Compton Limit Research Articles

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron

Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron. Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 μas angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest (u, v)-distances of 1.1 × 1012 K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be > 11 to explain the high brightness temperatures.

Multiband RadioAstron space VLBI imaging of the jet in quasar S5 0836+710

Context. Detailed studies of relativistic jets in active galactic nuclei (AGN) require high-fidelity imaging at the highest possible resolution. This can be achieved using very long baseline interferometry (VLBI) at radio frequencies, combining worldwide (global) VLBI arrays of radio telescopes with a space-borne antenna on board a satellite. Aims. We present multiwavelength images made of the radio emission in the powerful quasar S5 0836+710, obtained using a global VLBI array and the antenna Spektr-R of the RadioAstron mission of the Russian Space Agency, with the goal of studying the internal structure and physics of the relativistic jet in this object. Methods. The RadioAstron observations at wavelengths of 18 cm, 6 cm, and 1.3 cm are part of the Key Science Program for imaging radio emission in strong AGN. The internal structure of the jet is studied by analyzing transverse intensity profiles and modeling the structural patterns developing in the flow. Results. The RadioAstron images reveal a wealth of structural detail in the jet of S5 0836+710 on angular scales ranging from 0.02 mas to 200 mas. Brightness temperatures in excess of 1013 K are measured in the jet, requiring Doppler factors of ≥100 for reconciling them with the inverse Compton limit. Several oscillatory patterns are identified in the ridge line of the jet and can be explained in terms of the Kelvin–Helmholtz (KH) instability. The oscillatory patterns are interpreted as the surface and body wavelengths of the helical mode of the KH instability. The interpretation provides estimates of the jet Mach number and of the ratio of the jet to the ambient density, which are found to be Mj ≈ 12 and η ≈ 0.33. The ratio of the jet to the ambient density should be conservatively considered an upper limit because its estimate relies on approximations.

Long-Term and Rapid Radio Variability of the Blazar 3C 454.3 in 2010–2017

The article presents the results of observations of the blazar 3C 454.3 (J2253+1608), obtained in 2010–2017 on the RATAN-600 radio telescope of the Special Astrophysical Observatory at 4.6, 8.2, 11.2, and 21.7 GHz and on the 32-m Zelenchuk and Badary radio telescopes of the Quasar VLBI Network of the Institute of Applied Astronomy at 4.84 and 8.57 GHz. Long-term variability of the radio emission is studied, as well as variability on time scales of several days and intraday variability (IDV). Two flares were observed in the long-term light curve, in 2010 and in 2015–2017. The flux density at 21.7 GHz increased by a factor of ten during these flares. The delay in the maximum of the first flare at 4.85 GHz relative to the maximum at 21.7 GHz was six months. The time scale for variability on the descending branch of the first flare at 21.7 GHz was τvar = 1.2 yrs, yielding an upper limit on the linear size of the emitting region of 0.4 pc, corresponding to an angular size of 0.06 mas. The brightness temperature during the flare exceeded the Compton limit, implying a Doppler factor δ = 3.5, consistent with the known presence of a relativistic jet oriented close to the line of sight. No significant variability on time scales from several days to several weeks was found in five sets of daily observations carried out over 120 days. IDV was detected at 8.57 GHz on the 32-m telescopes in 30 of 61 successful observing sessions, with the presence of IDV correlated with the maxima of flares. The characteristic time scale for the IDV was from two to ten hours. A number of IDV light curves show the presence of a time delay in the maxima in the light curves for simultaneous observations carried out on the Badary and Zelenchuk antennas, which are widely separated in longitude. This demonstrates that the IDV most like arises in the interstellar medium.

RadioAstron Science Program Five Years after Launch: Main Science Results

The RadioAstron ground-space interferometer provides the highest angular resolution achieved now in astronomy. The detection of interferometric fringes from quasars with this angular resolution on baselines of 100–200 thousand km suggests the brightness temperatures which exceed the Compton limit by two orders of magnitude. Polarimetric measurements on ground-space baselines have revealed fine structure testifying to recollimation shocks on scales of 100–250 μas and a helical magnetic field near the base of radio emission in BL Lacertae. Substructure within a the scattering disk of pulsar emission on interferometer baselines (from 60000 to 250000 km) was discovered. This substructure is produced by action of the interstellar interferometer with an effective baseline of about 1 AU and the effective angular resolution of better than 1 μas. Diameters of scattering disks were measured for several pulsars, and distances to diffusing screens were evaluated. The ground-space observations of sources of the maser radiation in lines of water and hydroxyl have shown that the maser sources in star-forming regions remain unresolved on baselines, which considerably exceed the Earth diameter. These very compact and bright features with angular sizes of about 20–60 μas correspond to linear sizes of about 5–10 million km (several solar diameters).

Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron

RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R satellite, launched in 2011, performing Space Very Long Baseline Interferometry (SVLBI) observations supported by a global ground array of radio telescopes. With an apogee of about 350 000 km, it is offering for the first time the possibility to perform {\mu}as-resolution imaging in the cm-band. We present observations at 22 GHz of 3C 273, performed in 2014, designed to reach a maximum baseline of approximately nine Earth diameters. Reaching an angular resolution of 0.3 mas, we study a particularly low-activity state of the source, and estimate the nuclear region brightness temperature, comparing with the extreme one detected one year before during the RadioAstron early science period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study the kinematics of the jet in a 1.5-year time window. We find that the nuclear brightness temperature is two orders of magnitude lower than the exceptionally high value detected in 2013 with RadioAstron at the same frequency (1.4x10^13 K, source-frame), and even one order of magnitude lower than the equipartition value. The kinematics analysis at 43 GHz shows that a new component was ejected 2 months after the 2013 epoch, visible also in our 22 GHz map presented here. Consequently this was located upstream of the core during the brightness temperature peak. These observations confirm that the previously detected extreme brightness temperature in 3C 273, exceeding the inverse Compton limit, is a short-lived phenomenon caused by a temporary departure from equipartition. Thus, the availability of interferometric baselines capable of providing {\mu}as angular resolution does not systematically imply measured brightness temperatures over the known physical limits for astrophysical sources.

PKS 1954–388: RadioAstron Detection on 80,000 km Baselines and Multiwavelength Observations

AbstractWe present results from a multiwavelength study of the blazar PKS 1954–388 at radio, UV, X-ray, and gamma-ray energies. A RadioAstron observation at 1.66 GHz in June 2012 resulted in the detection of interferometric fringes on baselines of 6.2 Earth-diameters. This suggests a source frame brightness temperature of greater than 2 × 1012 K, well in excess of both equipartition and inverse Compton limits and implying the existence of Doppler boosting in the core. An 8.4-GHz TANAMI VLBI image, made less than a month after the RadioAstron observations, is consistent with a previously reported superluminal motion for a jet component. Flux density monitoring with the Australia Telescope Compact Array confirms previous evidence for long-term variability that increases with observing frequency. A search for more rapid variability revealed no evidence for significant day-scale flux density variation. The ATCA light-curve reveals a strong radio flare beginning in late 2013, which peaks higher, and earlier, at higher frequencies. Comparison with the Fermi gamma-ray light-curve indicates this followed ~ 9 months after the start of a prolonged gamma-ray high-state—a radio lag comparable to that seen in other blazars. The multiwavelength data are combined to derive a Spectral Energy Distribution, which is fitted by a one-zone synchrotron-self-Compton (SSC) model with the addition of external Compton (EC) emission.

Simple model dielectric functions for insulators

The Drude dielectric function is a simple way of describing the dielectric function of free electron materials, which have an uniform electron density, in a classical way. The Mermin dielectric function describes a free electron gas, but is based on quantum physics. More complex metals have varying electron densities and are often described by a sum of Drude dielectric functions, the weight of each function being taken proportional to the volume with the corresponding density. Here we describe a slight variation on the Drude dielectric functions that describes insulators in a semi-classical way and a form of the Levine-Louie dielectric function including a relaxation time that does the same within the framework of quantum physics. In the optical limit the semi-classical description of an insulator and the quantum physics description coincide, in the same way as the Drude and Mermin dielectric function coincide in the optical limit for metals. There is a simple relation between the coefficients used in the classical and quantum approaches, a relation that ensures that the obtained dielectric function corresponds to the right static refractive index.For water we give a comparison of the model dielectric function at non-zero momentum with inelastic X-ray measurements, both at relative small momenta and in the Compton limit. The Levine-Louie dielectric function including a relaxation time describes the spectra at small momentum quite well, but in the Compton limit there are significant deviations.

