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At least 811 records · Page 45Linked to original sources

arXiv2004

Intracluster Light in Nearby Galaxy Clusters: Relationship to the Halos of Brightest Cluster Galaxies

We present a detailed analysis of the surface brightness distribution of the brightest cluster galaxy (BCG) in each of 24 galaxy clusters at 0.03 -r_e relation than that of the elliptical fundamental plane. We interpret this outer component as a population of intracluster stars tracing the cluster potential. The two components are strongly aligned (|Δθ|<10\degr) in roughly 40% of the clusters. When they are not aligned, the components tend toward high degrees of misalignment, suggesting that accretion of infalling material may change the orientation of some BCGs for a time. The extent of the outer component and its similar elongation to published cluster galaxy distributions indicates that the evolution of the intracluster light is tied to the cluster as a whole rather than to the BCG.

astro-ph↗

arXiv2004

Implications of the Measured Image Size for the Radio Afterglow of GRB 030329

We use data on the image size of the radio afterglow of GRB 030329 (Taylor et al. 2004) to constrain the physical parameters of this explosion. Together with the observed broad band spectrum, this data over-constrains the physical parameters, thus enabling to test different GRB jet models for consistency. We consider two extreme models for the lateral spreading of the jet: model 1 with relativistic expansion in the local rest frame, and model 2 with little lateral expansion as long as the jet is highly relativistic. We find that both models are consistent with the data for a uniform external medium, while for a stellar wind environment model 1 is consistent with the data but model 2 is disfavored by the data. Our derivations can be used to place tighter constraints on the dynamics and structure of GRB jets in future afterglows, following a denser monitoring campaign for the temporal evolution of their image size.

astro-ph↗

arXiv2004

Neutrino Interactions in the Outflow from Gamma-Ray Burst Accretion Disks

We examine the composition of matter as it flows away from gamma ray burst accretion disks, in order to determine what sort of nucleosynthesis may occur. Since there is a large flux of neutrinos leaving the surface of the disk, the electron fraction of the outflowing material will change due to charged current neutrino interactions. We calculate the electron fraction in the wind using detailed neutrino fluxes from every point on the disk and study a range of trajectories and outflow conditions for several different accretion disk models. We find that low electron fractions, conducive to making r-process elements, only appear in outflows from disks with high accretion rates that have a significant region both of trapped neutrinos and antineutrinos. Disks with lower accretion rates that have only a significant region of trapped neutrinos can have outflows with very high electron fractions, whereas the lowest accretion rate disks with little trapping have outflow electrons fractions of closer to one half.

astro-ph↗

arXiv2014

Exploring the Small Magellanic Cloud to the Faintest X-ray Fluxes: Source Catalog, Timing and Spectral Analysis

We present the results of a pair of 100 ksec Chandra observations in the Small Magellanic Cloud to survey high mass X-ray binaries (HMXBs), stars and LMXBs/CVs down to Lx = 4.3 x 10^32 erg/s The two SMC Deep Fields are located in the most active star forming region of the bar, with Deep Field-1 positioned at the most pulsar-rich location identified from previous surveys. Two new pulsars were discovered in outburst: CXOU J004929.7-731058 (P=892s), CXOU J005252.2-721715 (P=326s), and 3 new HMXB candidates were identified. Of 15 Be-pulsars now known in the field, 13 were detected, with pulsations seen in 9 of them. Ephemerides demonstrate that 6 of the 10 pulsars known to exhibit regular outbursts were seen outside their periastron phase, and quiescent X-ray emission at Lx=10^33 - 10^34 is shown to be common. Comparison with ROSAT, ASCA, XMM-Newton catalogs resulted in positive identification of several previously ambiguous sources. Bright optical counterparts exist for 40 of the X-ray sources, of which 33 are consistent with early-type stars Mv<-2, B-V<0.2), and are the subject of a companion paper. The results point to an underlying HMXB population-density up to double that of active systems. The full catalog of 394 point-sources is presented along with detailed analyses of timing and spectral properties.

astro-ph↗

arXiv2018

The radiative transfer at second order: a full treatment of the Boltzmann equation with polarization

This article investigates the full Boltzmann equation up to second order in the cosmological perturbations. Describing the distribution of polarized radiation by a tensor valued distribution function, we study the gauge dependence of the distribution function and summarize the construction of the gauge-invariant distribution function. The Liouville operator which describes the free streaming of electrons, and the collision term which describes the scattering of photons on free electrons are computed up to second order. Finally, the remaining dependence in the direction of the photon momentum is handled by expanding in projected symmetric trace-free multipoles and also in the more commonly used normal modes components. The results obtained remain to be used for computing numerically the contribution in the cosmic microwave background bispectrum which arises from the evolution of second order perturbations, in order to disentangle the primordial non-Gaussianity from the one generated by the subsequent non-linear evolution.

