scholarly journals High-Energy Continuum Variability in Active Galactic Nuclei

1994 ◽  
Vol 159 ◽  
pp. 113-122
Author(s):  
Rick Edelson
Keyword(s):  
Accretion Disk ◽  
Thermal Emission ◽  
High Energy ◽  
Thermal Source ◽  
X Rays ◽  
Optical Monitoring ◽  
Bl Lac Objects ◽  
X Ray ◽  
Bl Lac ◽  
Bl Lacs

CGRO and IUE observations suggest that the strong, aperiodic variability seen in the Exosat long-look observations of AGN extends over a much wider energy band. Some BL Lac objects (but no Seyfert 1 galaxies) have shown X-ray variations which were so rapid that they violate the assumptions of isotropy inherent in the Eddington limit. In the ultraviolet, Seyfert 1s as a class show an anti-correlation between the variability amplitude and luminosity, while BL Lacs show a positive correlation. Furthermore, Seyfert 1s show strong flux-correlated spectral variability, while BL Lacs show little or none. All of this suggests that the high-energy continua of BL Lacs are beamed towards us, while the ultraviolet continua of Seyfert 1s are emitted isotropically.The November 1991 multi-waveband monitoring of the BL Lac PKS 2155−304 showed strong correlated variability, with the soft X-rays leading the ultraviolet by a few hours, and no measurable lag between the ultraviolet and optical down to a limit of ≲ 1.5 hr. This indicates that the X-rays from this BL Lac are not produced by Compton upscattering, and that the ultraviolet does not come directly from a thermal source such as an accretion disk. This also strongly constrains the relativistic jet model, suggesting that all of the radiation is produced in a flattened region like a shock front.Low temporal resolution ultraviolet/optical monitoring of the Seyfert 1 NGC 5548 in 1989 yielded a strong correlation with no measurable lag to a limit of ≲4 days, casting some doubt on the standard model of thermal emission from an accretion disk in Seyfert 1s. Upcoming X-ray/ultraviolet/optical monitoring of the Seyfert 1 NGC 4151 in December 1993 will have much faster sampling, to permit a strong test of both this model and the competing reprocessing model.

scholarly journals An X-ray Spectral Survey of BL Lac Objects

1989 ◽  
Vol 134 ◽  
pp. 191-193 ◽  
Author(s):  
P. Barr ◽  
P. Giommi ◽  
A. Pollock ◽  
G. Tagliaferri ◽  
D. Maccagni ◽  
...  
Keyword(s):  
High Energy ◽  
Oxygen Absorption ◽  
X Rays ◽  
Bl Lac Objects ◽  
Synchrotron Source ◽  
X Ray ◽  
High Energy Tail ◽  
Bl Lac ◽  
Spectral Survey ◽  
Bl Lacs

A wide variety of X-ray spectral forms has been reported in BL Lac objects. Concave spectra, i.e. a steep soft X-ray spectrum with a flat high energy tail, have been reported in a few of the brightest BL Lacs (e.g Urry 1986). Conversely, convex spectra (steep hard X-rays, flat soft X-ray spectrum) have also been reported, sometimes in the same objects (Madejski 1985, Barr et al 1988, George et al 1988). The high energy tails have usually been invoked as a signature of synchrotron-self-Compton emission. Two conflicting interpretations of the convex spectra have been made. Urry et al (1986) suggest absorption by a partially ionised medium, probably intrinsic to the BL Lac object, following the identification of an Oxygen absorption trough in the Einstein OGS spectrum of PKS 2155-304 by Canizares and Kruper (1984). Conversely, Barr et al (1988) attribute the hard X-ray steepening to energy loss mechanisms operating on a synchrotron source.

scholarly journals A new hard X-ray-selected sample of extreme high-energy peaked BL Lac objects and their TeV gamma-ray properties

2019 ◽  
Vol 486 (2) ◽  
pp. 1741-1762 ◽  
Author(s):  
L Foffano ◽  
E Prandini ◽  
A Franceschini ◽  
S Paiano
Keyword(s):  
Spectral Properties ◽  
Gamma Ray ◽  
Spectral Energy Distribution ◽  
Broad Band ◽  
High Energy ◽  
Spectral Energy ◽  
X Rays ◽  
Bl Lac Objects ◽  
X Ray ◽  
Bl Lac