Variability of Blazars and Blazar Models over 38 Years

Since 1978, when BL Lac objects and violently variable quasars were married to become “blazars”, physical interpretations of the phenomenon have evolved. Remarkably, though, the general picture of relativistic jets beaming their radiation in our direction, proposed that year by Blandford and Rees, remains intact. The main stress on theoretical models has come from observations that reveal ever more extreme variability requiring rampant particle acceleration on time-scales of minutes, often parsecs away from the central black hole. Here the author reviews many of the observations and theoretical ideas that have shaped his studies of blazars over about 40 years. This leads to his preferred scenario that blazar jets contain a helical magnetic field close to the black hole, turbulent plasma on parsecs scales, and both standing and moving shock waves. Particle acceleration can then occur in multiple stages involving the second-order Fermi process, magnetic reconnections, and modest jumps in energy at shock fronts. The most extreme variability, as well as brightness temperatures ~100 times the inverse Compton limit, probably require occasional exceptionally high bulk Lorentz factors. These can result, for example, from supersonic, relativistic turbulence, or ultra-relativistic flows propelled from sites of magnetic reconnection. Future efforts in these and other areas can determine whether these potential solutions are valid.

Radio and gamma-ray properties of extragalactic jets from the TANAMI sample

Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of -30{\deg}) gamma-ray bright AGN simultaneously with Fermi/LAT monitoring of their gamma-ray emission. We present the radio and gamma-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright gamma-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17% of galaxies detected by the LAT. Upper limits were established on the gamma-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The gamma-ray and radio luminosities are related by $L_\gamma \propto L_r^{0.89+-0.04}$. The brightness temperatures of the radio cores increase with the average gamma-ray luminosity, and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower gamma/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are strongly gamma-ray-fainter than the Fermi/LAT-detected ones, their gamma-ray luminosity should not be significantly lower than the upper limits calculated here.

A SEARCH FOR AGN INTRA-DAY VARIABILITY WITH KVN

Active galactic nuclei (AGN) are known for irregular variability on all time scales, down to intra-day variability with relative variations of a few percent within minutes to hours. On such short timescales, unexplored territory, such as the possible existence of a shortest characteristic time scale of activity and the shape of the high frequency end of AGN power spectra, still exists. We present the results of AGN single-dish fast photometry performed with the Korean VLBI Network (KVN). Observations were done in a "anti-correlated" mode using two antennas, with always at least one antenna pointing at the target. This results in an effective time resolution of less than three minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability, if any. We were able to derive high-quality light curves for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86 GHz, between May 2012 and April 2013. We performed a detailed statistical analysis in order to assess the levels of variability and the corresponding upper limits. We found upper limits on flux variability ranging from $\sim$1.6% to $\sim$7.6%. The upper limits on the derived brightness temperatures exceed the inverse Compton limit by three to six orders of magnitude. From our results, plus comparison with data obtained by the University of Michigan Radio Astronomy Observatory, we conclude that we have not detected source-intrinsic variability which would have to occur at sub-per cent levels.

Measurements and simulation of Faraday rotation across the Coma radio relic

The aim of this work is to probe the magnetic field properties in relics and infall regions of galaxy clusters using Faraday Rotation Measures. We present Very Large Array multi-frequency observations of seven sources in the region South-West of the Coma cluster, where the infalling group NGC4839 and the relic 1253+275 are located. The Faraday Rotation Measure maps for the observed sources are derived and analysed to study the magnetic field in the South-West region of Coma. We discuss how to interpret the data by comparing observed and mock rotation measures maps that are produced simulating different 3-dimensional magnetic field models. The magnetic field model that gives the best fit to the Coma central region underestimates the rotation measure in the South-West region by a factor ~6, and no significant jump in the rotation measure data is found at the position of the relic. We explore different possibilities to reconcile observed and mock rotation measure trends, and conclude that an amplification of the magnetic field along the South-West sector is the most plausible solution. Our data together with recent X-ray estimates of the gas density obtained with Suzaku suggest that a magnetic field amplification by a factor ~3 is required throughout the entire South-West region in order to reconcile real and mock rotation measures trends. The magnetic field in the relic region is inferred to be ~2 microG, consistent with Inverse Compton limits.

Multi-Epoch VLBI observations of the extreme scintillator J1819+3845

AbstractThe quasar J1819+3845 has shown extreme variability with flux density variations in the radio regime up to 600% in less than one hour. In case of intrinsic high varibility, the short time scale sets a limit on the size of the emitting region and allows to estimate its brightness temperature. This would exceed 1021 K in the case of J1819+3845. Even an high relativistic jet beamed and doppler boosted in our line of sight cannot explain such an extreme violation of the Inverse Compton limit (1012 K). The variability of this source has been proven to be due to scattering in the Interstellar medium by a number of different experiments. Such an explanation requires a closeby scattering screen (few parsecs) and it results in a brightness temperature of about 1014 K. Many observing campaigns have been carried on to map the innermost jet structures of J1819+3845. Here we present the results of a number of VLBI observations, including space VLBI, to search for the missing jet in this puzzling source.

