High-energy Gamma-ray Sources Research Articles

Microquasars as High-energy Gamma-ray Sources

Galactic microquasars are certainly one of the most recent additions to the field of high energy astrophysics and have attracted increasing interest over the last decade. However, the high energy part of the spectrum of microquasars is the most poorly known, mainly due the lack of sensitive instrumentation in the past. Microquasars are now primary targets for all of the observatories working in the X-ray and gamma-ray domains. They also appear as the possible counterparts for some of the unidentified sources of high-energy gamma-rays detected by the experiment EGRET on board the satellite COMPTON-GRO. This paper provides a general review of the main observational results obtained up to now as well as a summary of the scenarios for production of high-energy γ-rays at the present moment.

PKS 1622-253: A Weakly Accreting, Powerful Gamma-Ray Source

In this Letter, we discuss new deep radio observations of PKS 1622-253 and their implications for the energetics of the central engine that powers this strong high energy gamma-ray source. Combining archival infrared and optical measurements with new millimeter observations, we show that even though the accretion flow in PKS 1622-253 is under-luminous by quasar standards, a powerful super-luminal jet is launched with a higher kinetic luminosity than most EGRET blazars. Only a few percent of the total jet kinetic luminosity is required to power even the most powerful gamma ray flares that are observed. The implication is that a high accretion system is not required to power the strongest high energy gamma ray sources.

A possible black hole in the -ray microquasar LS 5039

The population of high energy and very high energy gamma-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, conforms a reduced but physically important sample. Most of these sources are extragalactic (e.g., blazars), while among the galactic ones there are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al. (2000), has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high energy gamma-ray sources. To model and understand how the energetic photons are produced and escape from LS 5039 it is crucial to unveil the nature of the compact object, which remains unknown. Here we present new intermediate-dispersion spectroscopy of this source which, combined with values reported in the literature, provides an orbital period of 3.90603+/-0.00017 d, a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6 R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These, combined with our dynamical solution and the assumption of pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss about the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the SN explosion scenario, and the behaviour of the TeV emission with the new orbital period.

A Multiwavelength Investigation of Unidentified Egret Sources

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.

Observation of EGRET gamma-ray sources by an Extensive Air Shower experiment

Ultra-high-energy (E$>$100 Tev) Extensive Air Showers (EASs) have been monitored for a period of five years (1997 - 2003), using a small array of scintillator detectors in Tehran, Iran. The data have been analyzed to take in to account of the dependence of source counts with respect to the zenith angle. During a calendar year different sources come in the field of view of the detector at varying zenith angles. Because of varying thickness of the overlaying atmosphere, the shower count rate is extremely dependent on zenith angle which have been carefully analyzed over time. High energy gamma-ray sources from EGRET third catalogue where observed and the data were analyzed using an excess method. A number of upper limits for a number of EGRET sources were obtained, including 6 AGNs or probably AGNs and 4 unidentified sources.

A New Population of Very High Energy Gamma-Ray Sources in the Milky Way

Very high energy gamma-rays probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used to image cosmic particle accelerators. A first sensitive survey of the inner part of the Milky Way with the High Energy Stereoscopic System (HESS) reveals a population of eight previously unknown firmly detected sources of very high energy gamma-rays. At least two have no known radio or x-ray counterpart and may be representative of a new class of "dark" nucleonic cosmic ray sources.

Electronic Structure of Liquid Mercury Using Compton Scattering Technique

The isotropic Compton profile of mercury has been measured, using 661.65 keV gamma-rays from a 20 Ci 137Cs source. To extract the true experimental Compton line shape, besides the usual systematic corrections we have incorporated for the first time the background correction due to bremsstrahlung radiation generated by photo and Compton electrons. Theoretical computations have been carried out, using the renormalised-free-atom (RFA) for the electron configuration 4f145d106s2 and free electron models. It is found that the present experimental data with bremsstrahlung background correction are in better agreement with the RFA calculations. This work suggests the incorporation of the bremsstrahlung background correction in Compton scattering experiments of heavy materials using high-energy gamma-ray sources.

An Optical Survey of the Position Error Contours of Unidentified High-Energy Gamma-Ray Sources at Galactic Latitude |b| &gt; 20°

We present results from our optical survey of the position error contours (error boxes) of unidentified EGRET sources at mid-to-high Galactic latitude. It is our intention to search for potential blazars that may have been missed in the original identification process of the three EGRET Catalogs and supplementary publications. We have first searched the error contours of unidentified sources at |b| > 20° for flat-spectrum radio sources using the NASA/IPAC Extragalactic Database. For each such radio source found, we conducted optical searches for counterparts using the Palomar 60 inch (1.5 m) telescope and the University of Wyoming's 2.3 and 0.6 m telescopes. Many of the radio sources have plausible optical counterparts, and spectroscopy will be conducted at a later date to determine which of these sources are quasars or active galaxies. Results show that several sources are optically variable and/or have flat or inverted radio-to-millimeter spectra and are thus potential blazars.

