The BATSE search for absorption lines

No absorption lines between 15 and 100 keV have been detected in burst spectra accumulated by BATSE. Almost 250 bursts have been inspected visually, and a computerized search has begun. Our simulations show that BATSE could indeed detect lines similar to those observed byGinga in strong bursts, and our tests of the detectors' actual capabilities demonstrate they function as understood. The apparent discrepancy between BATSE andGinga is not yet compelling, and therefore the BATSE nondetections do not invalidate theGinga detections. Nonetheless, the absence of BATSE line detections indicates a low line-occurrence frequency.     

Search for nearby host galaxies of short gamma-ray bursts detected and well localized by BATSE/IPN

We have collected the observational data accumulated before the Swift experiment to check the possible connection of short gamma-ray bursts (GRBs) with low-redshift galaxies. The BATSE/IPN experimental data on well-localized short GRBs and the SDSS DR5 and PSCz catalogs of galaxies are used. The PSCz sky coverage has allowed us to search for host galaxies for a sample of 34 short GRBs. One or more galaxies have been found in the error boxes of six bursts, but the probability of a chance coincidence for each of them is high. No excess of nearby galaxies in the total sample has been detected. The 90% confidence limit corresponds to the fact that no more than 7%of the short GRBs could originate in nearby galaxies of the PSCz sample. The estimated upper limit of several percent may be considered to be valid in the volume z = 0.015–0.025. Based on the results of our search, we have estimated the lower limits for the isotropic energies E _γiso of 31 short bursts from our sample. Their values lie within the range 1.0 × 10⁴⁷–2.7 × 10⁴⁹ erg. The possible fraction of the flares from magnetars in our sample of short GRBs is discussed. The SDSS sky coverage is currently insufficient to perform a similar analysis.     

Effects of variableC min on gamma-ray burstC max/C min distributions

Claims continue to be made that detector selection effects can explain the deviation of the gamma-ray burst brightness distributions from the -3/2 power law expected for homogeneous burst sources. However, these effects are insufficient to explain the BATSE observations. The BATSE sensitivity threshold does vary with time, independent of the burst brightness; however, a homogeneous distribution of standard candle sources would still produce a -3/2 power law. The variation in the threshold does affect inhomogeneous source models. As an example, the effect of a time-varyingC _min on theC _max/C _min distribution of an extended Galactic halo model is shown here. To fit the BATSEC _max/C _min distribution including a varyingC _min requires a larger observing distance (relative to the scale-height of the halo) than for a constantC _min; however, the observations can still be fit using the halo models.     

γ-ray bursts from QSOs?

One of the most amazing phenomena in astronomy, during the last twenty years, have been cosmic gamma-ray bursts (GRBs). The duration of these events vary from a few milliseconds to hundreds of seconds. We have never been able to identify the source of these bursts in other wavelengths. These objects have also never been seen in-rays after the initial bursts although there is some very weak statistical evidence that some of the bursts will repeat (Quashnock and Lamb 1993). The standard explanation for these bursts has been that they are somehow related to neutron stars in our own Galaxy. The latest results from the Burst and Transient Source Experiment aboard the Compton Gamma-Ray Observatory (Fishmanet al. 1994) show clearly that there is no excess concentration of these events (743 bursts) in the Galactic plane. After this, a more promising explanation is that the bursts are related to the Galactic halo or that the origin is extragalactic. In this letter we show that it is very probable that the origin of these events is the QSOs and that the radiation comes from the same synchrotron source as in the other observed wavelengths.     

Microlensing of tidal debris on the Magellanic great circle

Increasing evidence suggests that the Galactic halo is lumpy on kpc scales as a result of the accretion of at least a dozen small galaxies [Large and Small Magellanic Clouds (LMC/SMC), Sgr, Fornax, etc.]. Faint stars in such lumpy structures can significantly microlense a background star with an optical depth of 10⁻⁷–10⁻⁶, which is comparable to the observed value to the LMC. The observed microlensing events towards the LMC can be explained by a tidal debris tail from the progenitor of the Magellanic Clouds and Magellanic Stream. The LMC stars can either lense stars in the debris tail a few kpc behind the LMC, or be lensed by stars in the part of the debris tail in front of the LMC. The models are consistent with an elementary particle dominated Galactic halo without massive compact halo objects (MACHOs). They also differ from Sahu's LMC-self-lensing model by predicting a higher optical depth and event rate and lower concentration of events to the LMC centre.     

