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1.
Population studies of EGRET gamma-ray sources indicate that there is a distinctive population of bright sources at low galactic latitudes. The sources have a distribution consistent with that of young galactic objects, with a concentration toward the inner spiral arms. There is a subgroup that displays strong variability with timescales from days to months. Following an earlier suggestion by Kaufman Bernadó et al. (2002), we explore the possibility that these sources could be high-mass microquasars. Detailed models for the gamma-ray emission that include inverse Compton interactions of electrons in the relativistic jets and photons from all local fields (stellar UV photons, synchrotron photons, soft X-ray photons from the accretion disk, and hard X-ray photons from a corona) are presented. We conclude that microquasars are excellent candidates for the parent population of the subgroup of variable low-latitude EGRET sources.  相似文献   

2.
In this paper we briefly comment on the observational status of the possible physical association between unidentified EGRET sources and supernova remnants (SNRs) in our Galaxy. We draw upon recent results presented in the review by Torres et al. (2003), concerning molecular gas in the vicinity of all 19 SNRs found to be positionally coincident with EGRET sources at low Galactic latitudes. In addition, we present new results regarding the supernova remnant CTA 1. Our findings disfavor the possibility of a physical connection with the nearby (in projection) EGRET source. There remains possible, however, that the compact object produced in the supernova explosion be related with the observed γ-ray flux.  相似文献   

3.
Methanol masers are known to be associated with regions of massive star formation. By studying the distribution of these masers with respect to unidentified EGRET sources, one may be able to identify regions of enhanced star formation leading to either supernova remnants and/or pulsars. In this study, we test for positional coincidences between class II methanol masers and EGRET sources and determine the statistical significance of these coincidences.  相似文献   

4.
We describe simulations of the response of a gaseous disc to an active spiral potential. The potential is derived from an N -body calculation and leads to a multi-armed time-evolving pattern. The gas forms long spiral arms typical of grand-design galaxies, although the spiral pattern is asymmetric. The primary difference from a grand-design spiral galaxy, which has a consistent two-/four-armed pattern, is that instead of passing through the spiral arms, gas generally falls into a developing potential minimum and is released only when the local minimum dissolves. In this case, the densest gas is coincident with the spiral potential, rather than offset as in the grand-design spirals. We would therefore expect no offset between the spiral shock and star formation, and no obvious corotation radius. Spurs which occur in grand-design spirals when large clumps are sheared off leaving the spiral arms, are rare in the active, time-evolving spiral reported here. Instead, large branches are formed from spiral arms when the underlying spiral potential is dissolving due to the N -body dynamics. We find that the molecular cloud mass spectrum for the active potential is similar to that for clouds in grand-design calculations, depending primarily on the ambient pressure rather than the nature of the potential. The largest molecular clouds occur when spiral arms collide, rather than by agglomeration within a spiral arm.  相似文献   

5.
A major legacy of the EGRET experiment aboard the Compton Gamma-Ray Observatory (CGRO) is the detection of a large number of unidentified Gamma-ray sources. Out of the 271 EGRET sources are 170 not identified yet [Hartman et al. ApJS (123) (1999) 79]. To provide additional information on these unidentified EGRET sources, we derived simultaneous MeV fluxes or flux limits for each source by analyzing the contemporaneous COMPTEL data. Then we compare these MeV fluxes to the extrapolations of the published EGRET spectra. Our results can be grouped into 4 categories [Zhang et al. A&A (421) (2004) 983]: (1) non-constraining upper limits (∼120 sources), (2) hints (> 2 sigma) or detections with consistent MeV fluxes (∼16 sources), (3) enhanced MeV emission (2 sources), and (4) constraining MeV flux limits, requiring a spectral break (∼22 sources). In summary, for about 40 of the unidentified EGRET sources we can provide supplementary spectral information in the neighboring gamma-ray band, which – together with other source parameters – might provide clues to the underlying source nature.  相似文献   