A numerical model for the γ-ray emission of the microquasar LS 5039

The possible association between the microquasar LS 5039 and the EGRET source 3EG J1824-1514 suggests that microquasars could also be sources of high energy gamma-rays. In this paper, we explore, with a detailed numerical model, if this system can produce the emission detected by EGRET (>100 MeV) through inverse Compton (IC) scattering. Our numerical approach considers a population of relativistic electrons entrained in a cylindrical inhomogeneous jet, interacting with both the radiation and the magnetic fields, taking into account the Thomson and Klein-Nishina regimes of interaction. The computed spectrum reproduces the observed spectral characteristics at very high energy.

Identification of gamma-ray sources with Wolf-Rayet stars

An analysis of unidentified discrete sources of gamma-rays with energies E>100 MeV demonstrates that the spatial characteristics of this group of gamma-ray sources coincides with those of Wolf-Rayet stars. It is concluded that Wolf-Rayet stars are potential steady sources of high-energy gamma rays with mean luminosities L(>100 MeV)≈1035 erg/s.

Variability of EGRET Gamma‐Ray Sources

The variability of the high-energy gamma-ray sources in the Third EGRET Catalog is analyzed by a new method. We reanalyze the EGRET data to calculate a likelihood function for the flux of each source in each observation, for both detections and upper limits. These functions can be combined in a uniform manner with a simple model of the flux distribution to characterize the flux variation by a confidence interval for the relative standard deviation of the flux. The main result is a table of these values for almost all the cataloged sources. As expected, the identified pulsars are steady emitters and the blazars are mostly highly variable. The unidentified sources are heterogeneous, with greater variation at higher Galactic latitude. There is an indication that pulsar wind nebulae are associated with variable sources. There is a population of variable sources along the Galactic plane, concentrated in the inner spiral arms.

PSR J2021+3651: A Young Radio Pulsar Coincident with an Unidentified EGRET γ-Ray Source

We report on a deep search for radio pulsations toward five unidentified ASCA X-ray sources coincident with EGRET gamma-ray sources. This search has led to the discovery of a young and energetic pulsar using data obtained with the new Wideband Arecibo Pulsar Processor. PSR J2021+3651 is likely associated with the X-ray source AX J2021.1+3651, which in turn is likely associated with the COS B high energy gamma-ray source 2CG 075+00, also known as GeV J2020+3658 or 3EG J2021+3716. PSR J2021+3651 has a rotation period P = 104 ms and P_dot = 9.6x10^{-14}, implying a characteristic age ~17 kyr and a spin-down luminosity E_dot ~ 3.4x10^{36}ergs/s. The dispersion measure DM ~ 371 pc cm^{-3} is by far the highest of any observed pulsar in the Galactic longitude range 55 < l < 80. This DM suggests a distance d > 10 kpc, and a high gamma-ray efficiency of \~15%, but the true distance may be closer if there is a significant contribution to the DM from excess gas in the Cygnus region. The implied luminosity of the associated X-ray source suggests the X-ray emission is dominated by a pulsar wind nebula unresolved by ASCA.

Supermassive Black Holes in BL Lacertae Objects: Estimated Masses and Their Relation to Nuclear Luminosity

We compile a sample of 13 BL Lac objects for which rapid optical variabilities are reliably established and luminosities of the nuclei and the host galaxies have been well observed by the Hubble Space Telescope (HST). We present a new method for estimating the masses of the supermassive black holes (SMBHs) at the centers of these objects using the rapid optical variability, and find that the masses of the SMBHs range from 10(7.42) to 10(9.19) M(.). This mass range is quite similar to that of dark compact objects in galaxies. To further investigate the reliability of our method, we also compile another sample with nine high-energy gamma-ray emission sources of BL Lac objects and obtain the masses for the SMBHs in these objects using the Dermer & Gehrels method. We find that the masses derived from these two methods are quite similar. These results strongly imply that the estimation of masses of SMBHs by rapid optical variability is reliable. We examine the correlations of the masses with the luminosities of the nuclei and the host galaxies observed by HST, and find that the masses are significantly correlated with the luminosities of the nuclei, but not correlated with the luminosities of host galaxies at all. In addition, we examine the relation between the observed minimal time-scale of variation and the bolometric luminosities of the objects. We find that they obey the Abromowicz & Nobili relation. These results possibly imply that the luminosities of the nuclei are produced by accretion of the central SMBHs.

