scholarly journals Characterising the Extended Morphologies of BL Lacertae Objects at 144 MHz with LOFAR

2021 ◽  
Vol 257 (2) ◽  
pp. 30
Author(s):  
Seán Mooney ◽  
Francesco Massaro ◽  
John Quinn ◽  
Alessandro Capetti ◽  
Ranieri D. Baldi ◽  
...  
Keyword(s):  
Flux Density ◽  
Low Frequency ◽  
Photon Flux ◽  
Diffuse Emission ◽  
Bl Lac Objects ◽  
Bulk Motion ◽  
Extended Radio ◽  
Bl Lac ◽  
Spatially Extended ◽  
Extended Emission

Abstract We present a morphological and spectral study of a sample of 99 BL Lac objects using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lac objects, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115″, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulk motion in jets. The spatial extents and luminosities of the extended emission are consistent with the unification scheme for active galactic nuclei, where BL Lac objects correspond to low-excitation radio galaxies with the jet axis aligned along the line of sight. While extended emission is detected around the majority of BL Lac objects, the median 144–1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ∼42% on average to the total low-frequency flux density. A stronger correlation was found between the 144 MHz core flux density and the γ-ray photon flux (r = 0.69) than between the 144 MHz extended flux density and the γ-ray photon flux (r = 0.42). This suggests that the radio-to-γ-ray connection weakens at low radio frequencies because the population of particles that give rise to the γ-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially extended features.

scholarly journals Extended Emission in BL Lac Objects

1996 ◽  
Vol 175 ◽  
pp. 53-54
Author(s):  
M. Bondi ◽  
D. Dallacasa ◽  
C. Stanghellini ◽  
R. Della Ceca
Keyword(s):  
Magnetic Field ◽  
Statistical Analysis ◽  
Dynamic Range ◽  
Radio Sources ◽  
Parent Population ◽  
Bl Lac Objects ◽  
Extended Radio ◽  
Bl Lac ◽  
Bl Lacs ◽  
Extended Emission

The study of the extended emission and polarization properties of BL Lacs is an important step for the identification of their parent population. FRI radio sources, the supposed parent population of BL Lacs, have weaker extended radio luminosity and a dominant inferred magnetic field perpendicular to the jet, while FRII radio sources, the supposed parent population of quasars, have stronger extended radio power and an inferred magnetic field parallel to the jet. The only complete sample of radio selected BL Lacs (1 Jy sample, Stickel et al. 1991, ApJ, 374, 431) contains 34 objects. Unfortunately, about half of 1 Jy BL Lacs do not have very high dynamic range images, necessary to detect the low emissivity radio emission surrounding the bright compact source, either because the object was never observed, or because the observation was carried out at the beginning of 1980s with low sensitivity. In 1994 we started a programme using the VLA (A, B, and D configuration, see Table 1) and the WSRT (W in Table 1) to complete the high sensitivity radio imaging of the 1 Jy sample. We aim to investigate morphology and polarization properties, as well as the luminosity of the extended emission. This contribution presents the L band observations. The results are very preliminary, some of the data reduction is still in progress as well as the statistical analysis. The sources in Table 1 have been roughly classified as extended (E), or point-like (P) if no extended feature was detected. Among the 15 sources observed at the highest resolution 13 were classified as extended. In many sources we detect significantly much more extended flux than previously reported from earlier observations. Almost all the BL Lac objects we observed at the highest resolution show some extended features; furthermore, in a few cases, we detected emission on the arcminute scale. The power of the extended luminosity covers 3 orders of magnitude, 3 objects (0537–441, 0820+225, and 2240–260) have values typical of a FRII radio source. These new data will be used for an updated statistical analysis of the properties of the extended emission in the 1 Jy sample of BL Lac objects.

scholarly journals Gaia-assisted discovery of a detached low-ionisation BAL quasar with very large ejection velocities

2020 ◽  
Vol 634 ◽  
pp. A111 ◽  
Author(s):  
J. P. U. Fynbo ◽  
P. Møller ◽  
K. E. Heintz ◽  
J. N. Burchett ◽  
L. Christensen ◽  
...  
Keyword(s):  
Emission Line ◽  
Absorption Line ◽  
Emission Lines ◽  
Broad Absorption ◽  
Broad Absorption Line ◽  
Red Edge ◽  
First Case ◽  
Bl Lac ◽  
Spatially Extended ◽  
Extended Emission

We report on the discovery of a peculiar broad absorption line (BAL) quasar identified in our Gaia-assisted survey of red quasars. The systemic redshift of this quasar was difficult to establish because of the absence of conspicuous emission lines. Based on deep and broad BAL troughs of at least Si IV, C IV, and Al III, a redshift of z = 2.41 was established under the assumption that the systemic redshift can be inferred from the red edge of the BAL troughs. However, we observe a weak and spatially extended emission line at 4450 Å that is most likely due to Lyman-α emission, which implies a systemic redshift of z = 2.66 if correctly identified. There is also evidence for the onset of Lyman-α forest absorption bluewards of 4450 Å and evidence for Hα emission in the K band consistent with a systemic redshift of z = 2.66. If this redshift is correct, the quasar is an extreme example of a detached low-ionisation BAL quasar. The BALs must originate from material moving with very large velocities ranging from 22 000 km s−1 to 40 000 km s−1. To our knowledge, this is the first case of a systemic-redshift measurement based on extended Lyman-α emission for a BAL quasar. This method could also be useful in cases of sufficiently distant BL Lac quasars without systemic-redshift information.