gr-qc↗

arXiv2001

Gamma-ray burst: probe of a black hole

There is strong evidence for the existence of black holes(BHs) in some X-ray binaries and most galatic nuclei, based on different measuremental approaches, but black holes aren't finally identified for the lack of very firm observational evidence up to now. Because the direct evidence for BHs should come from determination of strong gravitational redshift, we hope an object can fall into the region near the BH horizon where radiation can be detected. Therefore the object must be compact stars as neutron stars(NSs), then the intense astrophysical processes will release very high energy radiation which is transient, fast-variant. These phenomena may point to gamma-ray bursts(GRBs) observed. And recent observations of iron lines suggest that afterglows of GRBs show the similar property in active galatic nuclei(AGNs), implying GRBs may originate from intense events related to black holes. In this letter, a model for GRBs and afterglows is proposed to obtain the range of gravitational redshifts($z_g$) of GRBs with known cosmological redshifts. Hence, we provide a new method that with a search for high-energy emission lines(X- or $γ$-rays) in GRBs, one can determine the gravitational redshift. We expect $z_g>0.5$ or even larger, so that we can rule out the possibility of other compact objects such as NSs, suggesting that the central progenitors of GRBs should be black holes.

astro-ph↗

arXiv2002

Milestones in the Observations of Cosmic Magnetic Fields

Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. (Another long paragraph is omitted due to the limited space here)

astro-ph↗

arXiv2003

Optical Flash of GRB 990123: constraints on the physical parameter of the reverse shock

The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best fitted physical parameters for GRB 990123 (Panaitescu & Kumar 2001) and the assumption that the parameters in the optical flash are the same as those in the afterglow, we show that: 1) the shell is thick but not thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3\times10^{-4} of the observed. In order to compensate this divergency, the physical parameters the electron energy ration and the magnetic ratio $ε_e$, $ε_B$ should be 0.61, 0.39 respectively, which are much different from those in the late afterglow.

astro-ph↗

arXiv2002

Dynamical Evolution of Gamma-Ray Burst Remnants with Evolving Radiative Efficiency

In previous works, a generic dynamical model has been suggested by Huang et al., which is shown to be correct for both adiabatic and radiative blastwaves, and in both ultra-relativistic and non-relativistic phases. In deriving their equations, Huang et al. have assumed that the radiative efficiency of the fireball is constant. They then applied their model directly to realistic cases where the radiative efficiency evolves with time. In this paper, we abandon the above assumption and re-derive a more accurate dynamical equation for gamma-ray burst remnants. Numerical results show that Huang et al.'s model is accurate enough in general cases.

astro-ph↗

arXiv2002

How fast could a proto-pulsar rotate?

According to two estimated relations between the initial period and the dynamo-generated magnetic dipole field of pulsars, we calculate the statistical distributions of pulsar initial periods. It is found that proto-pulsars are very likely to have rotation periods between 20 and 30 ms, and that most of the pulsars rotate initially at a period < 60 ms.

astro-ph↗

arXiv2002

What can the redshift observed in EXO 0748-676 tell us?

The mass-radius relations for bare and crusted strange stars are calculated with the bag model. Comparing these relations with the observed one derived from redshift, we address that the conclusion, that EXO 0748-676 can not be a strange star, is incorrect. Various strange star models can show that EXO 0748-676 could have a mass of (1.3 ~ 1.7) M_sun and a radius of (8.4 ~ 11.4) km. It is proposed that some nascent strange stars could be bare and have mass \~ 0.1 M_sun, whereas their masses increase during a long accretion history.

astro-ph↗

arXiv2002

Statistical Shapes of the Highest Pulses in Gamma--Ray Bursts

The statistical shapes of the highest pulse have been studied by aligned method. A wavelet package analysis technique and a developed pulse--finding algorithm have been applied to select the highest pulse from burst profiles observed by BATSE on board CGRO from 1991 April 21 to 1999 January 26. The results of this work show that the statistical shapes of the highest pulses are related to energy: the higher the energy, the narrower the pulse. However, the characteristic structures of the pulses have nothing to do with energy, which strongly supports the previous conclusion that the temporal profiles in different channels are self--similar. The characteristic structures of the pulses can be well described by a model proposed by Norris et al. (1996). The fitting parameters are: $t_r$=0.12, $t_d$=0.16, $\upsilon =1.09$, the ratio of $t_r$ to $t_d$ for the pulse is 0.75. The result leads to our conjecture that the mechanisms of bursts in different gamma-ray bands might be the same. The shock, either an internal or an external one, producing the pulse, might emit photons over the four energy channels in the same way.

astro-ph↗

arXiv2003

Rhombic Cell Analysis - A New Way of Probing the Large-Scale Structure of the Universe. I. General Considerations