MAXIMUM BRIGHTNESS TEMPERATURE OF AN INCOHERENT SYNCHROTRON SOURCE : INVERSE COMPTON LIMIT—A MISNOMER

We show that an upper limit of ~ 10^{12} K on the peak brightness temperature for an incoherent synchrotron radio source, commonly referred to in the literature as an inverse Compton limit, may not really be due to inverse Compton effects. We show that a somewhat tighter limit T_{eq} ~ 10^{11} is actually obtained for the condition of equipartition of energy between radiating particles and magnetic fields which happens to be a configuration of minimum energy for a self-absorbed synchrotron radio source. An order of magnitude change in brightness temperature from T_{eq} in either direction would require departures from equipartition of about eight orders of magnitude, implying a change in total energy of the system up to ~ 10^{4} times the equipartition value. Constraints of such extreme energy variations imply that brightness temperatures may not depart much from T_{eq}. This is supported by the fact that at the spectral turnover, brightness temperatures much lower than ~ 10^{11} K are also not seen in VLBI observations. Higher brightness temperatures in particular, would require in the source not only many orders of magnitude higher additional energy for the relativistic particles but also many order of magnitude weaker magnetic fields. Diamagnetic effects do not allow such extreme conditions, keeping the brightness temperatures close to the equipartition value, which is well below the limit where inverse Compton effects become important.

Compton limit of the Josephson supercurrent

Coulomb-blockade suppressed electron transport through tunnel junctions is a well-understood phenomenon. The complementary effect appears in Josephson point contacts due to the restricted horizon which equals the Compton wavelength of the Josephson junction. This leads to a strongly reduced critical current as well as a residual contact resistance due to quantum tunneling of the phase. When the lead capacitance becomes smaller than its natural unit ${e}^{2}/\ensuremath{\hbar}c\ensuremath{\approx}0.81\text{ }\text{pF}/\text{m}$, the Josephson phase transforms into the Bloch lattice.

Doppler boosting, superluminal motion, and the kinematics of AGN jets

We discuss results from a decade long program to study the fine-scale structure and the kinematics of relativistic AGN jets with the aim of better understanding the acceleration and collimation of the relativistic plasma forming AGN jets. From the observed distribution of brightness temperature, apparent velocity, flux density, time variability, and apparent luminosity, the intrinsic properties of the jets including Lorentz factor, luminosity, orientation, and brightness temperature are discussed. Special attention is given to the jet in M87, which has been studied over a wide range of wavelengths and which, due to its proximity, is observed with excellent spatial resolution. Most radio jets appear quite linear, but we also observe curved non-linear jets and non-radial motions. Sometimes, different features in a given jet appear to follow the same curved path but there is evidence for ballistic trajectories as well. The data are best fit with a distribution of Lorentz factors extending up to γ∼30 and intrinsic luminosity up to ∼1026 W Hz−1. In general, gamma-ray quasars may have somewhat larger Lorentz factors than non gamma-ray quasars. Initially the observed brightness temperature near the base of the jet extend up to ∼5×1013 K which is well in excess of the inverse Compton limit and corresponds to a large excess of particle energy over magnetic energy. However, more typically, the observed brightness temperatures are ∼2×1011 K, i.e., closer to equipartition.

Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71

We report on a densely time sampled polarimetric flux density monitoring of the BL Lac object S5 0716+71 at 86 GHz and 229 GHz. The source was observed with the IRAM 30 m telescope at Pico Veleta within a coordinated multi-frequency observing campaign, which was centred around a 500 ks INTEGRAL observation during November 10 to 16, 2003. The aim of this campaign was to search for signatures of inverse-Compton catastrophes through the observation of the broad-band variability of the source. At 86 GHz, S5 0716+71 showed no intra-day variability, but showed remarkable inter-day variability with a flux density increase of 34% during the first four observing days, which cannot be explained by source extrinsic causes. At this frequency, making use of a new calibration strategy, we reach a relative rms accuracy of the flux density measurements of 1.2%. Although the flux density variability at 229 GHz was consistent with that at 86 GHz, the larger measurement errors at 229 GHz do not allow us to detect, with high confidence, inter-day variations at this frequency. At 86 GHz, the linear polarization fraction of S5 0716+71 was unusually large . Inter-day variability in linear polarization at 86 GHz, with significance level ; % and , was observed during the first four observing days. From the total flux density variations at the synchrotron turnover frequency () we compute an apparent brightness temperature K at a redshift of 0.3, which exceeds by two orders of magnitude the inverse-Compton limit. A relativistic correction for with a Doppler factor brings the observed brightness temperature down to the inverse Compton limit. A more accurate lower limit of , consistent with previous estimates from VLBI observations, is obtained from the comparison of the 86 GHz synchrotron flux density and the upper limits for the synchrotron self-Compton flux density obtained from the INTEGRAL observations. The relativistic beaming of the emission by this high Doppler factor explains the non-detection of “catastrophic” inverse-Compton avalanches by INTEGRAL.

