Γ-ray Band Research Articles

Gamma‐Ray and Multi–Wave Band Emission from Gamma‐Ray–loud Blazars

Using multi-wave band data for 61 γ-ray-loud blazars (17 BL Lacertae objects and 44 flat-spectrum radio quasars [FSRQs]), we have studied the possible correlation between flux densities (FR, FK, FO, FX, and Fγ) in the radio, infrared, optical, X-ray, and γ-ray wave bands, in both the low and high states. For some blazars, it is hard to determine whether they are in a low or a high state because only one data point is available for each of them; initially, we exclude these blazars in our analysis. However, we include these blazars in later analysis by temporarily assuming them to be in a low state or a high state. Our main results are as follows. There are very strong correlations between FX and FO and between FO and FK in both low and high states. However, a strong correlation between FX and FK exists only in the low state. No definite correlation is found between γ-ray flux density and those of lower energy bands; however, there are hints of anticorrelation between Fγ and FX as well as Fγ and FO and a positive correlation between Fγ and FR. From these results we suggest that (1) photons from infrared to X-rays are emitted by the same particles via synchrotron radiation and (2) if γ-rays are mainly produced by an inverse Compton scattering mechanism, it seems that the up-scattered soft photons are from external photons rather than synchrotron photons.

Measurement of the Galactic X‐Ray/Gamma‐Ray Background Radiation: Contribution of Discrete Sources

The Galactic background radiation near the Scutum Arm was observed simultaneously with RXTE and OSSE in order to determine the spectral shape and the origin of the emission in the hard X-ray/soft gamma-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by 4 components: a high energy continuum dominating above 500 keV that can be characterized by a power law of photon index ~ 1.6 (an extrapolation from measurements above ~ 1 MeV); a positron annihilation line at 511 keV and positronium continuum; a variable hard X-ray/soft gamma-ray component that dominates between 10-200 keV (with a minimum detected flux of ~ 7.7 x 10^-7 photons cm^-2 s^-1 keV^-1 deg^-2 at 100 keV averaged over the field of view of OSSE) and that is well modeled by an exponentially cut off power law of photon index ~ 0.6 and energy cut off at ~ 41 keV; and finally a thermal plasma model of solar abundances and temperature of 2.6 keV that dominates below 10 keV. We estimate that the contribution of bright discrete sources to the minimum flux detected by OSSE was ~ 46% at 60 keV and ~ 20% at 100 keV. The remaining unresolved emission may be interpreted either as truly diffuse emission with a hard spectrum (such as that from inverse Compton scattering) or the superposition of discrete sources that have very hard spectra.

Hard X-ray/softγ-ray observations of the Galactic Background Radiation: unresolved discrete sources?

The Galactic background radiation near the Scutum Arm was observed simultaneously with the Rossi X-Ray Timing Explorer (RXTE) and the Oriented Scintillation Spectrometer Experiment (OSSE) in order to constrain the spectral shape and the origin of the emission in the hard X-ray/soft γ-ray band. The spectrum in the 3 keV to 1 MeV band is well modeled by 4 components: a high energy continuum dominating above 500 keV that can be characterized by a power law of photon index ∼1 .6 below 1 MeV; a positron annihilation line at 511 keV and positronium continuum; a variable hard X-ray/soft γ-ray component that dominates between 10-400 keV (with a minimum detected flux of ∼7.7 × 10—7 photons cm—2 s—1 keV—1 deg—2 at 100 keV averaged over the field of view of OSSE) and is well modeled by an exponentially cut off power law of photon index ∼0.6 and energy cut off at ∼41 keV; and finally a Raymond-Smith thermal plasma model of solar abundances and temperature ∼2.6 keV that dominates below 10 keV. We estimate that at 60 and 100 keV, the contribution of bright discrete sources to the minimum flux detected by OSSE was ∼49% and ∼20%, respectively. This implies that the spectrum of the residual, unresolved emission is harder than that measured with the above power law. This emission may be interpreted either as truly diffuse emission with a hard spectrum (such as that from inverse Compton scattering) or the superposition of discrete sources that have very hard spectra.