X-ray of gamma-ray bursts observable by HEAO-B

On grounds of Mazetset al. (1980a) conclusion that all gamma-ray bursts recorded were emitted at the distance 1 kpc from the Sun it is shown that bursts from the Galaxy and from another galaxies may be observable byEinstein Observatory (HEAO-B). The HEAO-B observing programs are considered and the numbers of possibly recorded events are estimated. It is concluded that the important information about sources of gamma-bursts may be obtained in this way.     

The Spin-X wide-field X-ray monitor of the Spectrum-X-Gamma astrophysical observatory

The Spin-X wide-field X-ray monitor of the Spectrum-X-Gamma astrophysical observatory, which is based on the principle of a coded-aperture telescope, is designed to detect and localize cosmic gammaray-burst (GRB) sources; to survey large areas of the sky in search of new transients; and to carry out long-term observations of bright Galactic sources, including X-ray bursters. The monitor consists of two noncoaxial identical modules, Spin-X1 and Spin-X2, which together cover 6.8% of the sky. The high-apogee, four-day orbit of the Spectrum-XG satellite allows the instrument to be in observing mode more than 50% of the time. Having simulated the rate of GRB detection by Spin-X, we show that extrapolating BATSE 50–300-keV average data on the number of GRBs, their duration, and their mean energy spectrum to the X-ray energy band leads to disagreement with the observed detection rate of GRBs in the X-ray band. The number of GRBs that can be detected and localized with an accuracy r≤3′ (3σ) (the error-circle radius) by Spin-X is estimated to be ten bursts per year. We present data on the Spin-X sensitivity achievable during long-term observations of persistent and transient sources and on its sensitivity to X-ray bursts from Galactic sources in the 2–30-keV energy band.     

Gamma—Ray Bursts：Afterglows and Central Engines

Gamma-ray bursts (GRBs) are the most intense transient gamma-ray events in the sky; this, together with the strong evidence (the isotropic and inhomogeneous distribution of GRBs detected by BASTE) that they are located at cosmological distances, makes them the most energetic events ever known. For example, the observed radiation energies of some GRBs are equivalent to the total convertion into radiation of the mass energy of more than one solar mass. This is thousand times stronger than the energy of a supernova explosion. Some unconventional energy mechanism and extremely high conversion efficiency for these mysterious events are required. The discovery of host galaxies and association with supernovae at cosmoligical distances by the recently launched satellite of BeppoSAX and ground based radio and optical telescopes in GRB afterglow provides further support to the cosmological origin of GRBs and put strong constraints on their central engine. It is the aim of this article to review the possible central engines, energy mechanisms, dynamical and spectral evolution of GRBs, especially focusing on the afterglows in multi-wavebands.     

The growth and assembly of a massive galaxy at z∼ 2

We study the stellar mass assembly of the Spiderweb galaxy  (MRC 1138−262)  , a massive   z = 2.2  radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by   z = 0  , increasing its stellar mass by up to a factor of ≃2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.     

Detection of the first thermonuclear X-ray burst from AX J1754.2-2754

When analyzing the archival data of the INTEGRAL observatory, we detected an intense X-ray burst recorded on April 16, 2005, by the JEM-X and IBIS/ISGRI telescopes from the weak and poorly studied source AX J1754.2-2754. Analysis of its time profiles and spectra allows this event to be attributed to type I X-ray bursts associated with thermonuclear explosions on the surfaces of neutron stars and the source itself to X-ray bursters. Peculiarities of the X-ray emission observed at the initial evolutionary phase of the burst point to a dramatic expansion and a corresponding cooling of the neutron star photosphere that took place at this time under the action of radiation pressure. Assuming the luminosity of the source at this phase to be the Eddington one, we have estimated the distance to the burst to be d = 6.6 ± 0.3 kpc (for a hydrogen atmosphere of the neutron star) and d = 9.2 ± 0.4 kpc (for a helium atmosphere).     

Analysis of the spatial distribution of gamma-ray bursts in their host galaxies

We compare the radial distributions of known localized gamma-ray bursts (GRBs) relative to the centers of their host galaxies with the distributions of known objects in nearby galaxies (supernovae of various types, X-ray binaries), the hypothetical dark-matter profiles, and the distribution of luminous matter in galaxies in the model of an exponential disk. By comparing the moments of empirical distributions, we show that the radial distribution of GRBs in galaxies differs significantly from that of other sources. We suggest a new statistical method for comparing empirical samples that is based on estimating the number of objects within a given radius. The exponential disk profile was found to be in best agreement with the radial distribution of GRBs. The distribution of GRBs relative to the centers of their host galaxies also agrees with the dark matter profile at certain model parameters.     

