Polarization evolution accompanying the very early sharp decline of gamma-ray burst X-ray afterglows

In the synchrotron radiation model, the polarization property depends on both the configuration of the magnetic field and the geometry of the visible emitting region. Some peculiar behaviours in the X-ray afterglows of gamma-ray bursts (GRBs) observed with Swift , such as energetic flares and a plateau followed by a sharp drop, might be highly linearly polarized because the outflows powering these behaviours may be dominated by Poynting flux. The breakdown of the symmetry of the visible emitting region may also be well hidden in the peculiar X-ray data and may give rise to interesting polarization signatures. In this paper, we focus on the polarization accompanying the very early sharp decline of GRB X-ray afterglows. We show that strong polarization evolution is possible in both the high latitude emission model and the dying central engine model, which are used to interpret this sharp X-ray decline. It is thus not easy to efficiently probe the physical origin of the very early X-ray sharp decline with future polarimetry. Strong polarization evolution is also possible in the decline phase of X-ray flares and in the shallow decline phase of X-ray light curves characterized by chromatic X-ray versus optical breaks. A detector such as the X-ray Telescope (XRT), but with polarization capability, on board a satellite like Swift would be suitable for testing our predictions.     

AMI limits on 15-GHz excess emission in northern H ii regions

We present observations between 14.2 and 17.9 GHz of 16 Galactic H  ii regions made with the Arcminute Microkelvin Imager. In conjunction with data from the literature at lower radio frequencies we investigate the possibility of a spinning dust component in the spectra of these objects. We conclude that there is no significant evidence for spinning dust towards these sources and measure an average spectral index of  α= 0.15 ± 0.07  (where   S ∝ν^−α  ) between 1.4 and 17.9 GHz for the sample.     

An analysis of infrared emission spectra from the regions near the Galactic Centre

We present consistent modelling of line and continuum infrared (IR) spectra in the region close to the Galactic Centre. The models account for the coupled effect of shocks and photoionization from an external source. The results show that the shock velocities range between ∼65 and 80 km s⁻¹ and the pre-shock densities between 1 cm⁻³ in the interstellar medium (ISM) to 200 cm⁻³ in the filamentary structures. The pre-shock magnetic field increases from 5 × 10⁻⁶ G in the surrounding ISM to ∼8 × 10⁻⁵ G in the arched filaments. The stellar temperatures are ∼38 000 K in the Quintuplet cluster and ∼27 000 K in the Arches Cluster. The ionization parameter is relatively low (<0.01) with the highest values near the clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the gaseous phase of Si is found throughout the whole observed region, indicating the presence of silicate dust. Grains including iron are concentrated throughout the arched filaments. The modelling of the continuum spectral energy distribution in the IR range indicates that a component of dust at temperatures of ∼100–200 K is present in the central region of the Galaxy. Radio emission appears to be thermal bremsstrahlung in the E2–W1 filaments crossing strip; however, a synchrotron component is not excluded. More data are necessary to resolve these questions.     

The Synchrotron-self-Compton Radiation Accompanying Shallow Decaying X-Ray Afterglow： the Case of GRB 940217

High energy emission (> tens MeV) of Gamma-Ray Bursts (GRBs) provides an important clue on the physical processes occurring in GRBs that may be correlated with the GRB early afterglow. A shallow decline phase has been well identified in about half of Swift Gamma-ray Burst X-ray afterglows. The widely considered interpretation involves a significant energy injection and possibly time-evolving shock parameter(s). We calculate the synchrotron-self-Compton (SSC) radiation of such an external forward shock and show that it could explain the well-known long term high energy (i.e., tens MeV to GeV) afterglow of GRB 940217. We propose that cooperation of Swift and GLAST will help to reveal the nature of GRBs.     

Observational limits on inverse Compton processes in gamma-ray bursts

Inverse Compton (IC) scattering is one of two viable mechanisms that can produce prompt non-thermal soft gamma-ray emission in gamma-ray bursts. IC requires low-energy seed photons and a population of relativistic electrons that upscatter them. The same electrons will upscatter the gamma-ray photons to even higher energies in the TeV range. Using the current upper limits on the prompt optical emission, we show that under general conservative assumption the IC mechanism suffers from an 'energy crisis'. Namely, IC will overproduce a very high energy component that would carry much more energy than the observed prompt gamma-rays, or alternatively it will require a low-energy seed that is more energetic than the prompt gamma-rays. Our analysis is general, and it makes no assumptions on the specific mechanism that produces the relativistic electron population.     

A refined model for spinning dust radiation

We present a comprehensive treatment of the spectrum of electric dipole emission from spinning dust grains, updating the commonly used model of Draine & Lazarian. Grain angular velocity distributions are computed using the Fokker–Planck equation; we revisit the drift and diffusion coefficients for the major torques on the grain, including collisions, grain-plasma interactions and infrared emission. We use updated grain optical properties and size distributions. The theoretical formalism is implemented in the companion code, spdust , which is publicly available. The effect of some environmental and grain parameters on the emissivity is shown and analysed.     