6.
The variability of γ-ray sources listed in the third EGRET catalog is studied using three variability indices. These indices are found to be statistically equivalent if the observed data are sufficiently accurate. Using the three indices,30 EGRET point sources which are positionally coincident with pulsars and 40 persistent unidentified sources at low latitudes are analyzed for their variability status. It is found that 14 of the 30 point sources may have genuine or plausible associations with pulsars, and 16 of the 40 persistent unidentified sources are possible pulsar candidates.  相似文献   

7.
The large majority of EGRET point sources remain to this day without an identified low-energy counterpart. Whatever the nature of the EGRET unidentified sources, faint unresolved objects of the same class must have a contribution to the diffuse gamma-ray background: if most unidentified objects are extragalactic, faint unresolved sources of the same class contribute to the background, as a distinct extragalactic population; on the other hand, if most unidentified sources are Galactic, their counterparts in external galaxies will contribute to the unresolved emission from these systems. Understanding this component of the gamma-ray background, along with other guaranteed contributions from known sources, is essential in any attempt to use gamma-ray observations to constrain exotic high-energy physics. Here, we follow an empirical approach to estimate whether a potential contribution of unidentified sources to the extragalactic gamma-ray background is likely to be important, and we find that it is. Additionally, we comment on how the anticipated GLAST measurement of the diffuse gamma-ray background will change, depending on the nature of the majority of these sources.  相似文献   

8.
Most of the unidentified gamma ray sources detected near the Galactic plane by EGRET aboard CGRO are expected to be gamma ray pulsars. We present a study about the detectability and identification of some unidentified EGRET sources with the MAGIC telescope. We list some unidentified gamma ray sources from the third EGRET catalogue to be detected with MAGIC taking into account some important conditions such as the variability parameter of the source, spectral index, inclusion in the GeV catalogue (ApJ 488, 1997, p. 872) and possible associations with known X-ray/radio sources located within the error box of the unidentified gamma ray source. We show the required observation time of these gamma ray pulsar candidates to be detected by MAGIC telescope considering reasonable values of cut-off energy. To be more realistic, we have chosen the zenith angle corresponding to the source culmination in the simulation of the effective area A since the observation time is function of the effective area. In addition to this study, it is very important to consider the extrapolated EGRET flux at MAGIC energies above 30 GeV of the gamma ray pulsar candidates taking the MAGIC sensitivity.  相似文献   

9.
The third EGRET catalog contains a large number of unidentified sources. This subset of objects is expected to include known gamma-ray emitters of Galactic origin such as pulsars and supernova remnants, in addition to an extragalactic population of blazars. However, current data allows the intriguing possibility that some of these objects may represent a new class of yet undiscovered gamma-ray sources. Many theoretically motivated candidate emitters (e.g. clumps of annihilating dark matter particles) have been suggested to account for these detections. We take a new approach to determine to what extent this population is Galactic and to investigate the nature of the possible Galactic component. By assuming that galaxies similar to the Milky Way should host comparable populations of objects, we constrain the allowed Galactic abundance and distribution of various classes of gamma-ray sources using the EGRET data set. We find it is highly improbable that a large number of the unidentified sources are members of a Galactic halo population, but that a distribution of the sources entirely in the disk and bulge is plausible. Finally, we discuss the additional constraints and new insights that GLAST will provide.  相似文献   

10.
In a novel approach in observational high-energy gamma-ray astronomy, observations carried out by imaging atmospheric Cherenkov telescopes provide necessary templates to pinpoint the nature of intriguing, yet unidentified EGRET gamma-ray sources. Using GeV-photons detected by EGRET and taking advantage of high spatial resolution images from H.E.S.S. observations, we were able to shed new light on the EGRET observed gamma-ray emission in the Kookaburra complex, whose previous coverage in the literature is somewhat contradictory. 3EG J1420–6038 very likely accounts for two GeV gamma-ray sources (E>1 GeV), both in positional coincidence with the recently reported pulsar wind nebulae (PWN) by HESS in the Kookaburra/Rabbit complex. PWN associations at VHE energies, supported by accumulating evidence from observations in the radio and X-ray band, are indicative for the PSR/plerionic origin of spatially coincident, but still unidentified Galactic gamma-ray sources from EGRET. This not only supports the already suggested connection between variable, but unidentified low-latitude gamma-ray sources with pulsar wind nebulae (3EG J1420–6038 has been suggested as PWN candidate previously), it also documents the ability of resolving apparently confused EGRET sources by connecting the GeV emission as measured from a large-aperture space-based gamma-ray instrument with narrow field-of-view but superior spatial resolution observations by ground-based atmospheric Cherenkov telescopes, a very promising identification technique for achieving convincing individual source identifications in the era of GLAST-LAT.   相似文献   

