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1.
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.  相似文献   

2.
We suggest that an extreme Kerr black hole with a mass ∼106 M⊙, a dimensionless angular momentum     and a marginally stable orbital radius     located in a normal galaxy may produce a γ -ray burst (GRB) by capturing and disrupting a star. During the capture period, a transient accretion disc is formed and a strong transient magnetic field ∼     lasting for     may be produced at the inner boundary of the accretion disc. A large amount of rotational energy of the black hole is extracted and released in an ultrarelativistic jet with a bulk Lorentz factor Γ larger than 103 via the Blandford–Znajek process. The relativistic jet energy can be converted into γ -radiation via an internal shock mechanism. The GRB duration should be the same as the lifetime of the strong transient magnetic field. The maximum number of sub-bursts is estimated to be     because the disc material is likely to break into pieces with a size about the thickness of the disc h at the cusp     The shortest risetime of the burst estimated from this model is ∼     The model GRB density rate is also estimated.  相似文献   

3.
We study the emission from an old supernova remnant (SNR) with an age of around 105 yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 105 yr, proton acceleration is efficient enough to emit TeV γ-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ-rays and X-rays are dominated by hadronic processes,  π0  -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 105 yr, there are few high-energy particles emitting TeV γ-rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1–10 TeV) to X-ray (2–10 keV) energy flux and found that it can be more than  ∼102  . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.  相似文献   

4.
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5.
Recent observations of the environments of gamma-ray bursts (GRBs) favour massive stars as their progenitors, which are likely to be surrounded by gas and dust. The visibility of the optical and UV emission of a GRB is expected to depend on the characteristics of both the dust and the GRB emission itself. A reasonable distribution of surrounding dust is capable of absorbing all the optical and UV emission of the optical flash and afterglow of a GRB, unless the optical flash has a peak isotropic luminosity L peak≳1049 erg s−1 . This means that dark bursts should exist and these bursts will have to be studied at infrared rather than optical wavelengths. In this paper details will be given about the infrared GRB dust emission. The reprocessed dust emission peaks at a rest-frame wavelength of about 8 μm. Forthcoming space telescopes, in particular the IRAC camera on board the Space Infrared Telescope Facility , could detect this emission out to a redshift of about two. However, an accurate position of the GRB afterglow must be provided for this emission to be identified, because the light curve of the reprocessed dust emission does not vary on time-scales less than several years.  相似文献   

6.
Measurement sensitivity in the energetic γ-ray region has improved considerably and is about to increase further in the near future, motivating a detailed calculation of high-energy (HE; ≥100 MeV) and very high-energy (VHE; ≥100 GeV) γ-ray emission from the nearby starburst galaxy NGC 253. Adopting the convection–diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, we determine the steady-state energy distributions of these particles by a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region. Doing so fully specifies the electron spectrum throughout the galactic disc and, with an assumed spatial profile of the magnetic field, the predicted radio emission from the full disc matches well the observed spectrum, confirming the validity of our treatment. The resulting radiative yields of both particles are calculated; the integrated HE and VHE fluxes from the entire disc are predicted to be   f (≥100 MeV) ≃ (1.8+1.5−0.8) × 10−8 cm−2 s−1  and   f (≥100 GeV) ≃ (3.6+3.4−1.7) × 10−12 cm−2 s−1  , with a central magnetic field value   B 0≃ 190 ± 10 μ  G. We discuss the feasibility of measuring emission at these levels with the space-borne Fermi and ground-based Cherenkov telescopes.  相似文献   

7.
We test the gamma-ray burst (GRB) correlation between temporal variability and peak luminosity of the γ-ray profile on a homogeneous sample of 36 Swift /Burst Alert Telescope (BAT) GRBs with firm redshift determination. This is the first time that this correlation can be tested on a homogeneous data sample. The correlation is confirmed, as long as the six GRBs with low luminosity (  <5 × 1050   erg s−1  in the rest-frame 100–1000 keV energy band) are ignored. We confirm that the considerable scatter of the correlation already known is not due to the combination of data from different instruments with different energy bands, but it is intrinsic to the correlation itself. Thanks to the unprecedented sensitivity of Swift /BAT, the variability/peak luminosity correlation is tested on low-luminosity GRBs. Our results show that these GRBs are definite outliers.  相似文献   