射电源法拉第旋率观测效应的模拟研究

由于受射电望远镜分辨率的限制,观测视线方向上可能有多源重叠现象.观测目标源的法拉第旋率(rotation measure,RM)及偏振角(polarization angle,PA)的测量值就会受到方向束内其它背景射电源的影响.通过模拟研究发现,背景射电源对目标源偏振参量测量的干扰形式与干扰源的RM值有关.只运用两三个...     

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).     

超新星遗迹G1.9+0.3射电流量密度的时间演化

对年轻超新星遗迹的射电观测有助于理解超新星遗迹的早期演化. 选取银河系最年轻的超新星遗迹\lk G1.9+0.3进行了研究. 收集了已有的射电流量密度测量, 转化到同一频率, 从而获得了G1.9+0.3的流量密度在过去近50 yr的演化. 发现流量密度在2008年之前几乎一直在增加, 随后开始减小, 流量密度达到峰值的年龄约为\lk 150--155 yr. 流量密度的增加可能由磁场放大或者粒子加速效率提高产生的高能电子增多导致. 根据流量密度到达峰值的年龄, 结合前人的数值模拟, 讨论了超新星抛射物的质量和超新星爆发释放的动能.     

Predicted γ-ray image of SN 1006 due to inverse Compton emission

We propose a method to synthesize the inverse Compton (IC) γ-ray image of a supernova remnant starting from the radio (or hard X-ray) map and using results of the spatially resolved X-ray spectral analysis. The method is successfully applied to SN 1006. We found that synthesized IC γ-ray images of SN 1006 show morphology in nice agreement with that reported by the High Energy Stereoscopic System (HESS) collaboration. The good correlation found between the observed very high energy γ-ray and X-ray/radio appearance can be considered as evidence of the fact that the γ-ray emission of SN 1006 observed by HESS is leptonic in origin, although a hadronic origin may not be excluded.     

High-energy afterglow emission from gamma-ray bursts

We calculate the very high-energy (sub-GeV to TeV) inverse Compton emission of GRB afterglows. We argue that this emission provides a powerful test of the currently accepted afterglow model. We focus on two processes: synchrotron self-Compton emission within the afterglow blast wave, and external inverse Compton emission which occurs when flare photons (produced by an internal process) pass through the blast wave. We show that if our current interpretations of the Swift X-ray telescope (XRT) data are correct, there should be a canonical high-energy afterglow emission light curve. Our predictions can be tested with high-energy observatories such as GLAST , Whipple, HESS and MAGIC. Under favourable conditions we expect afterglow detections in all these detectors.     

Deep 1.4-GHz observations of diffuse polarized emission

Polarized diffuse emission observations at 1.4 GHz in a high Galactic latitude area of the Northern celestial hemisphere are presented. The  3.2 × 3.2 deg²  field, centred at  RA = 10^h58^m, Dec. =+42°18' (B1950)  , has Galactic coordinates   l ∼ 172°, b ∼+63°  and is located in the region selected as northern target of the Balloon-borne Radiometers for Sky Polarization Observations experiment. Observations have been performed with the Effelsberg 100-m telescope. We find that the angular power spectra of the E and B modes have slopes of  β _E =−1.79 ± 0.13  and  β _B =−1.74 ± 0.12  , respectively. Because of the very high Galactic latitude and the smooth emission, a weak Faraday rotation action is expected, which allows both a fair extrapolation to cosmic microwave background polarization (CMBP) frequencies and an estimate of the contamination by the Galactic synchrotron emission. We extrapolate the E -mode spectrum up to 32 GHz and confirm the possibility to safely detect the CMBP E -mode signal in the Ka band found in another low-emission region. Extrapolated up to 90 GHz, the Galactic synchrotron B mode looks to compete with the cosmic signal only for models with a tensor-to-scalar perturbation power ratio   T / S < 0.001  , which is even lower than the T / S value of 0.01 found to be accessible in the only other high Galactic latitude area investigated to date. This suggests that values as low as   T / S = 0.01  might be accessed at high Galactic latitudes. Such low-emission values can allow a significant redshift of the best frequency to detect the CMBP B mode, also reducing the contamination by Galactic dust, and opening interesting perspectives to investigate inflation models.     

Broad-band non-thermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants

Molecular clouds are expected to emit non-thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. We consider here a situation in which a molecular cloud is located in the proximity of a supernova remnant which is efficiently accelerating cosmic rays and gradually releasing them in the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma-ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma-ray spectrum exhibits a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV and TeV gamma-ray sources.     

Spectral indices of Galactic radio loops between 1420, 820 and 408 MHz

In this paper, the average brightness temperatures and surface brightnesses at 1420, 820 and 408 MHz of the six main Galactic radio-continuum loops are derived, as are their radio spectral indices. The temperatures and surface brightnesses of the radio loops are computed using data taken from radio-continuum surveys at 1420, 820 and 408 MHz. We have demonstrated the reality of Loops V and VI and present diagrams of their spectra for the first time. We derived the radio spectral indices of Galactic radio loops from radio surveys at three frequencies (1420, 820 and 408 MHz) and confirm them to be non-thermal sources. Diameters and distances of Loops I–VI were also calculated. The results obtained are in good agreement with current theories of supernova remnant (SNR) evolution and suggest that radio loops may have a SNR origin.