A new way of probing the large-scale structure of the universe is proposed. Space is partitioned into cells the shape of rhombic dodecahedron. The cells are labelled ``filled'' or ``empty'' according as they contain galaxies or not. The cell size is so chosen as to have nearly equal numbers of filled and empty cells for the given galaxy sample. Two observables on each cell are definable: the number of its {\em like} neighbors, $n_1$, and a two-suffixed topological type $τ$, the suffixes being the numbers of its like and unlike {\em neighbor-groups}. The frequency distributions of $n_1$ and $τ$ in the observed set of filled (empty) cells are then considered as indicators of the morphology of the set. The method is applied to the CfA catalogue of galaxies as an illustration. Despite its limited size the data offers evidence 1) that the empty cells are more strongly clustered than the filled cells, and 2) that the filled cells, but not the empty cells, have a tendency of occurring in sheets. Further directions of development both in theory and application are indicated.

astro-ph↗

arXiv2003

Non-LTE Analysis for the Sodium Abundances of Metal-Poor Stars in the Galactic Disk and Halo

We performed an extensive non-LTE analysis of the neutral sodium lines of Na {\sc i} 5683/5688, 5890/5896, 6154/6161, and 8183/8195 for disk/halo stars of F--K type covering a wide metallicity range ($-4 \la$ [Fe/H] $\la 0.4$), based on our own data as well as those collected from the literature. For comparatively metal-rich disk stars ($-1 \la$ [Fe/H] $\la 0$) where the weaker 6154/6161 lines are best abundance indicators, we confirmed [Na/Fe] $\sim$ 0 with an ``upturn'' (i.e., a shallow/broad dip around $-0.5 \la$ [Fe/H] $\la 0$) as already reported by previous studies. Regarding the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines suffering considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly ``subsolar'' [Na/Fe] values down to $\sim -0.4$ (with a somewhat large scatter of $\sim \pm 0.2$ dex) on the average at the typical halo metallicity of [Fe/H] $\sim -2$, while they appear to rise again toward a very metal-poor regime recovering a near-solar ratio of [Na/Fe] $\sim$ 0 ([Fe/H] $\sim -3$ to -4). These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.

astro-ph↗

arXiv2003

Study of the neutron star structure in strong magnetic fields including the anomalous magnetic moments

We study the effects of strong magnetic fields on the neutron star structure. If the interior field of a star is on the same order of the surface field currently observed, the influences of the magnetic field on the star mass and radius are negligible. If one assumes that the internal magnetic field can be as large as that estimated from the scalar virial theorem, considerable effects can be induced. The maximum mass of stars is arisen substantially while the central density is largely suppressed. For two equal-mass stars the radius of the magnetic star can be larger by about 10% $\sim$ 20% than the nonmagnetic star.

astro-ph↗

arXiv2003

The binary black hole scenario for the BL Lacertae object AO 0235+16

Recent analysis of the long term radio lightcurve of the extremely variable BL Lacertae object AO 0235+16 by Raiteri et al. (2001) have revealed the presence of recurrent outbursts in this source with a period of ~ 5.7 +/- 0.5 yr. Periodicity analysis of the optical lightcurve also show evidence for a shorter period. Here we discuss whether such a behavior can be explained by a binary black hole model where the accretion disk of one of the supermassive black holes is precessing due to the tidal effects of the companion. We estimate the mass of the accreting hole and analyze under what constraints onto the secondary mass and the orbital parameters of the system it is possible to provide a viable interpretation of the available multiwavelength data.

astro-ph↗

arXiv2003

Strange stars: Can their crust reach the neutron drip density?

Electrostatic potential of electrons near the surface of static strange stars at zero temperature is studied within the frame of the MIT bag model. We find that for QCD parameters within rather wide ranges, and if the nuclear crust on the strange star is at a density leading to neutron drip, the electrostatic potential is insufficient to establish an outwardly directed electric field, which is crucial for the survival of such crusts. If a minimum gap width of 200 fm is called in for a more stringent constraint, our calculations completely rule out the possibility of such crusts' presence on strange stars. Therefore, our results prefer against the existence of neutron-drip crust in nature.

astro-ph↗

arXiv2003

Spectral properties of anomalous X-ray pulsars

In this paper, the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variation with spin-down rate $\dotΩ$ is considered. Firstly, based on an accretion-powered model, the influences of both magnetic field and mass accretion rate on the spectra properties of AXPs are addressed. Subsequently, the relation between the spectral property of AXPs and mass accretion rate $\dot{M}$ is investigated. The result shows that there exists a linear correlation between the photon index and mass accretion rate, and the spectral hardness increases with increasing $\dot{M}$. A possible emission mechanism for the explanation of spectral properties of AXPs is also discussed.

astro-ph↗