2MASS observation of BL Lac objects II

In this paper, we have searched for the Two Micron All Sky Survey (2MASS) counterparts of 1434 BL Lacs. Eight hundred and thirty-three of 1434 BL Lacs (～58%) have spatially coincident 2MASS counterparts. Fermi-detected BL Lacs (FBLs) have a much higher 2MASS detection rate than non-Fermi-detected BL Lacs (non-FBLs) (～95% vs 49%). We compare the near-infrared (NIR) apparent magnitudes, monochromatic luminosities and spectral indices of different subclasses of BL Lacs: (i) FBLs are significantly brighter and more luminous than non-FBLs; (ii) low-energy peaked BL Lacs (LBLs) are significantly more luminous than high-energy peaked BL Lacs (HBLs); (iii) the NIR spectral index, α_IR, shows a sequence of decrease from LBLs to intermediate-energy peaked BL Lacs (IBLs) then to HBLs. The K_S band luminosity and NIR spectral index are both strongly anti-correlated with the synchrotron peak frequency (ν_peak). About 66% of BL Lacs in our sample are located close to the power law line in the J-H–H-K_S diagram, which implies predominantly non-thermal radiation in the NIR range. We label 218 BL Lacs whose NIR spectra show convex curvatures probably due to the significant contaminations of host-galaxy starlight. Finally we investigate the correlation between NIR and gamma-ray emission using the 2MASS observational data.     

Mid-infrared variability of blazars: a view from NEOWISE survey

We studied mid-infrared (MIR) variability of a large sample of blazars detected by LAT/Fermi (called Fermi-detected blazars) and those not detected by LAT/Fermi (called non-Fermi-detected blazars) in order to investigate any difference in the variability characteristics between them as well as between FSRQs and BL Lacs. Making use of NEOWISE archival data from October 2013 to December 2017, we constructed the long-term 3.4 μm and 4.6 μm light curves of 2,573 blazars and computed their intrinsic variability amplitudes, \(\varPsi \). As a result, we found that (1) Fermi-detected blazars show higher \(\varPsi \) than non-Fermi-detected blazars; (2) Fermi-detected FSRQs show higher \(\varPsi \) than non-Fermi-detected FSRQs; (3) Fermi-detected BL Lacs show higher \(\varPsi \) than non-Fermi-detected BL Lacs; (4) FSRQs show higher \(\varPsi \) than BL Lacs. By comparing their distributions of “\(\mathit{var}\_\mathit{flg}\)” in the ALLWISE database, we also found that Fermi-detected blazars/FSRQs/BL Lacs tend to be more variable than non-Fermi-detected blazars/FSRQs/BL Lacs. For the Fermi-detected blazars, there are highly significant correlations between the flux densities and spectral indices in the MIR and gamma-ray bands. These results imply that the activity in the two bands is connected. Our results are consistent with several earlier results on the differences between Fermi-detected blazars and non-Fermi-detected blazars. We also give some possible explanations about the statistical results.     

Correlation between the Gamma-ray and the Near-infrared Emissions from Gamma-ray-loud Blazars in the Low State

We have studied the correlations between the near-IR and the γ-ray bands using 29 observed γ-ray-loud blazars (16 BL Lac objects and 13flat-spectrum radio quasars (FSRQs)) with near-IR and γ-ray spectralindices and fluxes. The results show that there are very strong correlations between near-IR mean spectral index α_IR and γ-ray mean spectral index α_γ and between near-IR and γ-ray fluxes. Which means that γ-ray radiation from the blazars should be mainly produced by the synchrotron self-Compton process. In addition, the γ-ray emission of BL Lac objects may be somewhat different from flat-spectrum radio quasars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correlation between X-ray and γ-ray for Fermi blazars

Using γ-ray band data detected by Fermi Large Area Telescope (LAT) and X-ray band data for 78 blazars, we find a medium correlation between X-ray flux and γ-ray flux in all states. A medium correlation is also found between X-ray (1 keV) mean spectral index α _x and γ-ray mean spectral index α _γ for BL Lacertae objects (BL Lacs), and there is no correlation for Flat Spectrum Radio Quasars (FSRQs). From these results, we suggest that the most likely radiation mechanism for the high energy gamma-rays would be synchrotron self-Compton (SSC), and that the gamma-ray emission mechanism may be somewhat different for BL Lacs and FSRQs.     

