摘要:

We compute X-ray spectra produced bynon-staticcoronae atop accretion discs around black holes. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma,&bgr;=&ngr;/c.We show that an outflowing corona atop a neutral reflector reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflectionRand the X-ray spectrum slope &Ggr; since both strongly depend on &bgr;. A similar correlation was observed and its shape is well fitted by the dynamic model. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419382

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The evolution of the &ggr;-ray emission from GROJ0422+32after its main outburst of August 1992 was followed by COMPTEL during 25 observation periods (VPs) between August 1992 and August 1997, where NPer 92 was in the FoV of COMPTEL, and for 3 VPs preceding the nova outburst. It is found that the NPer 92 &ggr;-ray emission is greatly confined to energies between 1.5 and 2.0 MeV. This emission is prominent during observations that correspond to phases between 0.0 and 0.5 of the ∼120 days period discovered in the nova optical emission. This periodical activity of the nova in form of mini-outbursts is supported by the detection of the &ggr;-ray emission from GROJ0422+32in seven episodes up to ∼1800 days (August 1997) after the main outburst, and by the All Sky Monitor’s (ASM, Rossi XTE) detection of X-ray mini-outbursts (four episodes). In one episode we have succeeded to derive a contemporaneous spectrum based on the measurements by COMPTEL, ASM and BATSE. The plausible mechanisms of the GROJ0422+32periodical mini-outbursts and of its characteristic &ggr;-ray emission are discussed. The possibility of the system being a triple is also evaluated. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419383

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We discuss the effects of a comptonizing corona on the appearance of the reflection components, and in particular of the reflection hump, in the X-rays spectra of accreting black holes. Indeed, in the framework of a thermal corona model, we expect that part of (or even all, depending on the corona covering factor) the reflection features should cross the hot plasma, and thus suffer Compton scattering, before being observed. We have studied in detail the dependence of these effects on the physical (i.e. the temperature and optical depth) and geometrical (i.e. the inclination angle) parameters of the corona, concentrating on the slab geometry. Due to the smoothing and shifting towards high energies of the comptonized reflection hump, the main effects on the emerging spectra appear mainly above 100 keV. We have also investigated the importance of such effects on the interpretation of the results obtained with the standard fitting procedures. We found that fitting Comptonization models, taking into account comptonized reflection, by the usual cut-off powerlaw+uncomptonizedreflection model, may lead to an underestimation of the reflection normalization and an overestimation of the high energy cut-off. We discuss and illustrate the importance of these effects by analyzing recent observational results as those of the galaxy NGC 4258. We also find that the comptonizing corona can produce and/or emphasize correlations between the reflection features characteristics (like the iron line equivalent width or the covering fraction) and the X-ray spectral index similar to those recently reported in the literature. We also underline the importance of these effects when dealing with accurate spectral fitting of the X-ray background. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419384

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We report on radio, near-infrared, optical and X-ray observations of the black hole candidate (BHC) XTEJ1550−564performed during its 2000 X-ray outburst. These observations have allowed us to sample the behavior of XTEJ1550−564in the X-ray Low Hard and Intermediate/Very High states. The radio emission in the Low Hard state most likely originates from a compact jet and the synchrotron emission from this jet may extend up to the optical range or beyond, therefore indicating that the total power of the compact jet is a significant fraction of the total luminosity of the system. In the Intermediate/Very High state the radio emission is quenched, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419385

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The Accretion-Ejection Instability has been proposed to explain the “ubiquitous” low frequency QPO of microquasars. This instability which occurs in the inner region of moderately magnetized disks has the the unique property that it can send toward the corona the energy and angular momentum from the disk. I will present recent results on the physics of the instability, and comparison with observations. In particular I will describe how it generates Alfven waves propagating the energy and angular momentum to the corona. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419386

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Since the first discovery of Cygnus X-1 in the mid 1960’s, the source has been the subject of intense soft &ggr;-ray (30 keV–10 MeV) observations by many balloon and satellite experiments. A large body of spectral and temporal information about the source has been gathered to date. While these results have significantly enhanced our understanding of the Cygnus X-1 system, they have also raised new questions that need to be addressed by future missions. This paper provides a brief summary of some of the important long-term temporal and spectral results obtained over the last thirty years, and discusses the current status and issues that need to be addressed by upcoming missions. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419387

