The circumstellar environment of Wolf–Rayet stars and gamma-ray burst afterglows

We study the evolution of the circumstellar medium of massive stars. We pay particular attention to Wolf-Rayet stars that are thought to be the progenitors of some long gamma-ray bursts (GRBs). We detail the mass-loss rates we use in our stellar evolution models and how we estimate the stellar wind speeds during different phases. With these details we simulate the interactions between the wind and the interstellar medium to predict the circumstellar environment around the stars at the time of core-collapse. We then investigate how the structure of the environment might affect the GRB afterglow. We find that when the afterglow jet encounters the free-wind/stalled-wind interface, rebrightening occurs and a bump is seen in the afterglow light curve. However, our predicted positions of this interface are too distant from the site of the GRB to reach while the afterglow remains observable. The values of the final wind density,   A _*  , from our stellar models are of the same order (≲1) as some of the values inferred from observed afterglow light curves. We do not reproduce the lowest   A _*  values below 0.5 inferred from afterglow observations. For these cases, we suggest that the progenitors could have been a WO-type Wolf–Rayet (WR) star or a very low-metallicity star. Finally, we turn our attention to the matter of stellar wind material producing absorption lines in the afterglow spectra. We discuss the observational signatures of two WR stellar types, WC and WO, in the afterglow light curve and spectra. We also indicate how it may be possible to constrain the initial mass and metallicity of a GRB progenitor by using the inferred wind density and wind velocity.     

Shallow Decay of X-ray Afterglows in Short GRBs： Energy Injection from a Millisecond Magnetar？

With the successful launch of Swift satellite,more and more data of early X-ray afterglows from short gamma-ray bursts have been collected.Some interesting features such as unusual afterglow light curves and unexpected X-ray flares are revealed.Especially,in some cases,there is a fiat segment in the X-ray afterglow light curve.Here we present a simplified model in which we believe that the flattening part is due to energy injection from the central engine.We assume that this energy injection arises from the magnetic dipole radiation of a millisecond pulsar formed after the merger of two neutron stars.We check this model with the short GRB 060313.Our numerical results suggest that energy injection from a millisecond magnetar could make part of the X-ray afterglow light curve flat.     

Merging strangeon stars

The state of supranuclear matter in compact stars remains puzzling, and it is argued that pulsars could be strangeon stars. What would happen if binary strangeon stars merge? This kind of merger could result in the formation of a hyper-massive strangeon star, accompanied by bursts of gravitational waves and electromagnetic radiation(and even a strangeon kilonova explained in the paper). The tidal polarizability of binary strangeon stars is different from that of binary neutron stars, because a strangeon star is self-bound on the surface by the fundamental strong force while a neutron star by the gravity, and their equations of state are different. Our calculation shows that the tidal polarizability of merging binary strangeon stars is favored by GW170817. Three kinds of kilonovae(i.e., of neutron, quark and strangeon) are discussed, and the light curve of the kilonova AT 2017 gfo following GW170817 could be explained by considering the decaying strangeon nuggets and remnant star spin-down. Additionally,the energy ejected to the fireball around the nascent remnant strangeon star, being manifested as a gamma-ray burst, is calculated. It is found that, after a prompt burst, an X-ray plateau could follow in a timescale of 10~2-10~3 s. Certainly, the results could be tested also by further observational synergies between gravitational wave detectors(e.g., Advanced LIGO) and X-ray telescopes(e.g., the Chinese HXMT satellite and e XTP mission), and especially if the detected gravitational wave form is checked by peculiar equations of state provided by the numerical relativistical simulation.     

Variability in GRB afterglows and GRB 021004

We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 s AB (after the burst) [GCN (2002) 1564]. Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with p=2.2 as the most likely explanation. According to this model our line of sight was towards a ‘cold spot’ that has lead to a relativity low γ-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, ν_c, could provide the best way to distinguish between our different models.     

Effect of rapid evolution of magnetic tilt angle on a newborn magnetar's dipole radiation

We study the electromagnetic radiation from a newborn magnetar whose magnetic tilt angle decreases rapidly. We calculate the evolution of the angular spin frequency, the perpendicular component of the surface magnetic field strength, and the energy loss rate through magnetic dipole radiation. We show that the spin-down of the magnetar experiences two stages characterized by two different timescales. The apparent magnetic field decreases with the decrease of the tilt angle. We further show that the energy loss rate of the magnetar is very different from that in the case of a fixed tilt angle. The evolution of the energy loss rate is consistent with the overall light curves of gamma-ray bursts which show a plateau structure in their afterglow stage. Our model supports the idea that some gamma-ray bursts with a plateau phase in their afterglow stage may originate from newborn millisecond magnetars.     