11.
The surface distribution of dark clouds along the major and minor axes were compiled to give a general formula expressing the surface distribution along both axes simultaneously. Absorption values were determined, assuming the parameters of the standard cloud model as a function of the distance from the two-dimensional distribution of dark clouds. Iso-cloud line numbers were drawn as a function of bothX andY. Two knots appeared in the new distribution function. This may announce the beign of the spiral arms. The knots may indicate the least possible number of the dark clouds per kiloparsec beyond which spiral structure may be possible. The projected distance of the two knots on the X-axis may correspond to some spiral features found by other investigators.  相似文献   

12.
Acceleration of charged particles by neutral gas turbulence in giant molecular clouds is considered. The gamma-ray emission from these clouds is estimated. It is shown that molecular clouds can be the counterparts of some of unidentified sources.  相似文献   

13.
The Whipple Observatory 10 m γ-ray telescope has been used to survey the error boxes of 24 EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. In no case is a statistically significant signal found in the EGRET error box which implies that, at least for this sample, the γ-ray spectra of these sources steepen between 100 MeV and 350 GeV.  相似文献   

14.
Statistical studies indicate that the 271 point sources of high-energy gamma rays belong to two groups: a galactic population and an isotropic extragalactic population. Many unidentified extragalactic sources are certainly blazars, and it is the intention of this work to uncover gamma-ray blazars missed by previous attempts. Until recently, searches for blazar counterparts to unidentified EGRET sources have focused on finding AGN that have 5-GHz radio flux densities S 5 near or above 1 Jy. However, the recent blazar identification of 3EG J2006-2321 (S 5 = 260 mJy) and other work suggest that careful studies of weaker flat-spectrum sources may be fruitful. In this spirit, error circles of four high-latitude unidentified EGRET sources have been searched for 5-GHz sources. The gamma-ray sources are 3EG J1133+0033, 3EG J1212+2304, 3EG J1222+2315, and 3EG J1227+4302. Within the error contours of each of the four sources are found six radio candidates; by observing the positions of the radio sources with the 0.81-m Tenagra II telescope it is determined that 14 of these 24 radio sources have optical counterparts with R < 22. Eight of these from two different EGRET sources have been observed in the B, V, and R bands in more than one epoch and the analysis of these data is ongoing. Any sources that are found to be variable will be the objects of multi-epoch polarimetry studies.  相似文献   

15.
In a cold dark matter (CDM) framework of structure formation, the dark matter haloes around galaxies assemble through successive mergers with smaller haloes. This merging process is not completely efficient, and hundreds of surviving halo cores, or subhaloes, are expected to remain in orbit within the halo of a galaxy like the Milky Way. While the dozen visible satellites of the Milky Way may trace some of these subhaloes, the majority are currently undetected. A large number of high-velocity clouds (HVCs) of neutral hydrogen are observed around the Milky Way, and it is plausible that some of the HVCs may trace subhaloes undetected in the optical. Confirming the existence of concentrations of dark matter associated with even a few of the HVCs would represent a dramatic step forward in our attempts to understand the nature of dark matter. Supersymmetric (SUSY) extensions of the Standard Model of particle physics currently suggest neutralinos as a natural well-motivated candidate for the non-baryonic dark matter of the universe. If this is indeed the case, then it may be possible to detect dark matter indirectly as it annihilates into neutrinos, photons or positrons. In particular, the centres of subhaloes might show up as point sources in gamma-ray observations. In this work, we consider the possibility that some of the unidentified EGRET γ-ray sources trace annihilating neutralino dark matter in the dark substructure of the Local Group. We compare the observed positions and fluxes of both the unidentified EGRET sources and the HVCs with the positions and fluxes predicted by a model of halo substructure, to determine up to what extent any of these three populations could be associated.  相似文献   