44Ti decay gamma-ray emission from young galactic supernova remnants

The discovery by COMPTEL of the 44Ti line emission at 1.16 MeV from the youngest known Galactic supernova remnant (SNR) Cas A has opened a new window for the investigation of SNR properties. This discovery also shows a way that could help to uncover missing young remnants of Galactic SNe that might have occurred some hundred years ago. Contrary to the situation at other wavelengths, in the γ-ray band the Galaxy is almost transparent, so that otherwise obscured supernova remnants may be detectable up to Av ∼ 103 in γ-ray line emission. This is one of the direct ways to complement historical observations of Galactic SNe. Here we present preliminary results of the 6 year 44Ti line emission survey performed by COMPTEL on-board the Compton Gamma-Ray Observatory (CGRO).

High‐Energy Break and Reflection Features in the Seyfert Galaxy MCG +8‐11‐11

We present the results from ASCA and OSSE simultaneous observations of the Seyfert 1.5 galaxy MCG +8-11-11 performed in 1995 August-September. The ASCA observations, split into two pointings of ~10 ks each, indicate a modest flux increase (~20%) in 3 days, possibly correlated to a softening of the 0.6-9 keV spectrum. The spectrum obtained combining data from both the observations is well described by a hard power law (Γ ~ 1.64 ± 0.03) absorbed by a column density slightly larger than the Galactic value. An iron line at 6.4 keV of EW ~ 400 eV is also required by the data. The simultaneous OSSE data are characterized by a much softer power law with photon index Γ=3.0+ 0.9−0.8 , strongly suggesting the presence of a spectral break in the hard X-ray/soft γ-ray band. In order to better investigate the complex hard X-ray spectrum of this source, a total spectrum has been produced by summing over all the observations of MCG +8-11-11 performed by OSSE so far. A joint fit to the complete OSSE data sets and our ASCA data clearly shows an exponential cutoff of the power law at ~300 keV (e-folding energy), as well as a quite strong reflection component. MCG +8-11-11 is then one of the few Seyfert galaxies for which a spectral break in the underlying continuum is detected unambiguously, fitting well in the X-ray/γ-ray description of co-added spectra of Seyfert galaxies. Though the break could be due to downscattering of X-rays on cold e+e- pairs in a nonthermal plasma, the inferred low compactness of this source favors instead thermal or quasi-thermal electron Comptonization in a structured corona as the leading process of high-energy radiation production.

Optical photometric monitoring of γ-ray loud blazars

New data from the optical monitoring of γ-ray loud blazars at the Torino Astronomical Observatory are presented. Observations have been taken in the Johnson's B, V, and Cousins' R bands with the REOSC telescope equipped with a pixel CCD camera. Many of the 22 monitored sources presented here show noticeable magnitude variations. Periods corresponding to pointings of the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory (CGRO) satellite are indicated on the light curves. The comparison of our data with those taken by CGRO in the γ-ray band will contribute to better understand the mechanism of the γ-ray emission. We finally show intranight light curves of 3C 66A and OJ 287, where microvariability was detected.

Towards a Better Understanding of Active Galactic Nuclei

Active Galactic Nuclei (AGN) are ideal sources for multi-wavelength studies as their emission can cover almost 20 orders of magnitude in frequency from the radio to the γ-ray band. After reviewing their basic properties, I will assess how well we know the multifrequency spectra of AGN as a class. I will then briefly illustrate how currently available and forthcoming sky surveys will help in addressing some of the open questions of AGN studies. Finally, an analysis of the problem of the missing Type 2 QSO will exemplify the dangers of monochromatic sky surveys for AGN.