8.
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.  相似文献   

9.
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts  (GRBs) 030226 ( z = 1.986), 021004 ( z = 2.335)  and  020124 ( z = 3.198)  . We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12−11Å, and a star formation rate of  10.6 ± 2.0 M⊙ yr−1  . We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all   z ≳ 2  GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the   z = 3  luminosity function (LF) under this assumption results in a characteristic luminosity of   R *= 24.6  and a faint-end slope of  α=−1.55  , consistent with the LF measured for Lyman-break galaxies.  相似文献   

10.
The afterglow of a gamma-ray burst (GRB) is commonly thought to be the result of continuous deceleration of a relativistically expanding fireball in the surrounding medium. Assuming that the expansion of the fireball is adiabatic and that the density of the medium is a power-law function of shock radius, i.e. n ext ∝  R − k , we study the effects of the first-order radiative correction and the non-uniformity of the medium on a GRB afterglow analytically. We first derive a new relation among the observed time, the shock radius and the Lorentz factor of the fireball: t ⊕ =  R /4(4− k ) γ2c, and also derive a new relation among the comoving time, the shock radius and the Lorentz factor of the fireball: t co = 2 R /(5− k ) γc. We next study the evolution of the fireball by using the analytic solution of Blandford &38; McKee. The radiation losses may not significantly influence this evolution. We further derive new scaling laws both between the X-ray flux and observed time and between the optical flux and observed time. We use these scaling laws to discuss the afterglows of GRB 970228 and GRB 970616, and find that if the spectral index of the electron distribution is p  = 2.5, implied from the spectra of GRBs, the X-ray afterglow of GRB 970616 is well fitted by assuming k  = 2.  相似文献   

11.
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12.
We report the serendipitous discovery of a population of low-mass, pre-main-sequence (PMS) stars in the direction of the Wolf–Rayet/O-star binary system γ 2  Vel and the Vela OB2 association. We argue that γ 2  Vel and the low-mass stars are truly associated and approximately coeval, and that both are at distances between 360 and 490 pc, disagreeing at the 2 σ level with the recent Hipparcos parallax of γ 2  Vel, but consistent with older distance estimates. Our results clearly have implications for the physical parameters of the γ 2  Vel system, but also offer an exciting opportunity to investigate the influence of high-mass stars on the mass function and circumstellar disc lifetimes of their lower mass PMS siblings.  相似文献   

13.
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).  相似文献   

14.
We present J , H and K -band spectroscopy of Cygnus A, spanning 1.0–2.4 μm in the rest-frame and hence several rovibrational H2, H recombination and [Fe  ii ] emission lines. The lines are spatially extended by up to 6 kpc from the nucleus, but their distinct kinematics indicate that the three groups (H, H2 and [Fe  ii ]) are not wholly produced in the same gas. The broadest line, [Fe  ii ] λ 1.644, exhibits a non-Gaussian profile with a broad base (FWHM≃1040 km s−1), perhaps because of the interaction with the radio source. Extinctions to the line-emitting regions substantially exceed earlier measurements based on optical H recombination lines.
Hard X-rays from the quasar nucleus are likely to dominate the excitation of the H2 emission. The results of Maloney, Hollenbach & Tielens are thus used to infer the total mass of gas in H2 v=1–0 S(1)-emitting clouds as a function of radius, for gas densities of 103 and 105 cm−3, and stopping column densities N H=1022–1024 cm−2. Assuming azimuthal symmetry, at least 2.3×108 M⊙ of such material is present within 5 kpc of the nucleus, if the line-emitting clouds see an unobscured quasar spectrum. Alternatively, if the bulk of the X-ray absorption to the nucleus inferred by Ueno et al. actually arises in a circumnuclear torus, the implied gas mass rises to ∼1010 M⊙. The latter plausibly accounts for 109 yr of mass deposition from the cluster cooling flow, for which within this radius.  相似文献   

15.
Recent ROSAT measurements show that the X-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields B  ∼ 1015 − 1016 G, which are relevant to models that invoke 'magnetars' to account for soft γ-ray emission from some repeating sources.  相似文献   

16.
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 y3/5 α7/5 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.  相似文献   