ABSTRACT Extreme high-energy peaked BL Lac objects (EHBLs) are an emerging class of blazars with exceptional spectral properties. The non-thermal emission of the relativistic jet peaks in the spectral energy distribution (SED) plot with the synchrotron emission in X-rays and with the gamma-ray emission in the TeV range or above. These high photon energies may represent a challenge for the standard modelling of these sources. They are important for the implications on the indirect measurements of the extragalactic background light, the intergalactic magnetic field estimate, and the possible origin of extragalactic high-energy neutrinos. In this paper, we perform a comparative study of the multiwavelength spectra of 32 EHBL objects detected by the Swift-BAT telescope in the hard X-ray band and by the Fermi-LAT telescope in the high-energy gamma-ray band. The source sample presents uniform spectral properties in the broad-band SEDs, except for the TeV gamma-ray band where an interesting bimodality seems to emerge. This suggests that the EHBL class is not homogeneous, and a possible subclassification of the EHBLs may be unveiled. Furthermore, in order to increase the number of EHBLs and settle their statistics, we discuss the potential detectability of the 14 currently TeV gamma-ray undetected sources in our sample by the Cherenkov telescopes.

scholarly journals TeV-peaked candidate BL Lac objects

2019 ◽  
Vol 491 (2) ◽  
pp. 2771-2778 ◽  
Author(s):  
L Costamante
Keyword(s):  
Gamma Ray ◽  
Spectral Energy Distribution ◽  
Flux Ratio ◽  
High Energy ◽  
Spectral Energy ◽  
Bl Lac Objects ◽  
X Ray ◽  
Cherenkov Telescopes ◽  
Bl Lac ◽  
Bl Lacs

ABSTRACT BL Lac objects can be extreme in two ways: with their synchrotron emission, peaking beyond 1 keV in their spectral energy distribution, or with their gamma-ray emission, peaking at multi-TeV energies up to and beyond 10–20 TeV, like 1ES 0229+200. This second type of extreme BL Lacs – which we can name TeV-peaked BL Lacs – is not well explained by the usual synchrotron self-Compton scenarios for BL Lacs. These sources are also important as probes for the intergalactic diffuse infrared background and cosmic magnetic fields, as well as possible sites of production of ultra-high-energy cosmic rays and neutrinos. However, all these studies are hindered by their still very limited number. Here I propose a new, simple criterium to select the best candidates for TeV observations, specifically aimed at this peculiar type of BL Lac objects by combining X-ray, gamma-ray, and infrared data. It is based on the observation of a clustering towards a high X-ray to GeV gamma-ray flux ratio, and it does not rely on the radio flux or X-ray spectrum. This makes it suitable to find TeV-peaked sources also with very faint radio emission. Taking advantage of the Fermi all-sky gamma-ray survey applied to the ROMA-BZCAT and Sedentary Survey samples, I produce an initial list of 47 TeV-peaked candidates for observations with present and future air-Cherenkov telescopes.

scholarly journals Optical Short-Term Variability in the X-Ray-Selected BL Lac Object IE 0317+186 and the Radio-Selected BL Lac Object ON231

1989 ◽  
Vol 134 ◽  
pp. 108-109
Author(s):  
F.Z. Cheng ◽  
J.F. Lu ◽  
G.Z. Xie ◽  
K.H. Li ◽  
Z.L. Li ◽  
...  
Keyword(s):  
Light Curves ◽  
Optical Monitoring ◽  
National Observatory ◽  
Image System ◽  
Short Term ◽  
Bl Lac Objects ◽  
X Ray ◽  
Bl Lac ◽  
Ccd Image ◽  
Short Term Variability

In order to compare X-ray-selected BL Lac objects with radio-selected BL Lac objects, we have carried out optical monitoring of some of these objects for about three years at Yunnan Observatory in China. All observations have been made with a CCD-image system at the f/13.3 Cassegrain focus of the 102-cm RCC telescope. The CCD-image system was developed by Ye et al. in Kitt Peak National Observatory of USA (Ye et al., 1985). The filters used were as follows: B-GG385(2mm)+BG12(1mm)+BG18(1mm), V-GG495(2mm)+BG18(2mm). After observing many times, more complete light curves have obtained for the X-ray-selected BL Lac object IE 0317+186 and the radio-selected BL Lac object ON 231, respectively(Fig 1 and Fig 2). Fig 1 shows that IE 0317+186 has a characteristic timescale of about 4.5hours with an amplitudes of ΔV≃0.65 mag. Fig 2 indicates that a timescale of short-term variability in ON 231 is about 70 min with an amplitudes of ΔB≃0.8 mag.

scholarly journals Deep X-ray view of the Class I YSO Elias 29 with XMM-Newton and NuSTAR

2019 ◽  
Vol 623 ◽  
pp. A67 ◽  
Author(s):  
I. Pillitteri ◽  
S. Sciortino ◽  
F. Reale ◽  
G. Micela ◽  
C. Argiroffi ◽  
...  
Keyword(s):  
Thermal Emission ◽  
Total Duration ◽  
Circumstellar Disks ◽  
High Energy ◽  
Class I ◽  
Young Stellar Objects ◽  
X Rays ◽  
Coronal Emission ◽  
Compact Structure ◽  
X Ray