The Infrared to Gamma‐Ray Pulse Shape of the Crab Nebula Pulsar

We analyze the pulse shape of the Crab Nebula pulsar in the near-infrared, optical, ultraviolet, X-ray, and γ-ray bands, including a previously unpublished ROSAT HRI pulse profile. We show that, in addition to the previously known trend for the fluences of the bridge and peak 2 to increase with energy relative to the fluence of peak 1, there is a small but statistically significant trend for both to decrease with energy relative to peak 1 over the near-infrared range. We find that the phase separation between the two peaks of the pulse profile decreases nearly continuously as a function of energy over seven decades of energy. We show that the peaks' full widths at half-maximum are significantly variable over this energy range, but without any clear pattern to the variability. We find that the differences between the energy dependences of the leading and trailing edge half-widths at half-maximum of both peaks found by Eikenberry et al. also continue over seven decades of energy. We show that the cusped shape of peak 2 reverses direction between the infrared/optical and X-ray/γ-ray bands, while the cusped shape of peak 1 shows weak evidence of reversing direction between the X-ray and γ-ray bands. Finally, we find that many of the pulse shape parameters show maxima or minima at energies of 0.5-1 eV, implying that an important change in the pulsar emission is occurring near this energy. Many of these complex phenomena are not predicted by current pulsar emission models, and offer new challenges for the development of such models.

X-Ray/Soft Gamma-Ray Observation of Centaurus A and Its Implication on the Emission Mechanism

We have studied Cen A (NGC 5128) in the X-ray band (3–20 keV) and soft γ-ray band (40–600 keV) with the Large Area Counter (LAC) of the Ginga satellite (1989 March and 1990 February) and with a balloon-borne low background detector (Welcome-1, 1991 November), respectively. The observed continuous spectra show a power-law shape (Γ∼ 1.8) with relatively heavy absorption (NH ∼ 1.5 × 1023 H cm−2) at the low-energy end and a possible break at ∼ 180 keV. We analyzed the total spectra as the sum of the direct power-law flux from the central engine, the Compton-scattered flux from a cold cloud near to the engine, and the iron fluorescence-line flux. By assuming that the geometry around the central engine remained unchanged during the two-year period, we studied two possible cold cloud geometries by comparing Monte-Carlo simulations with our observations: the first is the Compton-reflection model in which the cloud forms a slab covering 2π sr behind the central engine, the second is a case where the central source is totally surrounded by a cold cloud. We found that the latter geometry reproduces our data well; the observed power-law spectrum is then identified as direct flux from the central engine which undergoes a break at ∼ 180 keV.

The Radio-Gamma-Ray Connection: the Radio Properties of Gamma-Ray-Bright Blazars

Forty AGN have been detected at high significance level by EGRET in the γ-ray band. Previous studies based on radio observations near or at the times of the EGRET detections suggest that there is a causal connection between individual events in these two wavebands. Here we examine the question of whether the cm-λ and the γ-ray activity are related.

Long term variability of 3C279

Radio, mm, optical, UV and X-ray light curves of 3C 279 as well as the γ-ray light curve available since 1991 June are considered. Strong flux correlations are found between the X-ray and the radio-optical region. Variations at 100 MeV are well correlated to the optical and X-ray bands. The variability amplitude, as measured by an average logarithmic dispersion of the available measurements is small in the radio and increases systematically with increasing frequency up to the γ-ray band, with the possible exception of the X-ray amplitude.