γ-rays from binary system with energetic pulsar and Be star with aspherical wind: PSR B1259−63/SS2883

At least one massive binary system containing an energetic pulsar, PSR B1259−63/SS2883, has been recently detected in the TeV γ-rays by the HESS telescopes. These γ-rays are likely produced by particles accelerated in the vicinity of the pulsar and/or at the pulsar wind shock, in comptonization of soft radiation from the massive star. However, the process of γ-ray production in such systems can be quite complicated due to the anisotropy of the radiation field, complex structure of the pulsar wind termination shock and possible absorption of produced γ-rays which might initiate leptonic cascades. In this paper, we consider in detail all these effects. We calculate the γ-ray light curves and spectra for different geometries of the binary system PSR B1259−63/SS2883 and compare them with the TeV γ-ray observations. We conclude that the leptonic inverse-Compton model, which takes into account the complex structure of the pulsar wind shock due to the aspherical wind of the massive star, can explain the details of the observed γ-ray light curve.     

Search for constraints on Lorentz violation parameters by studying the polarization of afterglows from high-energy gamma-ray bursts

The birefringence of electromagnetic radiation from gamma-ray burst (GRB) afterglows due to the Lorentz violation (LV) of spacetime has been calculated. The dependence of this effect on the radiation wavelength and redshift z in the ΛCDM model of the Universe has been established. It is shown that polarization tests for GRB afterglows can be invoked to measure the LV parameters. LV causes oscillations in the parameters of the afterglow spectrum that can be in the ultraviolet spectral range and will depend significantly on redshift z. This can facilitate the recording of LV for an electromagnetic field and can lower significantly the energy threshold of its detection.     

Constraints on Extra-Dimensions and Variable Constants from Cosmological Gamma-Ray Bursts

The observation of the time delay between the soft emission and the high-energy radiation from cosmological gamma ray bursts can be used as an important observational test of multi-dimensional physical theories. The main source of the time delay is the variation of the electromagnetic coupling, due to dimensional reduction, which induces an energy dependence of the speed of light. For photons with energies around 1 TeV, the time delay could range from a few seconds in the case of Kaluza–Klein models to a few days for models with large extra-dimensions. Based on these results we suggest that the detection of the 18-GeV photon ∼4500 s after the keV/MeV burst in GRB 940217 provides a strong evidence for the existence of extra-dimensions. The time delay of photons, if observed by the next generation of high energy detectors, like, for example, the SWIFT and GLAST satellite based detectors, or the VERITAS ground-based TeV gamma-ray instrument, could differentiate between the different models with extra-dimensions.     

Orbital Decay of a Binary System Consisting of Spin-Down Neutron Stars

{W}e consider the gravitational radiation from two time variable mass stars, orbiting around each other under the influence of gravity. The total rates of the variation of the energy, angular momentum, semimajor axis, eccentricity and orbital period are obtained. The results could be important for the understanding of general relativistic effects in the case of the variation of the gravitational mass due to spinning down of the compact stars, which sensitively depends on the equations of state. The cases of the binary systems PSR 1913+16 and PSR 1534+12 are analyzed in detail, and, for different equations of state of nuclear matter, the corrections to the orbital decay due to gravitational radiation and to the spinning down of the pulsars are calculated. The results show that a future significant improvement in the observational techniques could lead to the observation of the specific general relativistic effect of mass variation of pulsars due to spinning down, via the study of orbital decay, even in slowly rotating binary systems.     

Turbulence induced additional deceleration in relativistic shock wave propagation: implications for gamma-ray burst

The late afterglow of gamma-ray burst is believed to be due to progressive deceleration of the forward shock wave driven by the gamma-ray burst ejecta propagating in the interstellar medium. We study the dynamic effect of interstellar turbulence on shock wave propagation. It is shown that the shock wave decelerates more quickly than previously assumed without the turbulence. As an observational consequence, an earlier jet break will appear in the light curve of the forward shock wave. The scatter of the jet-corrected energy release for gamma-ray burst, inferred from the jet-break, may be partly due to the physical uncertainties in the turbulence/shock wave interaction. This uncertainties also exist in two shell collisions in the well-known internal shock model proposed for gamma-ray burst prompt emission. The large scatters of known luminosity relations of gamma-ray burst may be intrinsic and thus gamma-ray burst is not a good standard candle. We also discuss the other implications.     

