Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution

We present a comprehensive study of the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced with a modest dispersion, consistent with observations, to take into account the radio detectability, which is important for blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but here we introduce the luminosity-dependent density evolution (LDDE) model, which is favored on the basis of the evolution of the X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses of the observed redshift and gamma-ray flux distributions of the EGRET blazars. Interestingly, we find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars and that the jet activity is universally correlated with the accretion history of AGNs. We then find that only 25–50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with XLF data. Therefore, unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer (by a factor of more than 3) than a previous estimate.     

Strong correlation between the peak power density in the time domain and the luminosity of long γ-ray bursts

This work presents a possible luminosity estimator for long γ-ray bursts (GRBs) based on their light curves. We use the method of variability analysis in the time domain to calculate the power density spectrum (PDS) for each of the 12 GRBs with known redshifts observed by CGRO/BATSE. The peak of the power density spectrum P is a measure of the intensity of variability of the given light curve and a strong correlation is found between P and the isotropic peak luminosity L of the GRB. It is a successor to the lag-luminosity relation of Norris et al. (2000) and the variability-luminosity relation of Reichart et al. (2001).     

Limits on the TeV flux of diffuse gamma rays as measured with the HEGRA air shower array

Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio I_γ/I_CR of the diffuse photon flux I_γ to the hadronic cosmic ray flux I_CR are determined for the energy region 20–100 TeV. The analysis uses a gamma–hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of I_γ/I_CR (at 54 TeV) <2.0×10⁻³ (at the 90% confidence level) for a sky region near the inner galaxy (20°< galactic longitude <60° and |galactic latitude |<5°). A method which is sensitive to both the isotropic and the non-isotropic component yields global upper limits of I_γ/I_CR (at 31 TeV) <1.2×10⁻² and I_γ/I_CR (at 53 TeV) <1.4×10⁻² (at the 90% confidence level).     

Formation of hard very high energy gamma-ray spectra of blazars due to internal photon–photon absorption

The energy spectra of TeV gamma-rays from blazars, after being corrected for intergalatic absorption in the extragalactic background light (EBL), appear unusually hard, a fact that poses challenges to the conventional models of particle acceleration in TeV blazars and/or to the EBL models. In this paper, we show that the internal absorption of gamma-rays caused by interactions with dense narrow-band radiation fields in the vicinity of compact gamma-ray production regions can lead to the formation of gamma-ray spectra of an almost arbitrary hardness. This allows significant relaxation of the current tight constraints on particle acceleration and radiation models, although at the expense of enhanced requirements to the available non-thermal energy budget. The latter, however, is not a critical issue, as long as it can be largely compensated by the Doppler boosting, assuming large (>10) Doppler factors of the relativistically moving gamma-ray production regions. The suggested scenario of formation of hard gamma-ray spectra predicts detectable synchrotron radiation of secondary electron–positron pairs which might require a revision of the current 'standard paradigm' of spectral energy distributions of gamma-ray blazars. If the primary gamma-rays are of hadronic origin related to pp or   p γ  interactions, the 'internal gamma-ray absorption' model predicts neutrino fluxes close to the detection threshold of the next generation high-energy neutrino detectors.     

General relativistic effects in the γ-ray radiation from pulsars

General relativistic and quantum-mechanical effects in the γ-ray radiation from neutron stars are studied in this paper. For three values of the magnetic dipole moment, we calculated the upper limit of the escaping energy of the γ-photon as a function of the angle between the line of sight and the magnetic axis, with and without these effects. We conclude that for low magnetic moments, the quantum-mechanical effects are unimportant but the general relativistic effects are to reduce significantly the upper limit. For high magnetic moments, the two effects nearly cancel out and the net effect is small. We discuss the implications of these results on the γ-ray spectrum.     

Influence of the pulsar parameters on γ-ray burst afterglows

By applying the fireball model of γ-ray burst with a central pulsar, the radiation fluxes of the afterglows of two γ-ray bursts, GRB970228 and GRB000301c, are calculated. The results of the calculation agree very well with the observations. The differing characters of the light curves with a “break” in the optical waveband R of the afterglows of two bursts are interpreted, in terms of differing pulsar parameter values.     

Measuring ΩM and ΩΛ with long‐duration gamma‐ray bursts

Gamma‐ray bursts (GRBs) are one of the most luminous events in the Universe. In addition, the Universe itself is almost transparent to γ ‐rays, making GRBs detectable up to very high redshifts. As a result, GRBs are very suitable to probe the cosmological parameters. This work shows the potential of long‐duration GRBs for measuring the cosmological parameters Ω_M and Ω_Λ by comparing the observed log N ‐log P distribution with the theoretical one. Provided that the GRBs rate and luminosity function are well determined, the best values and 1σ confidence intervals obtained are Ω_M = 0.22^+0.05_–0.03 and Ω_Λ = 1.06^+0.05_–0.10. Finally, a set of simulations show the ability of the method to measure Ω_M and Ω_Λ (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measuring the proper motion of the pulsar Geminga using γ-ray data

We have measured the right ascension of Geminga at different epochs by comparing the absolute phases between the different γ-ray observations. These values show that the right-ascension component of the proper motion of Geminga is 0.226±0.03 arcsec per year, which is consistent with that of G″, the most probable optical counterpart, thus independently confirming the correspondence between G″ and Geminga.     