17.
The problem of the efficiency of particle acceleration for a paraboloidal poloidal magnetic field is considered within the approach of steady axisymmetric magnetohydrodynamic (MHD) flow. For the large Michel magnetization parameter σ it is possible to linearize the stream equation near the force-free solution and to solve the problem self-consistently as was done by Beskin, Kuznetsova & Rafikov for a monopole magnetic field. It is shown that, on the fast magnetosonic surface (FMS), the particle Lorentz factor γ does not exceed the standard value  σ1/3  . On the other hand, in the supersonic region, the Lorentz factor grows with the distance z from the equatorial plane as  γ≈ ( z / R L)1/2  up to the distance   z ≈σ2 R L  , where   R L= c /ΩF  is the radius of the light cylinder. Thus, the maximal Lorentz factor is  γmax≈σ  , which corresponds to almost the full conversion of the Poynting energy flux into the particle kinetic one.  相似文献   

18.
We study the evolution of supernova remnants in a low-metallicity medium   Z /Z⊙= 10−4 to 10−2  in the early universe, using one-dimensional hydrodynamics with non-equilibrium chemistry. Once a post-shock layer is able to cool radiatively, a dense shell forms behind the shock. If this shell becomes gravitationally unstable and fragments into pieces, next-generation stars are expected to form from these fragments. To explore the possibility of this triggered star formation, we apply a linear perturbation analysis of an expanding shell to our results and constrain the parameter range of ambient density, explosion energy and metallicity where fragmentation of the shell occurs. For the explosion energy of  1051 erg (1052 erg)  , the shell fragmentation occurs for ambient densities higher than  ≳102 cm−3 (10 cm−3  ), respectively. This condition depends little on the metallicity in the ranges we examined. We find that the mode of star formation triggered occurs only in massive  (≳108 M⊙)  haloes.  相似文献   

19.
The gamma-ray burst (GRB) 021211 had a simple light curve, containing only one peak and the expected Poisson fluctuations. Such a burst may be attributed to an external shock, offering the best chance for a unified understanding of the gamma-ray burst and afterglow emissions. We analyse the properties of the prompt (burst) and delayed (afterglow) emissions of GRB 021211 within the fireball model. Consistency between the optical emission during the first 11 min (which, presumably, comes from the reverse shock heating of the ejecta) and the later afterglow emission (arising from the forward shock) requires that, at the onset of deceleration (∼2 s), the energy density in the magnetic field in the ejecta, expressed as a fraction of the equipartition value  (ɛ B )  , is larger than in the forward shock at 11 min by a factor of approximately 103. We find that synchrotron radiation from the forward shock can account for the gamma-ray emission of GRB 021211; to explain the observed GRB peak flux requires that, at 2 s,  ɛ B   in the forward shock is larger by a factor 100 than at 11 min. These results suggest that the magnetic field in the reverse shock and early forward shock is a frozen-in field originating in the explosion and that most of the energy in the explosion was initially stored in the magnetic field. We can rule out the possibility that the ejecta from the burst for GRB 021211 contained more than 10 electron–positron pairs per proton.  相似文献   

20.
Classical novae are important producers of radioactive nuclei, such as 7Be, 13N, 18F, 22Na and 26Al. The disintegration of these nuclei produces positrons (except for 7Be) that through annihilation with electrons produce photons of energies 511 keV and below. Furthermore, 7Be and 22Na decay producing photons with energies of 478 and 1275 keV, respectively, well in the γ-ray domain. Therefore, novae are potential sources of γ-ray emission. We have developed two codes in order to analyse carefully the γ-ray emission of individual classical novae: a hydrodynamical one, which follows both the accretion and the explosion stages, and a Monte Carlo one, able to treat both production and transfer of γ-ray photons. Both codes have been coupled in order to simulate realistic explosions. The properties of γ-ray spectra and γ-ray light curves (for the continuum and for the lines at 511, 478 and 1275 keV) have been analysed, with a special emphasis on the difference between carbon–oxygen and oxygen–neon novae. Predictions of detectability of individual novae by the future SPI spectrometer on board the INTEGRAL satellite are made. Concerning 26Al, its decay produces photons of 1809 keV but this occurs on a time-scale much longer than the typical time interval between nova outbursts in the Galaxy, making it undetectable in individual novae. The accumulated emission of 26Al from many Galactic novae has not been modelled in this paper.  相似文献   

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