Spectral fit of Cygnus X-1 in high energy— a self-consistent study

We fit the spectra of Cyg X-1 using two component advective flows with Keplerian accretion disks on the equatorial plane surrounded by sub-Keplerian disks when standing shocks are present. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multi-colour black body emission from the Keplerian disk are Comptonized by the thermal and non-thermal electrons. By varying Keplerian and sub-Keplerian rates we are able to reproduce the observed soft and hard states as far as X-ray region is concerned and ‘low γ-ray intensity’ and ‘high γ-ray intensity’ states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect as is observed in Cyg X-1.      

Non-thermal emission processes in massive binaries

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons, the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars. Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet binary—will be considered in details. Finally, strategies for future developments in this field will be discussed.     

Fermi耀变体的gamma射线多波段辐射与射电辐射的关系

从Fermi 3期源表(3FGL)中选择了一个含935个耀变体(blazar)的样本, 包括415个平谱射电源(Flat Spectrum Radio Quasar, FSRQ), 520个蝎虎天体(BL Lac object, BL Lac), 其中高同步峰BL Lac (HBL) 233个, 中同步峰BL Lac (IBL) 144个, 低同步峰BL Lac (LBL) 143个. 研究了总样本、FSRQ、BL Lac及其子类HBL、LBL的射电1.4GHz 与gamma射线在0.1、0.3、1、3、10GeV处辐射流量密度的关系. 结果显示: 所有样本的射电1.4GHz与5个波段gamma射线的辐射流量都有强相关, 相关系数在0.48--0.81之间, 机会概率均小于$10^{-4; 对于不同的样本相关系数随着gamma射线辐射频率的变化有不同的变化趋势, 所有样本在5个波段的相关系数平均值随gamma射线频率的增加而减小. 该结果暗示, 随着频率的升高, blazar的gamma射线辐射主导机制在发生变化, 在相同频率处, 不同类型天体的辐射主导机制存在差异; HBL的gamma射线辐射主要由同步自康普顿主导, 而LBL的其他成份比HBL的更复杂; FSRQ的gamma射线起源较BL Lac的复杂.     

BL Lac objects in the synchrotron proton blazar model

We calculate the spectral energy distribution (SED) of electromagnetic radiation and the spectrum of high-energy neutrinos from BL Lac objects in the context of the synchrotron proton blazar model. In this model, the high-energy hump of the SED is due to accelerated protons, while most of the low-energy hump is due to synchrotron radiation by co-accelerated electrons. To accelerate protons to sufficiently high energies to produce the high-energy hump, rather high magnetic fields are required. Assuming reasonable emission region volumes and Doppler factors, we then find that in low-frequency peaked BL Lacs (LBLs), which have higher luminosities than high-frequency peaked BL Lacs (HBLs), there is a significant contribution to the high-frequency hump of the SED from pion photoproduction and subsequent cascading, including synchrotron radiation by muons. In contrast, in HBLs we find that the high-frequency hump of the SED is dominated by proton synchrotron radiation. We are able to model the SED of typical LBLs and HBLs, and to model the famous 1997 flare of Markarian 501. We also calculate the expected neutrino output of typical BL Lac objects, and estimate the diffuse neutrino intensity due to all BL Lacs. Because pion photoproduction is inefficient in HBLs, as protons lose energy predominantly by synchrotron radiation, the contribution of LBLs dominates the diffuse neutrino intensity. We suggest that nearby LBLs may well be observable with future high-sensitivity TeV γ-ray telescopes.     

Models of Very-High-Energy Gamma-Ray Emission from the Jets of Microquasars: Orbital Modulation

The recent detection of very-high-energy (GeV – TeV) γ-ray emission from the Galactic black-hole candidate and microquasar LS 5039 has sparked renewed interest in jet models for the high-energy emission in those objects. In this work, we have focused on models in which the high-energy emission results from synchrotron and Compton emission by relativistic electrons in the jet (leptonic jet models). Particular attention has been paid to a possible orbital modulation of the high-energy emission due to azimuthal asymmetries caused by the presence of the companion star. Both orbital-phase dependentγγ absorption and Compton scattering of optical/UV photons from the companion star may lead to an orbital modulation of the gamma-ray emission. We make specific predictions which should be testable with refined data from HESS and the upcoming GLAST mission.     