scholarly journals Cm-Wavelength Flux Variability: Constraints on AGN Modeling

1994 ◽  
Vol 159 ◽  
pp. 497-497
Author(s):  
M.F. Aller ◽  
H.D. Aller ◽  
P.A. Hughes
Keyword(s):  
Flux Density ◽  
Linear Polarization ◽  
Total Flux ◽  
Bl Lac Objects ◽  
Bl Lacertae ◽  
Flux Variability ◽  
Bl Lac ◽  
Bl Lacs

To study whether the radio properties of BL Lacertae type objects and QSOs differ, we initiated a program in 1979 to monitor the total flux density and linear polarization at 14.5, 8.0 and 4.8 GHz of the strongest then known BL Lac objects which met the Hewitt-Burbidge criteria (Ledden, private communications) plus 3 subsequently identified high declination BL Lacs (Biermann et al. 1981). Results based on the behavior of the 45 brightest sample members are compared here with the properties of the QSOs in the flux-limited Pearson-Readhead sample (Pearson and Readhead 1988).

scholarly journals A cosmic collider: Was the IceCube neutrino generated in a precessing jet-jet interaction in TXS 0506+056?

2019 ◽  
Vol 630 ◽  
pp. A103 ◽  
Author(s):  
S. Britzen ◽  
C. Fendt ◽  
M. Böttcher ◽  
M. Zajaček ◽  
F. Jaron ◽  
...  
Keyword(s):  
Flux Density ◽  
Gamma Ray ◽  
Viewing Angle ◽  
Bl Lac Objects ◽  
Neutrino Event ◽  
Bl Lac ◽  
Before And After ◽  
Cosmic Neutrino ◽  
Two Jets ◽  
First Time

Context. The neutrino event IceCube−170922A appears to originate from the BL Lac object TXS 0506+056. To understand the neutrino creation process and to localize the emission site, we studied the radio images of the jet at 15 GHz. Aims. Other BL Lac objects show properties similar to those of TXS 0506+056, such as multiwavelength variability or a curved jet. However, to date only TXS 0506+056 has been identified as neutrino emitter. The aim of this paper is to determine what makes the parsec-scale jet of TXS 0506+056 specific in this respect. Methods. We reanalyzed and remodeled 16 VLBA 15 GHz observations between 2009 and 2018. We thoroughly examined the jet kinematics and flux-density evolution of individual jet components during the time of enhanced neutrino activity between September 2014 and March 2015, and in particular before and after the neutrino event. Results. Our results suggest that the jet is very strongly curved and most likely observable under a special viewing angle of close to zero. We may observe the interaction between jet features that cross each other’s paths. We find subsequent flux-density flaring of six components passing the likely collision site. In addition, we find a strong indication for precession of the inner jet, and model a precession period of about 10 yr via the Lense-Thirring effect. We discuss an alternative scenario, which is the interpretation of observing the signature of two jets within TXS 0506+056, again hinting toward a collision of jetted material. We essentially suggest that the neutrino emission may result from the interaction of jetted material in combination with a special viewing angle and jet precession. Conclusions. We propose that the enhanced neutrino activity during the neutrino flare in 2014–2015 and the single EHE neutrino IceCube-170922A could have been generated by a cosmic collision within TXS 0506+056. Our findings seem capable of explaining the neutrino generation at the time of a low gamma-ray flux and also indicate that TXS 0506+056 might be an atypical blazar. It seems to be the first time that a potential collision of two jets on parsec scales has been reported and that the detection of a cosmic neutrino might be traced back to a cosmic jet-collision.

scholarly journals Extended radio emission in BL Lac objects

1999 ◽  
Vol 139 (3) ◽  
pp. 601-616 ◽  
Author(s):  
P. Cassaro ◽  
C. Stanghellini ◽  
M. Bondi ◽  
D. Dallacasa ◽  
R. Della Ceca ◽  
...  
Keyword(s):  
Radio Emission ◽  
Bl Lac Objects ◽  
Extended Radio ◽  
Bl Lac

scholarly journals Very High Energy gamma-rays from blazars

2013 ◽  
Vol 9 (S304) ◽  
pp. 119-124
Author(s):  
Helene Sol
Keyword(s):  
Gamma Ray ◽  
Spectral Energy Distribution ◽  
Low Frequency ◽  
High Energy ◽  
Spectral Energy ◽  
Bl Lac Objects ◽  
Cherenkov Telescopes ◽  
Bl Lac ◽  
Very High Energy ◽  
Very High