Milliarcsecond-Scale Spectral Properties and Jet Motions in M 87

Abstract We have combined high resolution VLBI Space Observatory Programme (VSOP) data at 1.6 and 4.8 GHz with Very Long Baseline Array (VLBA) data at higher frequencies and with similar resolutions to study the spectral properties of the core of M 87 with milliarcsec resolution. The VSOP data allows a more accurate measurement of the turnover frequency, and hence more reliable determination of associated physical parameters of the source. Comparison of the images with previously published images yields no evidence for significant motions of components in the parsec-scale jet. In addition, the brightness temperatures obtained from modelfits to the core are well below the inverse Compton limit, suggesting that the radio emission we are observing is not strongly Doppler boosted.

Evidence for a Population of Beamed Radio‐intermediate Quasars

Whether radio intermediate quasars possess relativistic jets as radio-loud quasars is an important issue in the understanding of the origin of radio emission in quasars. In this letter, using the two-epoch radio data obtained during Faint Image of Radio Sky at Twenty centimeter sky (FIRST) and NOAO VLA Sky Survey (NVSS), we identify 89 radio variable sources in the Sloan Digital Sky Survey. Among them, more than half are radio intermediate quasars (RL=f2cm/f2500A<250). For all objects with available multiple band radio observations, the radio spectra are either flat or inverted. The brightness temperature inferred from the variability is larger than the synchrotron-self Compton limit for a stationary source in 87 objects, indicating of relativistic beaming. Considering the sample selection and viewing angle effect, we conclude that relativistic jets probably exist in a substantianl fraction of radio intermediate quasars.

Polar Outflows in Six Broad Absorption Line Quasars

Using radio observations by FIRST and NVSS, we build a sample of 151 radio-variable quasars selected from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Six (and probably another two) of these objects are classified as broad absorption line (BAL) quasars and show radio flux variations of a few tens of percent within 1.5-5 yr. Such large amplitudes of variations imply brightness temperatures much higher than the inverse Compton limits (10(12) K) in all these BAL quasars, suggesting the presence of relativistic jets beaming toward the observer. The angles between the outflow and the jet are constrained to be less than similar to 20 degrees. Such BAL quasars with polar outflows are beyond the simple unification models of BAL and non-BAL quasars, which hypothesize BAL quasars as otherwise normal quasars seen nearly edge-on.

Diffractive interstellar scintillation of the quasar J1819+3845 at ${\vec{\lambda}}$21 cm

We report the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at cm which resemble diffractive interstellar scintillations observed in pulsars. The observations were taken with the Westerbork Synthesis Radio Telescope on a dozen occasions in the period between Aug. 2002 and Jan. 2005. The data were sampled at both high temporal and high frequency resolution and have an overall simultaneous frequency span of up to 600 MHz. In constructing the light curves and dynamic spectra the confusion from background sources has been eliminated. The timescale (down to 20 min) and the bandwidth (frequency decorrelation bandwidth of 160 MHz) of the observed variations jointly imply that the component of the source exhibiting this scintillation must possess a brightness temperature well in excess of the inverse Compton limit. A specific model in which both the source and scintillation pattern are isotropic implies a brightness temperature K, where previous estimates place the distance to the scattering medium in the range pc, yielding a minimum brightness temperature >20 times the inverse Compton limit. An independent estimate of the screen distance using the 21 cm scintillation properties alone indicates a minimum screen distance of pc and a brightness temperature above K. There is no evidence for anisotropy in the scattering medium or source from the scintillation characteristics, but these estimates may be reduced by a factor comparable to the axial ratio if the source is indeed elongated. The observed scintillation properties of J1819+3845 at 21 cm are compared with those at 6 cm, where a significantly larger source size has been deduced for the bulk of the emission by Dennett-Thorpe & de Bruyn (2003). However, opacity effects within the source and the different angular scales probed in the regimes of weak and strong scattering complicate this comparison.