A Study of the γ-ray and X-ray Radiations of BL Lac Objects

The X-ray and γ-ray flux densities of 18 γ-loud BL Lac objects as well as their average spectral indices of X-ray wave band (1 keV) and γ-ray wave band (>100 MeV) are collected, and the correlations among the various quantities are examined. The results indicate: (1) The X-ray flux density and the γ-ray flux density exhibit rather strong correlations in all the states, high, low or mediate. (2) Between the mean spectral indices in X-ray and γ-ray wave bands there is a comparatively intense anti-correlation. (3) Between the average spectral indices and the X-ray and γ- flux densities there is no evident correlation either in the high or the mediate state. (4) Between the average spectral index of γ-ray wave band and the flux density of X-rays in low state there is a rather strong anticorrelation. Between the mean spectral index of X-ray wave band and the γ-flux density in low state there also exists a weak correlation. The results of our analysis support the viewpoint that both the X-ray and γ-ray emissions of BL Lac objects come from the synchrotron radiation and synchrotron self Compton (SSC) radiation with one and the same distribution of relativistic electrons.     

The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background

We present a calculation of the blazar contribution to the extragalactic diffuse γ -ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ -ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies).
The observed log  N –log  S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Z _max=3. For Z _max=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background.
We predict a significant flattening of the γ -ray log  N –log  S function in the next two decades of flux below the EGRET threshold.     

Gamma-Ray and VLBI Radio Emission from Gamma-ray-loud Blazars

Based on the γ-ray data of 51 γ-ray-loud blazars in the third EGRET catalog and VLBI radio data at 2.29 GHz in high state. we studied the correlations between the VLBI and γ-rayluminosities and the fluxes using the partial correlation analysis method. The results show that there is a strong correlation between γ-ray and VLBI radio fluxes and a significant correlation between γ-ray and VLBI radio luminosities. These results are likely to provide a possible support for self-Compton model of the γ-ray emission in blazars. It also shows that the γ-ray radiation may come from the region within the order of 1 pc which is consistent with the results given by luminosity variation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electromagnetic and corpuscular emission from the solar flare of 1991 June 15: Continuous acceleraton of relativistic particles

Data on X-,γ-ray, optical and radio emission from the 1991 June 15 solar flare are considered. We have calculated the spectrum of protons that producesγ-rays during the gradual phase of the flare. The primary proton spectrum can be described as a Bessel-function-type up to 0.8 GeV and a power law with the spectral index ≈3 from 0.8 up to 10 GeV or above. We have also analyzed data on energetic particles near the Earth. Their spectrum differed from that of primary protons producingγ-ray line emission. In the gradual phase of the flare additional pulses of energy release occurred and the time profiles of cm-radio emission andγ-rays in the 0.8–10 MeV energy band and above 50 MeV coincided. A continuous and simultaneous stochastic acceleration of the protons and relativistic electrons at the gradual phase of the flare is considered as a natural explanation of the data.     

Low Luminosity BL Lacs

There are radio sources that have prominent synchrotron cores, and in some, e.g. BL Lacs, the non-thermal emission is still prominent at infrared/optical wavelengths. We have been analysing the Spectral Energy Distributions(SEDs) of a sample of radio-loud flat-spectrum low-luminosity objects that show a remarkable distribution of optical activity: from `Seyfert-like' objects, BL Lac objects to `normal' elliptical galaxies. The aim is to investigate if there is any correlation between differences in the SED and the optical classification. The results indicate that part of the optical diversity is not intrinsic, and that it is correlated with the frequency at which the synchrotron emission begins to decline. This revised version was published online in July 2006 with corrections to the Cover Date.     

A unifying view of the spectral energy distributions of blazars

We collect data at well-sampled frequencies from the radio to the γ-ray range for the following three complete samples of blazars: the Slew survey, the 1-Jy samples of BL Lacs and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The fraction of objects detected in γ-rays ( E  ≳ 100 MeV) is ∼ 17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, γ-ray detected sources do not differ either from other sources in each sample, or from all the γ-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities or of the broad-band spectral indices (radio to optical and radio to X-ray). We compute average spectral energy distributions (SEDs) from radio to γ-rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the γ-ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most γ-ray blazars, is presented in a separate paper.     