Flares and Flickering in the Cataclysmic Variable AE Aquarii

Detected from the radio to the γ-ray bands, AE Aquarii is an extraordinary cataclysmic variable. It is a relatively bright (V ∼11.4), non-eclipsing binary with an orbital period of 9h.9. The mass-donor secondary star is of approximate spectral type K5V and contributes about ∼ 80% of the total flux in the optical. While the mass ratio is well determined at q = M2/M1 = 0.646, the individual stellar masses are poorly known since the inclination is unknown. AE Aqr’s flickering behavior is unique among CVs, alternating from periods of very violent activity to near total quiescence. Large amplitude flares have been detected in the radio, optical and UV. Previous work has shown the optical flare spectrum to be similar to stellar flare spectra.Four nights of exactly simultaneous Hα spectroscopy and photometry were obtained on the 2.5 and 1.5m telescopes at Mt. Wilson on 27-30 July 1982. The simultaneity allowed us to renomalize the spectroscopy to agree with the photometry, giving us spectrophotometric data (spectral resolution ∼ 50 km s−, time resolution ∼ 68). A total of ∼ 17h of data were obtained, resulting in 10,098 spectra.

Ultra-high-energy cosmic rays in the Magellanic Clouds

Ultra-high-energy (UHE) γ-rays have been detected from several X-ray binaries. UHE γ-rays from sources in the LMC will interact in the microwave background producing electrons which emit synchrotron radiation in the X-ray and γ-ray bands. This radiation might be observable.

Hard emission at late times from SN 1987A

view Abstract Citations (176) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hard Emission at Late Times from SN 1987A Woosley, S. E. ; Pinto, Philip A. ; Hartmann, Dieter Abstract Using a model for the explosion that has been remarkably successful in predicting and explaining the evolution of SN 1987A during its first one and one-half years we calculate the future photometric evolution of the supernova in the UV-optical-IR, X-ray, and γ-ray bands using Monte Carlo techniques. Special attention is given to the contribution from radioactive isotopes other than ^56^Co, notably ^57^Co, ^44^Ti, and ^22^Na, and to the possible appearance, probably within the next year, of X-rays from an accreting magnetic neutron star at the center of the supernova. The signature of a radio pulsar like the Crab is also considered. In both cases the time history of the bolometric light curve may be a more sensitive diagnostic than the existence of either pulses or hard emission, but will become confused in the near future by contributions from the rare radioactivities. A source as faint as the pulsed emission of the Crab pulsar will never be discernable in the bolometric light curve. Pulsed hard emission is best sought near 30 keV near the end of 1989 (and thereafter), although more sensitive instrumentation than hitherto employed may be necessary. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/168019 Bibcode: 1989ApJ...346..395W Keywords: Astronomical Models; Light Curve; Stellar Composition; Stellar Spectrophotometry; Supernova 1987a; Computational Astrophysics; Monte Carlo Method; Neutron Stars; Pulsars; X Ray Sources; Astrophysics; GAMMA RAYS: GENERAL; PULSARS; STARS: INDIVIDUAL ALPHANUMERIC: SN 1987A; STARS: SUPERNOVAE; STARS: X-RAYS full text sources ADS | data products SIMBAD (4) NED (2)

Prospects for precision physical experiments in optics

Until recently the sources of electromagnetic waves in the radio and γ-ray bands were much more monochromatic than optical sources. The advent of stable lasers with frequency reproducibility ~1014 has uncovered new possibilities of performing experiments of interest in physics. To illustrate these possibilities, results are cited of the measurements of the quadratic Doppler effect in a gas, the magnetic hyperfine structure of a vibrational-rotational transition of methane, and the recoil effect. A number of experiments that lasers will make possible in the near future are discussed, namely, high-accuracy measurements of the Rydberg constant using two-photon absorption, measurement of the quadratic Doppler shift, measurements of the gravitational frequency shift of the earth's field, and the possibility of observing parity nonconservation in atomic transitions.

Arrival Times of 100 to 400 keV Pulses from NP0532

THE observation of electromagnetic radiation from the pulsar in the Crab Nebula (NP0532) has been reported over at least 16 decades in energy from the radio band to the high energy γ-ray band1–8. In general, a double pulse structure is evident over this range in energy, although variations in the relative intensities and widths of the two pulses are found. In the optical, X-ray and γ-ray bands the pulsed emission is also seen to spread out into the region between the primary and secondary pulses.