An Active Galactic Nucleus Identification for 3EG J2006−2321

We present a multiwavelength analysis of the high-energy gamma-ray source 3EG J2006-2321 (l = 1882, b = -2626). The flux of this source above 100 MeV is shown to be variable on timescales of days and months. Optical observations and careful examination of archived radio data indicate that its most probable identification is with PMN J2005-2310, a flat-spectrum radio quasar with a 5 GHz flux density of 260 mJy. Study of the V = 19.3 optical counterpart indicates a redshift of 0.833 and variable linear polarization. No X-ray source has been detected near the position of PMN J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper limit provides for a spectral energy distribution with global characteristics similar to those of known gamma-ray blazars. Taken together, these data indicate that 3EG J2006-2321, listed as unidentified in the Third EGRET Catalog, is a member of the blazar class of active galactic nuclei (AGNs). The 5 GHz radio flux density of this blazar is the lowest of the 68 EGRET-detected AGNs. The fact that EGRET has detected such a source has implications for unidentified EGRET sources, particularly those at high latitudes (|b| > 30°), many of which may be blazars.

3EG J2016+3657: Confirming an EGRET Blazar behind the Galactic Plane

We recently identified the blazar-like radio source G74.87+1.22 (B2013+370) as the counterpart of the high-energy gamma-ray source 3EG J2016+3657 in the Galactic plane. However, since most blazar identifications of EGRET sources are only probabilistic in quality even at high Galactic latitude, and since there also exists a population of unidentified Galactic EGRET sources, we sought to obtain additional evidence to support our assertion that 3EG J2016+3657 is a blazar. These new observations consist of a complete set of classifications for the 14 brightest ROSAT X-ray sources in the error circle, of which B2013+370 remains the most likely source of the gamma-rays. We also obtained further optical photometry of B2013+370 itself which shows that it is variable, providing additional evidence of its blazar nature. Interestingly, this field contains, in addition to the blazar, the plerionic supernova remnant CTB 87, which is too distant to be the EGRET source, and three newly discovered cataclysmic variables, all five of these X-ray sources falling within 16' of each other. This illustrates the daunting problem of obtaining complete identifications of EGRET sources in the Galactic plane.

A compact 3H(p,γ) 4He 19.8-MeV gamma-ray source for energy calibration at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) is a new 1000-t D 2O Cherenkov solar neutrino detector. A high-energy gamma-ray source is needed to calibrate SNO beyond the 8B solar neutrino endpoint of 15 MeV. This paper describes the design and construction of a source that generates 19.8-MeV gamma rays using the 3H(p,γ) 4He reaction (“pT”), and demonstrates that the source meets all the physical, operational and lifetime requirements for calibrating SNO. An ion source was built into this unit to generate and to accelerate protons up to 30 keV, and a high-purity scandium tritide target with a scandium–tritium atomic ratio of 1 : 2.0±0.2 was included. This pT source is the first self-contained, compact, and portable high-energy gamma-ray source ( E γ >10 MeV) .

A Systematic Search for Short‐Term Variability of EGRET Sources

The Third EGRET Catalog of High-Energy Gamma-Ray Sources contains 170 unidentified sources, and there is great interest in the nature of these sources. One means of determining source class is the study of flux variability on timescales of days; pulsars are believed to be stable on these timescales, while blazars are known to be highly variable. In addition, previous work has demonstrated that 3EG J0241-6103 and 3EG J1837-0606 are candidates for a new gamma-ray source class. These sources near the Galactic plane display transient behavior but cannot be associated with any known blazars. Although many instances of flaring active galactic nuclei have been reported, the EGRET database has not been systematically searched for occurrences of short-timescale (~1 day) variability. These considerations have led us to conduct a systematic search for short-term variability in EGRET data, covering all viewing periods through proposal cycle 4. Six 3EG catalog sources are reported here to display variability on short timescales; four of them are unidentified. In addition, three noncatalog variable sources are discussed.

V.H.E. gamma ray observation of AGN by the CAT telescope

The CAT imaging Telescope in the french Pyreness has successfully completed three years of observation of High Energy gamma ray sources, concentrated on the Crab used as a standard calibration source and other potentially emitting sources. Among those, the short term highly variable AGN : Mrk421 and Mrk501, have been extensively followed from 1997 through 1999, and many others have been searched for emission above the experimental energy threshold of 250 GeV. The coincidence between the high states of activity at many wavelength observed either by satellite-born or ground-based detectors may provide insight into the accelerating processes spanning a very wide energy range.

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High‐Energy Gamma‐Ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l,b=89,25). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. In addition there are no flat-spectrum radio sources in the vicinity. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed radio emission of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine.

Discovery of a new population of high-energy gamma-ray sources in the Milky Way

One of the great mysteries of the high-energy gamma-ray sky is the group of approximately 170 unidentified point sources found along the Galactic plane. They are more numerous than all other high-energy gamma-ray sources combined and, despite 20 years of effort, no clear counterparts have been found at other wavelengths. Here we report a new population of such objects. A cluster of approximately 20 faint sources appears north of the Galactic Centre, which is part of a broader class of faint objects at mid-latitudes. In addition, we show in a model-independent way that the mid-latitude sources are distinct from the population of bright unidentified sources along the Galactic plane. The distribution on the sky indicates that the faint mid-latitude sources are associated with the Gould belt of massive stars and gas clouds at approximately 600 light years distance, as has been previously suggested.