A Search for Tev Emission From X-ray Selected Agn with Milagro

A TeV emission component has been detected from several active galactic nuclei (AGN). The majority of TeV detections of AGN are for nearby (z < 0.13) X-ray selected BL Lacs in flaring states. The variability of these sources makes continuous monitoring necessary to detect flaring activity. The Milagro gamma-ray observatory is a wide-field (∼2 sr) instrument that operates continuously allowing daily observations of the Northern hemisphere. No significant detections of unidentified sources have been made to date. The observations are used to set limits on the time-integrated behavior of a set of X-ray selected BL Lacs. Costamante and Ghisellini [A&A 384 (2002) 56] present a selection of BL Lacs with X-ray and radio flux levels favorable for TeV emission and make predictions for the TeV flux levels. Twenty-seven objects selected from this sample are the subjects of a search for TeV emission on time scales from 8 days to 2.8 years in data taken between December 15, 2000, and September 8, 2003. None of the selected BL Lacs are detected on any of the time scales. Flux limits will be presented for each galaxy and time scale and comparisons will be made with the predicted emission. For the Milagro Collaboration.     

Identifying Variable γ-ray Sources Through Radio Observations

We present preliminary results of a campaign undertaken with different radio interferometers to observe a sample of the most variable unidentified EGRET sources. We expect to detect which of the possible counterparts of the γ-ray sources (any of the radio emitters in the field) varies in time with similar timescales as the γ-ray variation. If the γ-rays are produced in a jet-like source, as we have modelled theoretically, synchrotron emission is also expected at radio wavelengths. Such radio emission should appear variable in time and correlated with the γ-ray variability.     

High-energy gamma-ray emission from solar flares: Constraining the accelerated proton spectrum

Using a multi-component model to describe the γ-ray emission, we investigate the flares of December 16, 1988 and March 6, 1989 which exhibited unambiguous evidence of neutral pion decay. The observations are then combined with theoretical calculations of pion production to constrain the accelerated proton spectra. The detection of π⁰ emisson alone can indicate much about the energy distribution and spectral variation of the protons accelerated to pion producing energies. Here both the intensity and detailed spectral shape of the Doppler-broadened π⁰ decay feature are used to determine the spectral form of the accelerated proton energy distribution. The Doppler width of this γ-ray emission provides a unique diagnostic of the spectral shape at high energies, independent of any normalisation. To our knowledge, this is the first time that this diagnostic has been used to constrain the proton spectra. The form of the energetic proton distribution is found to be severely limited by the observed intensity and Doppler width of the π⁰ decay emission, demonstrating effectively the diagnostic capabilities of the π⁰ decay γ-rays. The spectral index derived from the γ-ray intensity is found to be much harder than that derived from the Doppler width. To reconcile this apparent discrepancy we investigate the effects of introducing a high-energy cut-off in the accelerated proton distribution. With cut-off energies of around 0.5–0.8 GeV and relatively hard spectra, the observed intensities and broadening can be reproduced with a single energetic proton distribution above the pion production threshold.     

H.E.S.S. observations of LS 5039

Recent observations of the binary system LS 5039 with the High Energy Stereoscopic System (H.E.S.S.) revealed that its Very High Energy (VHE) γ-ray emission is modulated at the 3.9 days orbital period of the system. The bulk of the emission is largely confined to half of the orbit, peaking around the inferior conjunction epoch of the compact object. The flux modulation provides the first indication of γ-ray absorption by pair production on the intense stellar photon field. This implies that the production region size must be not significantly greater than the gamma-gamma photosphere size (∼1 AU), thus excluding the large scale collimated outflows or jets (extending out to ∼1000 AU). A hardening of the spectrum is also observed at the same epoch between 0.2 and a few TeV which is unexpected under a pure absorption scenario and could rather arise from variation with phase in the maximum electron energy and/or the dominant VHE γ-ray production mechanism. This first-time observation of modulated γ-ray emission allows precise tests of the acceleration and emission models in binary systems. Mathieu de Naurois for the H.E.S.S. Collaboration.     

Radio-to-TeV γ-ray emission from PSR B1259–63

We discuss the implications of the recent X-ray and TeV γ-ray observations of the PSR B1259–63 system (a young rotation powered pulsar orbiting a Be star) for the theoretical models of interaction of pulsar and stellar winds. We show that previously considered models have problems to account for the observed behaviour of the system. We develop a model in which the broad band emission from the binary system is produced in result of collisions of GeV–TeV energy protons accelerated by the pulsar wind and interacting with the stellar disk. In this model the high energy γ-rays are produced in the decays of secondary neutral pions, while radio and X-ray emission are synchrotron and inverse Compton emission produced by low-energy (≤100 MeV) electrons from the decays of secondary charged π ^± mesons. This model can explain not only the observed energy spectra, but also the correlations between TeV, X-ray and radio emission components.