A new γ-ray pulsar: PSR 0740-28

We made a period folding analysis on two sets of COS-B observations and obtained a clearly pulsed phase structure in the γ-ray photons from the direction of PSR 0740-28. The phase diagrams from the two different data sets have a similar structure and the period and period rate are close to radio expectations. The probability of the two phase structures arising by chance is less than 2×10⁻⁵. Hence we believe PSR 0740-28 is a new γ-ray pulsar.     

Observational Characteristics of γ-ray Bursts and Their Early X-ray Afterglows

Data obtained by the on-board X-ray telescope of the Swift satellite show that a shallow decay component is present in the light curve of the early X-ray afterglows of some γ-ray bursts (GRBs), but not in others. The physical mechanism of this component is debatable. We have made a comparative study on the observational characteristics of the two kinds of GRBs for a sample of 29 GRBs. Our results demonstrate that the two kinds of GRBs have no significant difference in the burst duration, γ-ray flux, spectral index, hardness ratio and peak energy. However, a significant difference exists in the early X-ray afterglows of the bursts: the bursts with a shallow decay component tend to have a softer and fainter X-ray afterglow than those without a shallow decay component. The efficiency of the γ-ray radiation is also very different for the two kinds of bursts: it is obviously higher for the bursts with a shallow decay component than those without. These results seem to suggest that the progenitors and central engines of the two kinds of GRBs are similar, and that the appearance of the shallow decay component is probably due to the surrounding medium.     

Application of singular value decomposition (SVD) in high energy γ-ray astronomy (E0 0.1 TeV)

Recent advances in TeV γ-ray astronomy have been made using imaging techniques which reject in excess of 99% of the cosmic ray background while simultaneously detecting more than 50% of the genuine γ-ray signal. Here we describe an advanced signal processing technique, Singular Value Decomposition (SVD) and discuss its successful application to the TeV γ-ray database obtained by the Whipple Collaboration     

Internal absorption of gamma-rays in relativistic blobs of active galactic nuclei

We investigate the production of gamma-rays in the inverse Compton (IC) scattering process by leptons accelerated inside relativistic blobs in jets of active galactic nuclei. Leptons are injected homogeneously inside the spherical blob and initiate IC e ^± pair cascade in the synchrotron radiation (produced by the same population of leptons, SSC model), provided that the optical depth for gamma-rays is larger than unity. It is shown that for likely parameters internal absorption of gamma-rays has to be important. We suggest that new type of blazars might be discovered by the future simultaneous X-ray and γ-ray observations, showing peak emissions in the hard X-rays, and in the GeV γ-rays. Moreover, the considered scenario might be also responsible for the orphan X-ray flares recently reported from BL Lac type active galaxies.      

Cascades in the Earth’s magnetosphere initiated by photons with the parameters of the highest energy AGASA events

We investigate the cascading effects of extremely high energy (EHE) photons in the Earth’s magnetosphere assuming that these photons arrive with the parameters of the highest energy AGASA events (energies, arrival directions). For the location of the AGASA Observatory, we determine the directions in the sky from which photons can cascade with a high (low) probability. In the case of the primary photons with the parameters of the events with the energies >10²⁰ eV, we compute the average cascade spectra of secondary photons entering the Earth’s atmosphere, and estimate their fluctuations around these average values by selecting the events with the largest and smallest number of secondary cascade photons. It is shown that most photons with the parameters of the highest energy AGASA events should initiate cascades in the Earth’s magnetosphere with a high probability even though they tend to arrive from directions in the sky for which the perpendicular component of the magnetic field is weaker. On the other hand, if these events are caused by the photons with lower energies, then the fluctuations in their shower development in the magnetosphere and the atmosphere should be higher than in the case of photons with the energies estimated by the AGASA experiment.     

GeV emission from neutron-rich internal shocks of some long γ-ray bursts

In the neutron-rich internal shocks model for γ-ray bursts (GRBs), the Lorentz factors (LFs) of ion shells are variable, and so are the LFs of accompanying neutron shells. For slow neutron shells with a typical LF of approximate tens, the typical β-decay radius is  ∼10¹⁴–10¹⁵ cm  . As GRBs last long enough  [ T ₉₀ > 14(1 + z ) s]  , one earlier but slower ejected neutron shell will be swept successively by later ejected ion shells in the range  ∼10¹³–10¹⁵ cm  , where slow neutrons have decayed significantly. Part of the thermal energy released in the interaction will be given to the electrons. These accelerated electrons will mainly be cooled by the prompt soft γ-rays and give rise to GeV emission. This kind of GeV emission is particularly important for some very long GRBs and is detectable for the upcoming satellite Gamma-Ray Large Area Space Telescope (GLAST).     

Synchrotron emission in the fast cooling regime: which spectra can be explained?

We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks.     

Relationship between pulse width and energy in GRB 060124: from X-ray to γ-ray bands

GRB 060124 is the first event that both prompt and afterglow emission were observed simultaneously by the three Swift instruments. Its main peak also triggered Konus-Wind and HETE-II. Therefore, investigation on both the temporal and spectral properties of the prompt emission can be extended to X-ray bands. We perform a detailed analysis on the two well identified pulses of this burst, and find that the pulses are narrower at higher energies, and both X-rays and γ-rays follow the same w–E relation for an individual pulse. However, there is no a universal power-law index of the w–E relation among pulses. We find also that the rise-to-decay ratio r/d seems not to evolve with E and the r/d values are well consistent with that observed in typical GRBs. The broadband spectral energy distribution also suggests that the X-rays are consistent with the spectral behavior of the γ-rays. These results indicate that the X-ray emission tracks the γ-ray emission and the emissions in the two energy bands are likely to be originated from the same physical mechanism.