factor of the emitting source Γ and the magnetic field strength B in the emission region (with a weak dependence). We identify a small sample of four bursts that satisfy the following three criteria: (1) they all have simultaneous optical and γ-ray detections in multiple observational time intervals, (2) they all show temporal correlations between the optical and γ-ray light curves and (3) the optical emission is consistent with belonging to the same spectral component as the γ-ray emission. For all the time intervals of these four bursts, it is inferred that   R ≥ 10¹⁴  (Γ/300)^3/4 ( B /10⁵ G)^1/4  cm. For a small fraction of the sample, the constraint can be pinned down to   R ≈ 10¹⁴–10¹⁵ cm  for  Γ∼ 300  . For a second sample of bursts with prompt optical non-detections, only upper limits on R can be obtained. We find no inconsistency between the R -constraints for this non-detection sample and those for the detection sample.     

Inverse Compton e± pair cascade model for the γ-ray production in massive binary LSI +61° 303

We apply an inverse Compton   e ^±  pair cascade model for γ-ray production in the massive binary system LSI +61° 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. γ-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. γ-ray spectra expected in such a model have different shape than those ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anticorrelation between γ-ray fluxes in the GeV (1–10 GeV) and TeV (>200 GeV) energy ranges with the peak of the TeV emission at the phase ∼0.5 (the peak half-width ranges between the phases ∼0.4–0.9, for the inclination of the binary system equal to 60°, and ∼0.4–0.1 for 30°). The fine features of TeV γ-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothesis of acceleration of electrons already in the inner part of the microquasar jets.     

The cross-correlation between galaxies and groups: probing the galaxy distribution in and around dark matter haloes

Some massive binaries should contain energetic pulsars which inject relativistic leptons from their inner magnetospheres and/or pulsar wind regions. If the binary system is compact enough, then these leptons can initiate inverse Compton (IC) e^± pair cascades in the anisotropic radiation field of a massive star. γ-rays can be produced in the IC cascade during its development in a pulsar wind region and above a shock in a massive star wind region where the propagation of leptons is determined by the structure of a magnetic field around the massive star. For a binary system with specific parameters, we calculate phase-dependent spectra and fluxes of γ-rays escaping as a function of the inclination angle of the system and for different assumptions on injection conditions of the primary leptons (their initial spectra and location of the shock inside the binary). We conclude that the features of γ-ray emission from such massive binaries containing energetic pulsars should allow us to obtain important information on the acceleration of particles by the pulsars, and on interactions of a compact object with the massive star wind. Predicted γ-ray light curves and spectra in the GeV and TeV energy ranges from such binary systems within our Galaxy and Magellanic Clouds should be observed by future AGILE and GLAST satellites and low-threshold Cherenkov telescopes, such as MAGIC, HESS, VERITAS or CANGAROO III.     

TeV gamma-rays from accreting magnetars in massive binaries

This paper focuses on neutron stars (NS) of the magnetar type inside massive binary systems. We determine the conditions under which the matter from the stellar wind can penetrate the inner magnetosphere of the magnetar. At a certain distance from the NS surface, the magnetic pressure can balance the gravitational pressure of the accreting matter, creating a very turbulent, magnetized transition region. It is suggested that this region provides good conditions for the acceleration of electrons to relativistic energies. These electrons lose energy due to the synchrotron process and inverse Compton (IC) scattering of the radiation from the nearby massive stellar companion, producing high-energy radiation from X-rays up to ∼TeV γ-rays. The primary γ-rays can be further absorbed in the stellar radiation field, developing an IC  e^±  pair cascade. We calculate the synchrotron X-ray emission from primary electrons and secondary  e^±  pairs and the IC γ-ray emission from the cascade process. It is shown that quasi-simultaneous observations of the TeV γ-ray binary system LSI +61 303 in the X-ray and TeV γ-ray energy ranges can be explained with such an accreting magnetar model.     

High-energy emission from millisecond pulsars: polar cap models

The viability of polar cap models for high-energy emission from millisecond pulsars is discussed. It is shown that in millisecond pulsars, polar gap acceleration along the last open field lines is radiation-reaction limited, that is, the maximum energy to which particles can be accelerated is determined by balancing the energy-loss rate (due to curvature radiation) with the gap-acceleration rate. The maximum Lorentz factor is limited by curvature radiation and is not sensitive to the specific acceleration model. However, the distance (from the polar cap) at which the Lorentz factor achieves the limit is model dependent, and can be between one-hundredth (for the vacuum gap) and above one-tenth (for the space-charge limited gap) of a stellar radius distant from the polar cap for a pulsar period P =2 ms and a surface magnetic field B =7.510⁴ T. Because of the radiation reaction constraint and the relatively weak magnetic field, both the expected multiplicity (number of pairs per primary particle) and the Lorentz factor of the outflowing one-dimensional magnetospheric e^± plasma from the polar gap are considerably lower than those for normal pulsars. Assuming space-charge limited flow, the location of the pair production front (PPF) is estimated to occur at about one stellar radius above the polar cap, which is significantly higher than that for normal pulsars. If the observed X-ray emission originates in the region near or above the PPF, the wide hollow-cone can reproduce the observed wide double-peaked feature of the light curves without using the aligned rotator assumption.     