Observations of magnetic fields in the Milky Way and in nearby galaxies with a Square Kilometre Array

The role of magnetic fields in the dynamical evolution of galaxies and of the interstellar medium (ISM) is not well understood, mainly because such fields are difficult to directly observe. Radio astronomy provides the best tools to measure magnetic fields: synchrotron radiation traces fields illuminated by cosmic-ray electrons, while Faraday rotation and Zeeman splitting allow us to detect fields in all kinds of astronomical plasmas, from lowest to highest densities. Here, we describe how fundamental new advances in studying magnetic fields, both in our own Milky Way and in other nearby galaxies, can be made through observations with the proposed Square Kilometre Array. Underpinning much of what we propose is an all-sky survey of Faraday rotation, in which we will accumulate tens of millions of rotation measure measurements toward background radio sources. This will provide a unique database for studying magnetic fields in individual Galactic supernova remnants and Hii regions, for characterizing the overall magnetic geometry of our Galaxy’s disk and halo, and for understanding the structure and evolution of magnetic fields in galaxies. Also of considerable interest will be the mapping of diffuse polarized emission from the Milky Way in many narrow bands over a wide frequency range. This will allow us to carry out Faraday tomography of the Galaxy, yielding a high-resolution three-dimensional picture of the magnetic field within a few kpc of the Sun, and allowing us to understand its coupling to the other components of the ISM. Finally, direct synchrotron imaging of a large number of nearby galaxies, combined with Faraday rotation data, will allow us to determine the magnetic field structure in these sources, and to test both the dynamo and primordial field theories for field origin and amplification.     

Contribution of the Large Magellanic Cloud to the Galactic warp

Multi-scale interaction between the LMC, the Galactic halo, and the disk is examined with N-body simulations, and precise amplitudes of the Galactic warp excitation are obtained. The Galactic models are constructed most realistically to satisfy available observational constraints on the local circular velocity, the mass, surface density and thickness of the disk, the mass and size of the bulge, the local density of the halo matter at the solar radius, and the mass and orbit of the LMC. The mass of the halo within R=50 kpc is set to about 5×10¹¹ M_⊙. Since the observational estimate of the mass distributed in outer region has large ambiguity, two extreme cases are examined; M(<170 kpc)=2.1 and 0.9×10¹² M_⊙. LMC is orbiting in a ellipse with apocentric radii of 100 kpc, thus the main difference between our two models is the mass density in the satellite orbiting region, so that our study can clarify the role of the halo on excitation of the warp.By using hybrid algorithm (SCF–TREE) I have succeeded to follow the evolution with millions of particles. The orbiting satellite excites density enhancement as a wake, and the wake exerts a tidal force on the disk. Because of the additional torque from the wakes in the halo, the amplitudes of the induced warps are much larger than the classical estimate by Hunter and Toomre [ApJ 155 (1969) 747], who considered only the direct torque from the LMC. The obtained amplitudes of m=0, 1, 2 warps in the larger halo model show very good agreement with the observed amplitude in the Milky Way. This result revives the LMC as a possible candidate of the origin of the Galactic warp. Our smaller halo model, however, yield only weak warps in all the harmonic modes. Therefore, the halo still has significant influence on excitation of warp even in the interaction scenario for excitation of warps.     

Star Formation in Violent and Normal Evolutionary Phases

Mergers of massive gas-rich galaxies trigger violent starbursts that - over timescales of > 100 Myr and regions > 10 kpc - form massive and compact star clusters comparable in mass and radii to Galactic globular clusters. The star formation efficiency is higher by 1–2 orders of magnitude in these bursts than in undisturbed spirals, irregulars or even BCDs. We ask the question if star formation in these extreme regimes is just a scaled-up version of the normal star formation mode of if the formation of globular clusters reveals fundamentally different conditions. This revised version was published online in September 2006 with corrections to the Cover Date.     