Spectroscopy of the γ-ray burst GRB 021004: a structured jet ploughing through a massive stellar wind

We present spectra of the afterglow of the γ-ray burst GRB 021004 taken with the ISIS spectrograph on the William Herschel Telescope (WHT) and with the Focal Reducer/Low Dispersion Spectrograph 1 (FORS1) on the Very Large Telescope (VLT) at three epochs spanning 0.49–6.62 d after the burst. We observe strong absorption probably coming from the host galaxy, alongside absorption in H  i , Si  iv and C  iv with blueshifts of up to 2900 km s⁻¹ from the explosion centre, which we assume originates close to the progenitor. We find no significant variability of these spectral features. We investigate the origin of the outflowing material and evaluate various possible progenitor models. The most plausible explanation is that these result in the fossil stellar wind of a highly evolved Wolf–Rayet (WR) star. However, ionization from the burst itself prevents the existence of H  i , Si  iv and C  iv close to the afterglow surface where the fast stellar wind should dominate, and large amounts of blueshifted hydrogen are not expected in a WR star wind. We propose that the WR star wind is enriched by a hydrogen-rich companion, and that the GRB has a structured jet geometry in which the γ-rays emerge in a small opening angle within the wider opening angle of the cone of the afterglow. This scenario is able to explain both the spectral-line features and the irregular light curve of this afterglow.     

A quark nova in the wake of a core-collapse supernova:a unifying model for long duration gamma-ray bursts and fast radio bursts

By appealing to a quark nova(QN;the explosive transition of a neutron star to a quark star) in the wake of a core-collapse supernova(CCSN) explosion of a massive star,we develop a unified model for long duration gamma-ray bursts(LGRBs) and fast radio bursts(FRBs).The time delay(years to decades)between the SN and the QN,and the fragmented nature(i.e.,millions of chunks) of the relativistic QN ejecta are key to yielding a robust LGRB engine.In our model,an LGRB light curve exhibits the interaction of the fragmented QN ejecta with turbulent(i.e.,filamentary and magnetically saturated) SN ejecta which is shaped by its interaction with an underlying pulsar wind nebula(PWN).The afterglow is due to the interaction of the QN chunks,exiting the SN ejecta,with the surrounding medium.Our model can fit BAT/XRT prompt and afterglow light curves simultaneously with their spectra,thus yielding the observed properties of LGRBs(e.g.,the Band function and the X-ray flares).We find that the peak luminositypeak photon energy relationship(i.e.,the Yonetoku law),and the isotropic energy-peak photon energy relationship(i.e.,the Amati law) are not fundamental but phenomenological.FRB-like emission in our model results from coherent synchrotron emission(CSE) when the QN chunks interact with non-turbulent weakly magnetized PWN-SN ejecta,where conditions are prone to the Weibel instability.Magnetic field amplification induced by the Weibel instability in the shocked chunk frame sets the bunching length for electrons and pairs to radiate coherently.The resulting emission frequency,luminosity and duration in our model are consistent with FRB data.We find a natural unification of high-energy burst phenomena from FRBs(i.e.,those connected to CCSNe) to LGRBs including X-ray flashes(XRFs) and X-ray rich GRBs(XRR-GRBs) as well as superluminous SNe(SLSNe).We find a possible connection between ultra-high energy cosmic rays and FRBs and propose that a QN following a binary neutron star merger can yield a short duration GRB(SGRB) with fits to BAT/XRT light curves.     

Colliding Winds in Binaries: Observations

Colliding winds in binaries are discussed mainly from an observational point of view. Collisions are especially energetic in the case of hot, luminous stars, which drive strong, fast winds. Emphasis is therefore devoted to binaries containing Wolf-Rayet stars. The subject is divided up into (1) continuum radiation (X-ray and non-thermal radio from the hot bow shock head, IR from dust formed in some WC + O binaries far downstream in the collision shock cone) and (2) line radiation (optical and UV, both from various regions downstream from the bow shock head). The latter is particularly useful in providing constraints on the velocity field and hence ultimately the geometry of the wind collision and the binary system itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Optical observations of GRB 050401 afterglow : a case for double-jet model

The afterglow of GRB 050401 presents several novel and interesting features. (i) An initially faster decay in optical band than in X-rays. (ii) A break in the X-ray light curve after ∼0.06 d with an unusual slope after the break. (iii)The X-ray afterglow does not show any spectral evolution across the break while the R -band light curve does not show any break. We have modelled the observed multiband evolution of the afterglow of GRB 050401 as originating in a two-component jet, and interpreting the break in X-ray light curve as due to lateral expansion of a narrow collimated outflow which dominates the X-ray emission. The optical emission is attributed to a wider jet component. Our model reproduces all the observed features of multiband afterglow of GRB 050401. We present optical observations of GRB 050401 using the 104-cm Sampurnanand Telescope at the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital. Results of the analysis of multiband data are presented and compared with GRB 030329, the first reported case of double jet.     