Scattered emission from a relativistic outflow and its application to gamma-ray bursts

We investigate a scenario of photon scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the gamma-ray bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, ∼10⁵²–10⁵⁶ erg. The scattered emission from a faster shell could appear as a late short γ-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.     

Gamma-ray bursts from unstable Poynting-dominated outflows

Poynting-flux driven outflows from magnetized rotators are a plausible explanation for gamma-ray burst engines. We suggest a new possibility for how such outflows might transfer energy into radiating particles. We argue that, in a region near the rotation axis, the Poynting flux drives non-linearly unstable large-amplitude electromagnetic waves (LAEMW) that 'break' at radii     where the MHD approximation becomes inapplicable. In the 'foaming' (relativistically reconnecting) regions formed during the wave breaks, the random electric fields stochastically accelerate particles to ultrarelativistic energies which then radiate in turbulent electromagnetic fields. The typical energy of the emitted photons is a fraction of the fundamental Compton energy     with     plus additional boosting due to the bulk motion of the medium. The emission properties are similar to synchrotron radiation, with a typical cooling time ∼10⁻³ s. During the wave break, the plasma is also bulk accelerated in the outward radial direction and at larger radii can produce afterglows due to interactions with the external medium. The near equipartition fields required by afterglow models may be due to magnetic field regeneration in the outflowing plasma (similar to field generation by LAEMW in laser–plasma interactions) and mixing with the upstream plasma.     

Evidence for energy injection and a fine-tuned central engine at optical wavelengths in GRB 070419A

We present a comprehensive multiwavelength temporal and spectral analysis of the 'fast rise exponential decay' GRB 070419A. The early-time emission in the γ-ray and X-ray bands can be explained by a central engine active for at least 250 s, while at late times the X-ray light curve displays a simple power-law decay. In contrast, the observed behaviour in the optical band is complex (from 10² up to 10⁶ s). We investigate the light-curve behaviour in the context of the standard forward/reverse shock model; associating the peak in the optical light curve at ∼450 s with the fireball deceleration time results in a Lorenz factor  Γ≈ 350  at this time. In contrast, the shallow optical decay between 450 and 1500 s remains problematic, requiring a reverse shock component whose typical frequency is above the optical band at the optical peak time for it to be explained within the standard model. This predicts an increasing flux density for the forward shock component until   t ∼ 4 × 10⁶ s  , inconsistent with the observed decay of the optical emission from   t ∼ 10⁴ s  . A highly magnetized fireball is also ruled out due to unrealistic microphysic parameters and predicted light-curve behaviour that is not observed. We conclude that a long-lived central engine with a finely tuned energy injection rate and a sudden cessation of the injection is required to create the observed light curves, consistent with the same conditions that are invoked to explain the plateau phase of canonical X-ray light curves of γ-ray bursts.     

The brightness temperature problem in extreme intra-day variable quasars: a model for PKS 0405-385

I re-examine the brightness temperature problem in PKS 0405-385, which is an extreme intra-day variable radio quasar with an inferred brightness temperature of  ∼5 × 10¹⁴ K  at 5 GHz, well above the Compton catastrophe limit of  ∼10¹¹ K  that is reached when the synchrotron photon energy density exceeds the energy density of the magnetic field. If one takes into account the uncertainty in the distance to the ionized clouds responsible for interstellar scintillation causing rapid intra-day variability in PKS 0405-385, it is possible that the brightness temperature could be as low as  ∼10¹³ K  at 5 GHz, or even lower. The radio spectrum can be fitted by optically thin emission from mono-energetic electrons, or an electron spectrum with a low-energy cut-off such that the critical frequency of the lowest energy electrons is above the radio frequencies of interest. If one observes optically thin emission along a long narrow emission region, the average energy density in the emission region can be many orders of magnitude lower than calculated from the observed intensity if one assumed a spherical emission region. I discuss the physical conditions in the emission region and find that the Compton catastrophe can then be avoided using a reasonable Doppler factor. I also show that MeV to 100-GeV gamma-ray emission at observable flux levels should be expected from extreme intra-day variable sources such as PKS 0405-385.