X-ray emission is a characteristic feature of young stellar objects (YSOs) and the result of the interplay between rotation, magnetism, and accretion. For this reason high energy phenomena are key elements to understand the process of star formation, the evolution of their circumstellar disks, and eventually the formation of planets. We investigated the X-ray characteristics of the Class I YSO Elias 29 with joint XMM-Newton and NuSTAR observations of total duration 300 ks and 450 ks, respectively. These are the first observations of a very young (<1 Myr) stellar object in a band encompassing simultaneously both soft and hard X-rays (0.3 − 10 keV in XMM-Newton and ≈3 − 80 keV in NuSTAR). The quiescent spectrum is well described by one thermal component at ∼4.2 keV absorbed by NH ∼ 5.5  ×  1022 cm−2. In addition to the hot Fe complex at 6.7 keV, we observed fluorescent emission from Fe at ∼6.4 keV, confirming the previous findings. The line at 6.4 keV is detected during quiescent and flaring states and its flux is variable. The equivalent width is found varying in the range ≈0.15 − 0.5 keV. These values make unrealistic a simple model with a centrally illuminated disk and suggest a role of the cavity containing Elias 29 and possible reverberation processes that could occur in it. We observed two flares that have durations of 20 ks and 50 ks, respectively, and we observed the first flare with both XMM-Newton and NuSTAR. For this flare, we used its peak temperature and timing as diagnostics to infer a loop size of about 1 − 2 R⊙ in length, which is about 20%–30% of the stellar radius. This implies a relatively compact structure. We systematically observed an increase in NH of a factor five during the flares. This behavior has been observed during flares previously detected in Elias 29 with XMM-Newton and ASCA. The phenomenon suggests that the flaring regions could be buried under the accretion streams and at high stellar latitudes because the X-rays from flares pass through gas denser than the gas along the line of sight of the quiescent corona. In a different scenario, a contribution from scattered soft photons to the primary coronal emission could mimic a shallower NH in the quiescent spectrum. In the spectrum of the full NuSTAR exposure, we detect hard X-ray emission in the band ≈20 − 80 keV which is in excess with respect to the thermal emission and that is significant at a level of ≥2σ. We speculate that the hard X-ray emission could be due to a population of energetic electrons accelerated by the magnetic field along the accretion streams. These particles, along with X-ray photons with E >  7.11 keV, could be responsible for pumping up the Fe fluorescence when hitting cold Fe in the circumstellar disk.

scholarly journals X-Rays from Active Galactic Nuclei

1983 ◽  
Vol 6 ◽  
pp. 491-498 ◽  
Author(s):  
A.C. Fabian
Keyword(s):  
Active Galactic Nuclei ◽  
Galactic Nuclei ◽  
Seyfert Galaxies ◽  
X Rays ◽  
Bl Lac Objects ◽  
X Ray ◽  
Bl Lac

Recent X-ray observations of active galactic nuclei and Seyfert galaxies in particular are briefly reviewed. The application of the efficiency limit to rapidly varying luminous sources such as NGC 6814 is discussed. It is argued that the variability and probable MeV spectral turnover imply that most of the electrons which radiate the observed flux are only mildly relativistic. A possible link between the steep soft X-ray spectra and featureless optical continua of BL Lac objects is considered.

scholarly journals The γ-ray sky seen at X-ray energies

2020 ◽  
Vol 638 ◽  
pp. A128 ◽  
Author(s):  
E. J. Marchesini ◽  
A. Paggi ◽  
F. Massaro ◽  
N. Masetti ◽  
R. D’Abrusco ◽  
...  
Keyword(s):  
Gamma Ray ◽  
Large Fraction ◽  
Large Area ◽  
Bl Lac Objects ◽  
X Ray ◽  
Positional Uncertainty ◽  
Bl Lac ◽  
Frequency Behavior ◽  
Bl Lacs ◽  
Selection Of

Context. Nearly 50% of all sources detected by the Fermi Large Area Telescope are classified as blazars or blazar candidates, one of the most elusive classes of active galaxies. Additional blazars can also be hidden within the sample of unidentified or unassociated γ-ray sources (UGSs) that constitute about one-third of all gamma-ray sources detected to date. We recently confirmed that the large majority of Fermi blazars of the BL Lac subclass have an X-ray counterpart. Aims. Using the X-ray properties of a BL Lac training set and combining these with archival multifrequency information, we aim to search for UGSs that could have a BL Lac source within their γ-ray positional uncertainty regions. Methods. We reduced and analyzed the Swift X-ray observations of a selected sample of 327 UGSs. We then compared the X-ray fluxes and hardness ratios of all sources detected in the pointed fields with those of known Fermi BL Lacs. Results. We find at least one X-ray source, lying within the γ-ray positional uncertainty at 95% confidence level, for 223 UGSs and a total of 464 X-ray sources in all fields analyzed. The X-ray properties of a large fraction of them, eventually combined with radio, infrared, and optical information, exhibit BL Lac multi-frequency behavior, thus allowing us to select high-confidence BL Lac candidates; some of them were recently observed during our optical spectroscopic campaign which confirmed their nature. Conclusions. We find that out of 50 X-ray sources that were confirmed as BL Lacs through optical spectroscopy, 12 do not show canonical mid-infrared or radio BL Lac properties. This indicates that the selection of X-ray BL Lac candidates is a strong method to find new counterparts within Fermi UGSs. Finally, we pinpoint a sample of 32 Swift/XRT candidate counterparts to Fermi UGSs that are most likely BL Lac objects.