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

We discuss several recent developments in gamma-ray burst and afterglow theory, related in particular to the interaction with the environment and how this bears on the progenitor issue. Pair production induced by the initial gamma-rays in the nearby environment will modify the initial spectrum and the afterglow light curve, and the magnitude of these changes provides a diagnostic for the external density. The illumination of the progenitor remnant and/or the surroundings by the X-ray afterglow continuum can produce substantial Fe K-alpha line and edge emission, with implications for the progenitor model. TeV neutrinos are expected to be produced both by successful (&ggr;-ray detectable) and choked (&ggr;-ray dark) fireball jets as they make their way out of a massive progenitor star, whose fluence is detectable with planned km3detectors. Two mechanisms, IC and proton synchrotron, may play a role in afterglow energetics, and would have detectable signatures in the X-ray light curves as well as in the GeV range. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419388

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The spectra obtained above 100 MeV by the EGRET experiment aboard the Compton Gamma-Ray Observatory for a handful of gamma-ray bursts has given no indication of any spectral attenuation that might preclude detection of bursts at higher energies. With the discovery of optical afterglows and counterparts to bursts in the last few years, enabling the determination of significant redshifts for these sources, it is anticipated that profound spectral attenuation will arise in the GLAST energy band of 30 MeV-300 GeV for many if not most bursts. An important goal will be to discriminate between such extrinsic absorption, due to the cosmic infra-red background, and that which arises internally in GRBs. This paper explores expectations for the spectral properties in the GLAST band for bursts, in particular how attenuation of photons by pair creation internal to the source modifies the spectrum to produce distinctive signatures. The energy of spectral breaks and the associated spectral indices provide valuable information that can constrain the bulk Lorentz factor of the GRB outflow at a given time. Moreover, distinct temporal behavior is present for internal attenuation, and is easily distinguished from extrinsic absorption. These characteristics define palpable observational goals for both the GLAST mission and ground-based Cˇerenkov telescopes, and strongly impact the observability of bursts above 1 GeV. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419389

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The recent realization that energy conversion in &ggr;-ray bursts is most likely via internal shocks rather than via external shocks provides additional information about the inner engine: the relativistic flow must be irregular, it must be variable on a short time scale, it must be able to turn off to a very low level and then turn on again, and it must be active for up to a few hundred seconds and possibly much longer. An acceptable model requires that the central engine evolves into a configuration which is stable enough to survive the violent gravitational instabilities associated with the merging/collapse of compact objects, while still keeping enough binding energy to power the burst and in some cases to be active again after some period ofquiescence. At present, it is unclear if these separated emission episodes observed in some &ggr;-ray burst light curves are consequences of the same physical process, and if the time separation is due to some intrinsic property of the central source or of its environment. The hypothesis of an intermittent central engine, although intriguing, has to be tested against observations. The feasibility of different models in the production of episodes of quiescence are discussed. Some key theoretical issues are highlighted, along with the types of observations that would determine whether or not the central engine goes dormant for a period of time comparable to the duration of the gaps. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419390

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The redshift where the first stars formed is an important and unknown milestone in cosmological structure formation. The evidence linking gamma ray bursts (GRBs) with star formation activity implies that the first GRBs occurred shortly after the first stars formed. Gamma ray bursts and their afterglows may thus offer a unique probe of this epoch, because they are bright from gamma ray to radio wavelengths and should be observable to very high redshift. Indeed, our ongoing near-IR followup programs already have the potential to detect bursts at redshiftz∼10.In these proceedings, we discuss two distinct ways of using GRBs to probe the earliest star formation. First, direct GRB counts may be used as a proxy for star formation rate measurements. Second, high energy cutoffs in the GeV spectra of gamma ray bursts due to pair production with high redshift optical and ultraviolet background photons contain information on early star formation history. The second method is observationally more demanding, but also more rewarding, because each observed pair creation cutoff in a high redshift GRB spectrum will tell us about the integrated star formation history prior to the GRB redshift. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1419391

出版商:AIP    

年代:1901

数据来源: AIP