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.     

Delayed X-ray emission from fallback in compact-object mergers

When double neutron star or neutron star–black hole binaries merge, the final remnant may comprise a central solar-mass black hole surrounded by a  ∼0.01–0.1 M_⊙  torus. The subsequent evolution of this disc may be responsible for short γ-ray bursts (SGRBs). A comparable amount of mass is ejected into eccentric orbits and will eventually fallback to the merger site after ∼0.01 s. In this paper, we investigate analytically the fate of the fallback matter, which may provide a luminous signal long after the disc is exhausted. We find that matter in the eccentric tail returns at a super-Eddington rate and eventually (≳0.1 s) is unable to cool via neutrino emission and accrete all the way to the black hole. Therefore, contrary to previous claims, our analysis suggests that fallback matter is not an efficient source of late-time accretion power and unlikely to cause the late-flaring activity observed in SGRB afterglows. The fallback matter rather forms a radiation-driven wind or a bound atmosphere. In both the cases, the emitting plasma is very opaque and photons are released with a degraded energy in the X-ray band. We therefore suggest that compact binary mergers could be followed by an 'X-ray renaissance', as late as several days to weeks after the merger. This might be observed by the next generation of X-ray detectors.     

Bubbles in planetary nebulae and clusters of galaxies: jet bending

We study the bending of jets in binary stellar systems. A compact companion accretes mass from the slow wind of the mass-losing primary star, forms an accretion disc and blows two opposite jets. These fast jets are bent by the slow wind. Disregarding the orbital motion, we find the dependence of the bending angle on the properties of the slow wind and the jets. Bending of jets is observed in planetary nebulae which are thought to be the descendants of interacting binary stars. For example, in some of these planetary nebulae, the two bubbles (lobes) which are inflated by the two opposite jets are displaced to the same side of the symmetry axis of the nebula. Similar displacements are observed in bubble pairs in the centre of some clusters and groups of galaxies. We compare the bending of jets in binary stellar systems with that in clusters of galaxies.     

Constraints on the late X-ray radiation from the low-energy gamma-ray burst of December 3, 2003: INTEGRAL data

Comparison of the INTEGRAL upper limits on the hard X-ray flux before and after the low-energy GRB 031203 with the XMM measurements of the dust-scattered radiation at lower energies suggests that a significant fraction of the total burst energy could be released in the form of soft X-ray radiation at an early afterglow stage with a characteristic duration of ～100–1000 s. The overall time evolution of the afterglow from GRB 031203 may have not differed qualitatively from the behavior of standard (i.e., more intense) bursts studied by the SWIFT observatory. The available data also admit the possibility that the dust-scattered radiation was associated with an additional soft component in the spectrum of the gamma-ray burst itself.     

太阳的空间观测与研究Ι—太阳电磁波 辐射的观测与研究概况

太阳空间观测为揭示太阳新的观测现象与研究开拓了新的途径。空间观测具有全波段、全时段、全方位以及无大气抖动和大气散射光等观测优点。本文着重探讨了太阳空间长波射电观测、Ｘ射线观测、紫外线观测的成就与研究结果。这些波段（包括光学）的爆发均起因于太阳大气中被加速的荷能电子与太阳等离子体、磁场相互作用而产生的电磁辐射,其能量约占太阳耀斑总量的１／４,即１０２５Ｊ。     

Ring-Shaped Jets in Gamma-Ray Bursts

When the axis of a gamma-ray burst (GRB) does not coincide with the spin axis of its source, there may result a ring-shaped jet. Using some refined jet dynamics, we calculate multi-wavelength afterglow light curves for such ring-shaped jets. In the R-band we find an obvious break in the afterglow light curve due to the beaming effect and the break is affected by many parameters, such as the electron energy fraction ξe, the magnetic energy fraction ξ2B, the width of ring A0 and the medium number density n. The overall light curve can be divided into three power-law stages, I.e., an ultra-relativistic stage, an after-break stage and a deep Newtonian stage. For each stage the power-law index is larger in the ring-shaped jet than in the corresponding conical jet.