Formation of galactic subsystems in light of the magnesium abundance in field stars: The halo

Data from our compiled catalog of spectroscopically determined magnesium abundances in dwarfs and subgiants with accurate parallaxes are used to select Galactic halo stars according to kinematic criteria and to identify presumably accreted stars among them. Accreted stars are shown to constitute the majority in the Galactic halo. They came into the Galaxy from disrupted dwarf satellite galaxies. We analyze the relations between the relative magnesium abundances, metallicities, and Galactic orbital elements for protodisk and accreted halo stars. We show that the relative magnesium abundances in protodisk halo stars are virtually independent of metallicity and lie within a fairly narrow range, while presumably accreted stars demonstrate a large spread in relative magnesium abundances up to negative [Mg/Fe]. This behavior of protodisk halo stars suggests that the interstellar matter in the early Galaxy mixed well at the halo formation phase. The mean metallicity of magnesium-poor ([Mg/Fe] < 0.2 dex) accreted stars has been found to be displaced toward the negative values when passing from stars with low azimuthal velocities (|Θ| < 50 km s^?1) to those with high ones at Δ[Fe/H] ≈ ?0.5 dex. The mean apogalactic radii and inclinations of the orbits also increase with increasing absolute value of |Θ|, while their eccentricities decrease. As a result, negative radial and vertical gradients in relative magnesium abundances are observed in the accreted halo in the absence of correlations between the [Mg/Fe] ratios and other orbital elements, while these correlations are found at a high significance level for genetically related Galactic stars. Based on the above properties of accreted stars and our additional arguments, we surmise that as the masses of dwarf galaxies decrease, the maximum SN II masses and, hence, the yield of α-elements in them also decrease. In this case, the relation between the [Mg/Fe] ratios and the inclinations and sizes of the orbits of accreted stars is in complete agreement with numerical simulations of dynamical processes during the interaction of galaxies. Thus, the behavior of the magnesium abundance in accreted stars suggests that the satellite galaxies are disrupted and lose their stars en masse only after dynamical friction reduces significantly the sizes of their orbits and drags them into the Galactic plane. Less massive satellite galaxies are disrupted even before their orbits change appreciably under tidal forces.     

IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array

Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB detection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger effective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay between the burst onset and the start of ground observations. We also consider the possibility of using GBM on Fermi as a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM localization is problematic, the small field-of-view for IACTs can potentially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20–30 months with our baseline instrument model, assuming consistently rapid pursuit of GRB alerts, and provided that spectral breaks below ~100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year.     

Time-Dependent Synchrotron and Compton Spectra from Microquasar Jets

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very-high-energy γ-rays from the microquasar LS 5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, and adiabatic and radiative cooling in the jets of Galactic microquasars.     

The correlation between the local density of galaxies and the presence of Gamma‐Ray Bursts

In order to explore the characteristics of the environment surrounding Gamma‐Ray Bursts (GRBs), we have studied the correlation between the local density of galaxies and the presence of bursts. Using the Main Galaxy sample of the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) and the Current BATSE GRB Catalog, we calculate the local surface density of galaxies within a radius of <2° surrounding each burst. It turns out that there is no correlation between the local density of galaxies and the presence of GRBs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

MS 1603.6+2600: an accretion disc corona source?

We have observed the eclipsing low-mass X-ray binary MS 1603.6+2600 with Chandra for 7 ks. The X-ray spectrum is well fit with a single absorbed power law with an index of ∼2. We find a clear sinusoidal modulation in the X-ray light curve with a period of  1.7 ± 0.2 h  , consistent with the period of 1.85 h found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst, then the source can only be an accretion disc corona source at a distance of ∼11–24 kpc (implying a height above the Galactic disc of ∼8–17 kpc). It has also been proposed in the literature that MS 1603.76+2600 is a dipper at ∼75 kpc. We argue that, in this dipper scenario, the observed optical properties of MS 1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high-inclination, low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case, the source can be a nearby soft X-ray transient accreting at a quiescent rate, as was proposed by Hakala et al., or a high-inclination source at ∼15–20 kpc.     

Extended emission from short gamma-ray bursts detected with SPI-ACS/INTEGRAL

The short duration (T ₉₀ < 2 s) gamma-ray bursts (GRBs) detected in the SPI-ACS experiment onboard the INTEGRAL observatory are investigated. Averaged light curves have been constructed for various groups of events, including short GRBs and unidentified short events. Extended emission has been found in the averaged light curves of both short GRBs and unidentified short events. It is shown that the fraction of the short GRBs in the total number of SPI-ACS GRBs can range from 30 to 45%, which is considerably larger than has been thought previously.