X-Ray Emissions from Solar Flares and from Celestial Sources

1967 ◽  
Vol 1 (1) ◽  
pp. 4-5 ◽  
Author(s):  
T. A. Chubb
Keyword(s):  
Solar Flares ◽  
Thermal Emission ◽  
High Energy ◽  
Geiger Counter ◽  
X Rays ◽  
Large Measure ◽  
X Ray ◽  
Ion Chamber

One of the interesting questions in solar X-ray astronomy is the question as to whether the hard X-ray emission which occurs during major solar flares is a thermal or nonthermal phenomenon. The evidence for non-thermal emission has been based in large measure on a balloon-borne experiment by Peterson and Winckler, which constituted the first detection of high energy flare X-rays. In the Peterson-Winckler experiment, the incident solar X-rays were measured by both an ion chamber and a Geiger counter photometer, and from the ratio of responses, the hardness character of the incident X-rays was reduced. It was concluded that the observed result could have been explained in terms of the sudden non-thermal production of a group of electrons with energy of the order of 500 kilovolts.

scholarly journals X-Ray Observations of Active Galactic Nuclei

1983 ◽  
Vol 104 ◽  
pp. 347-347
Author(s):  
C. Megan Urry ◽  
Richard F. Mushotzky ◽  
Allyn F. Tennant ◽  
Elihu A. Boldt ◽  
Stephen S. Holt
Keyword(s):  
Luminosity Function ◽  
Galactic Nuclei ◽  
Seyfert Galaxies ◽  
Source Population ◽  
Bl Lac Objects ◽  
X Ray ◽  
Relativistic Beaming ◽  
Bl Lac ◽  
Ultraviolet Observations ◽  
Bl Lacs

HEAO 1 A2 and Einstein SSS spectral observations of Seyfert galaxies and BL Lac objects suggest that in both cases, the X-ray emission is due to relativistic particles. The five BL Lac objects have very soft spectra and at higher energies (above 10 keV) may have hard tails. Combining our X-ray data with radio, infrared, optical, and ultraviolet observations, we can fit the BL Lac spectra with the familiar synchrotron self-Compton model if we allow for relativistic beaming (Urry and Mushotzky 1982, Urry et al. 1982). We show that Doppler beaming of an underlying (Seyfert-like) source population flattens the observed luminosity function, and we emphasize that the relative numbers of BL Lacs and quasars in given spectral intervals are strong functions of selection effects, the degree of Doppler beaming, and the form of the intrinsic luminosity function.

NEW INSIGHT INTO GAMMA-RAY BLAZARS FROM THE FERMI-LAT

2010 ◽  
Vol 19 (06) ◽  
pp. 831-839 ◽  
Author(s):  
◽  
BENOIT LOTT
Keyword(s):  
Gamma Ray ◽  
High Energy ◽  
Test Statistic ◽  
Large Area ◽  
Bl Lac Objects ◽  
Spectrum Radio ◽  
Sky Survey ◽  
Bl Lac ◽  
Ngc 1275 ◽  
Bl Lacs

The first three months of sky-survey operation with the Large Area Telescope (LAT) on board the Fermi satellite revealed 132 bright sources at |b| > 10° with test statistic greater than 100 (corresponding to about 10σ). Two methods, based on the CGRaBS, CRATES and BZCat catalogs, indicated high-confidence associations of 106 of these sources with known AGNs. This sample is referred to as the LAT Bright AGN Sample (LBAS). It contains two radio galaxies, namely Centaurus A and NGC 1275, and 104 blazars consisting of 58 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, and four blazars with unknown classification. Remarkably, the LBAS includes 10 high-energy-peaked BL Lacs. Only 33 of the sources, plus two at |b| < 10°, were previously detected with EGRET, probably due to variability. The analysis of the gamma-ray properties of the LBAS sources reveals that the average GeV spectra of BL Lac objects are significantly harder than the spectra of FSRQs. Other spectral and variability blazar properties are discussed. Some prominent Fermi-detected radiogalaxies are presented.

Sign in / Sign up

Export Citation Format

Share Document