AbstractThe extragalactic very high energy (VHE) gamma-ray sky is dominated at the moment by more than fifty blazars detected by the present imaging atmospheric Cherenkov telescopes (IACT), with a majority (about 90%) of high-frequency peaked BL Lac objects (HBL) and a small number of low-frequency peaked and intermediate BL Lac objects (LBL and IBL) and flat spectrum radio quasars (FSRQ). A significant variability is often observed, with time scales from a few minutes to months and years. The spectral energy distribution (SED) of these blazars typically shows two bumps from the radio to the TeV range, which can usually be described by leptonic or hadronic processes. While elementary bricks of the VHE emission scenarios seem now reasonably well identified, a global picture of these sources, describing the geometry and dynamics of the VHE zone, is not yet available. Multiwavelength monitoring and global alert network will be important to better constrain the picture, especially with the perspective of CTA, a major project of the next generation in ground-based gamma-ray astronomy.

scholarly journals Probing the origin of low-frequency radio emission in PG quasars with the uGMRT – I

2020 ◽  
Vol 499 (4) ◽  
pp. 5826-5839
Author(s):  
Silpa S ◽  
P Kharb ◽  
L C Ho ◽  
C H Ishwara-Chandra ◽  
M E Jarvis ◽  
...  
Keyword(s):  
Radio Emission ◽  
Spectral Index ◽  
Low Frequency ◽  
Spectral Indices ◽  
Strong Correlations ◽  
Frequency Data ◽  
Diffuse Emission ◽  
Very Large Array ◽  
Extended Radio ◽  
Activity State

ABSTRACT We present the results from 685 MHz observations with the upgraded Giant Metrewave Radio Telescope (uGMRT) of 22 quasars belonging to the Palomar-Green (PG) quasar sample. Only four sources reveal extended radio structures on ∼10–30 kpc scales, while the rest are largely a combination of a radio core unresolved at the uGMRT resolution of ∼3–5 arcsec, surrounded by diffuse emission on few kpc to ∼10 kpc scales. A few sources reveal signatures of barely resolved jets and lobes in their spectral index images that are created using the 685 MHz uGMRT data and similar resolution GHz-frequency data from the Very Large Array. On the basis of their position on the radio–IR correlation as well as the spectral index images, we find that the radio emission in the two radio-loud (RL) quasars and nearly one-third of the radio-quiet (RQ) quasars is active galactic nucleus (AGN) dominated whereas the remaining sources appear to have significant contributions from stellar-related processes along with the AGN. While the two RL sources exhibit inverted spectral index in their cores, the RQ sources exhibit a range of spectral indices varying from flat to steep (−0.1 $\gtrsim $ αR  $\gtrsim $ −1.1) indicating the presence of unresolved jets/lobes or winds. Except for a significant correlation between the 685 MHz radio luminosity and the Eddington ratio, we do not find strong correlations between other 685 MHz radio properties and black hole (BH) properties in the RQ PG sources. This lack of correlations could be explained by the contribution of stellar-related emission, or radio emission from previous AGN activity episodes that may not be related to the current BH activity state.

Extended Emission in BL Lac Objects

1996 ◽  
pp. 53-54
Author(s):  
M. Bondi ◽  
D. Dallacasa ◽  
C. Stanghellini ◽  
R. Della Ceca
Keyword(s):  
Bl Lac Objects ◽  
Bl Lac ◽  
Extended Emission

scholarly journals SHALON observations of High-Frequency- and Low-Frequency-Peaked BL Lac objects

2013 ◽  
Vol 409 ◽  
pp. 012112
Author(s):  
V G Sinitsyna ◽  
A Y Alaverdyan ◽  
M S Andreeva ◽  
S S Borisov ◽  
I A Ivanov ◽  
...  
Keyword(s):  
High Frequency ◽  
Low Frequency ◽  
Bl Lac Objects ◽  
Bl Lac

scholarly journals A Comparison of the X-ray Spectra of Quasars and BL Lac Objects

1989 ◽  
Vol 134 ◽  
pp. 197-198
Author(s):  
D. M. Worrall ◽  
B. J. Wilkes
Keyword(s):  
Power Law ◽  
Radio Spectrum ◽  
Spectral Indices ◽  
Mean Power ◽  
Bl Lac Objects ◽  
X Ray ◽  
Bl Lac ◽  
Infrared Frequencies

Quasars with similar core-compact radio properties can be classified by their differences at optical and infrared frequencies. Their X-ray properties might be expected to be similar if the synchrotron self-Compton mechanism relates their radio and X-ray emission. We have compared the 0.2–3.5 keV mean power-law energy spectral indices, , for 4 quasar classes: 12 Highly Polarized QSOs (HPQs), 19 Flat Radio Spectrum, core-compact, low-polarization, QSOs (FRS QSOs), 24 radio-selected BL Lac objects, and 7 X-ray-selected BL Lac objects.

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