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 ζe, the magnetic energy fraction ζB^2, the width of ring △θ 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.
Observations indicate that Ellerman bombs （EBs） and chromospheric microflares both occur in the lower solar atmosphere,and share many common features,such as temperature enhancements,accompanying jet-like mass motions,short life-time,and so on.These strongly suggest that EBs and chromospheric microflares could both probably be induced by magnetic reconnection in the lower solar atmosphere.With gravity,ionization and radiation considered,we perform two-dimensional numerical simulations of magnetic reconnection in the lower solar atmosphere.The influence of different parameters,such as intensity of the magnetic field and anomalous resistivity,on the results are investigated.Our result demonstrates that the temperature increases are mainly due to the joule dissipation caused by magnetic reconnection.The spectral profiles of EBs and chromospheric microflares are calculated with the non-LTE radiative transfer theory and compared with observations.It is found that the typical features of the two phenomena can be qualitatively reproduced.
Xiao-Yan Xu 1,2,Cheng Fang 2,Ming-De Ding 2 and Dan-Hui Gao 2 1 Purple Mountain Observatory,Chinese Academy of Science,Nanjing 210008,China 2 Department of Astronomy,Nanjing University,Nanjing 210093,China
Warm absorption is a common phenomenon in Seyfert 1 s and quasars, but is rare in Seyfert 2s. We report the detection of warm absorbers with high energy resolution in the Seyfert 2 galaxy IRAS 18325-5926 for the first time with Chandra HETGS spectra. An intrinsic absorbing line system with an outflow velocity - 400 km s^-1 was found, which is contributed by two warm absorbers with FWHM of 570 km s^-1 and 1360 km s^-1, respectively. The two absorbers were adjacent, and moving transversely across our line of sight. We constrained the distance between the center and the absorbers to be a small value, suggesting that the absorbers may originate from the highly ionized accretion disk wind ejected five years ago. The perspective of this type 2 Seyfert provides the best situation in which to investigate the vertical part of the funnel-like outflows. Another weak absorbing line system with zero redshift was also detected, which could be due to Galactic absorption with very high temperature or an intrinsic outflow with a very high velocity - 6000 km s^-1.
Stellar ages and metallicities are crucial for understanding the formation and evolution of elliptical galaxies. However, due to the age-metallicity degeneracy, it is hard to measure these two parameters accurately with broad-band photometry. In this paper, we observed high-resolution spectra for a sample of 20 nearby elliptical galaxies （EGs） with the NAOC 2.16 m telescope, and determined stellar ages and metallicities by using the empirical population synthesis and Lick/IDS index system methods. We found that stellar ages from these two methods are consistent with each other for purely old EGs; however, stellar metallicities show a zeropoint offset of 0.5 Z⊙. Our results confirm that stellar populations in low-density environment galaxies are more diverse compared to their high-density counterparts. We also investigated the element abundance-galaxy mass relation for nearby elliptical galaxies.
Early-type galaxies （ETGs） are very important for understanding the formation and evolution of galaxies. Recent observations suggest that ETGs are not simply old stellar spheroids as we previously thought. Widespread recent star formation, cool gas and dust have been detected in a substantial fraction of ETGs. We make use of the radial profiles of 9 - r color and the concentration index from the Sloan Digital Sky Survey database to pick out 31 peculiar ETGs with central blue cores. By analyzing the photometric and spectroscopic data, we suggest that the blue cores are caused by star formation activities rather than the central weak active galactic nucleus. From the results of stellar population synthesis, we find that the stellar population of the blue cores is relatively young, spreading from several Myr to less than one Gyr. In 14 galaxies with H I observations, we find that the average gas fraction of these galaxies is about 0.55. The bluer galaxies show a higher gas fraction, and the total star forma- tion rate （SFR） correlates very well with the H I gas mass. The star formation history of these ETGs is affected by the environment, e.g. in the denser environment the H I gas is less and the total SFR is lower. We also discuss the origin of the central star formation of these early-type galaxies.
We present galactic spectroscopic data from a pencil beam of 10.75×7.5 centered on the X-ray cluster RXJ0054.0–2823 at z=0.29.We study the spectral evolution of galaxies from z=1 down to the cluster redshift in a magnitude-limited sample at R≤23,for which the statistical properties of the sample are well understood.We divide emission-line galaxies into star-forming galaxies,Low Ionization Nuclear Emission line Regions（LINERs） ,and Seyferts by using emission-line ratios of[OII],Hβ,and[OIII],and derive stellar fractions from population synthesis models. We focus our analysis on absorption and low-ionization galaxies.For absorption-line galaxies,we recover the well-known result that these galaxies have had no detectable evolution since z～0.6-0.7,but we also find that in the range z=0.65-1,at least 50% of the stars in bright absorption systems are younger than 2.5 Gyr.Faint absorption-line galaxies in the cluster at z=0.29 also had significant star formation during the previous 2-3 Gyr,but their brighter counterparts seem to be only composed of old stars.At z～0.8,our dynamically young cluster had a truncated red-sequence.This result seems to be consistent with a scenario where the final assembly of E/S0 took place at z1.In the volume-limited range 0.35≤z≤0.65,we find that 23% of the early-type galaxies have LINER-like spectra with Hβin absorption and have a significant component of A stars.The vast majority of LINERs in our sample have significant populations of young and intermediate-aged stars and are thus not related to AGNs,but to the population of‘retired galaxies’recently identified by Cid Fernandes et al.in the Sloan Digital Sky Survey（SDSS） .Early-type LINERs with various fractions of A stars and E＋A galaxies appear to play an important role in the formation of the red sequence.