16.
为改善文献上惯用的表现银河系分子谱线巡视结果的完全平滑了方位信息的径向分布方法,我们发展了原子气体或分子云参量的分环银经分布图,(X—l)_i图,它在某种程度上给出了方位信息。用现存旋臂模型结合这种图我们得到的银道面旋臂区和臂间区的E(HI),E(CO),E(~(13)CO)和N/S(~(13)CO)的两维对比度约为1—2。  相似文献   

17.
We present results of a population synthesis study of radio-loud and radio-quiet γ-ray pulsars from the Galactic plane and the Gould Belt. The simulation includes the Parkes multibeam pulsar survey, realistic beam geometries for radio and γ-ray emission from neutron stars and the new electron density model of Cordes and Lazio. Normalizing to the number of radio pulsars observed by a set of nine radio surveys, the simulation suggests a neutron star birth rate of 1.4 neutron stars per century in the Galactic plane. In addition, the simulation predicts 19 radio-loud and 7 radio-quiet γ-ray pulsars from the plane that EGRET should have observed as point sources. Assuming that during the last 5 Myr the Gould Belt produced 100 neutron stars, only 10 of these would be observed as radio pulsars with three radio-loud and four radio-quiet γ-ray pulsars observed by EGRET. These results are in general agreement with the recent number of about 25 EGRET error boxes that contain Parkes radio pulsars. Since the Gould Belt pulsars are relatively close by, the selection of EGRET radio-quiet γ-ray pulsars strongly favors large impact angles, β, in the viewing geometry where the off-beam emission from curvature radiation provides the γ-ray flux. Therefore, the simulated EGRET radio-quiet γ-ray pulsars, being young and nearby, most closely reflect the current shape of the Gould Belt suggesting that such sources may significantly contribute to the EGRET unidentified γ-ray sources correlated with the Gould Belt.  相似文献   

18.
During the past decade we have compiled a large molecular line data base of massive star forming regions in the southern Milky Way. These regions are confined into giant molecular clouds that trace the galactic spiral arms. Their radial distribution has a pronounced peak midway between the Sun and the galactic center, which in the IV quadrant corresponds to the location of the Norma Spiral arm. We study in some detail one of the foremost regions of massive star formation in the Norma arm, using millimeter continuum and line emission maps obtained with the SEST, APEX, and ASTE telescopes. It is a multiple system evolving along a complete GMC core, candidate for future ALMA observations.  相似文献   

19.
Some unidentified EGRET sources have been reported to have probable X-ray counterparts. Periodicities in the X-ray data of those sources, if found, may help to strengthen the identification and to reveal their nature. We performed a detailed search of periodicities with a photon-counting method, the H-test, in the XMM and ASCA data of RX J0007.0+7302, which is the most probable X-ray counterpart to the EGRET source 3EG J0010+7309. Although no periods with enough significance were found, a possible one, at 0.1275433± 0.0000001 s (MJD 52327.03399), is quite intriguing based on results of cross-checking the two data sets. We suggest future analysis with other data to search the vicinity of this period.  相似文献   

20.
Star formation     
Various topics on star formation, centered on the observed properties of young stars and their environment, are reviewed. (a) In our Galaxy, young stellar objects are generally associated with giant molecular clouds. (b) Giant molecular clouds cannot be in free-fall collapse. They are probably stabilized by magnetic fields, which are then likely to dominate the dynamical evolution of the clouds themselves. (c) Star formation occurs mostly in spiral arms. The role of spiral density waves is however not yet clearly understood. (d) The formation of massive stars can perturb the evolution of the progenitor cloud, and possibly trigger the sequential formation of OB subgroups. (e) There is a large number of clouds in the Galaxy associated only with low and intermediate mass young stars. These clouds are not perturbed by the presence of massive stars, and are probably the best source of information on the primary triggering mechanism, active on a galactic scale, and on the initial conditions for star formation.Paper presented at the European Workshop on Planetary Sciences, organised by the Laboratorio di Astrofisica Spaziale di Frascati, and held between April 23–27, 1979, at the Accademia Nazionale del Lincei in Rome, Italy.  相似文献   

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