Ari Laor

The X-ray source in active galactic nuclei (AGNs) is commonly referred to as the corona by analogy to stellar coronae. The similarities between the two suggest that the heating mechanism of AGN coronae is magnetic reconnection—as in cool stars—but this has not yet been directly observed. This work presents the first observational evidence for a magnetic reconnection flare in an AGN corona. We report on a flare in NGC 3783, which was observed with XRISM/Xtend and XMM-Newton/EPIC exhibiting distinct temporal evolution in soft (2.0 keV) X-rays. An ultrafast outflow (UFO) was detected during the event. The flare features the Neupert effect—a temporal signature of the hard light curve correlating with the time derivative of the soft light curve, which shows that the flare is powered by magnetic reconnection. The Neupert effect is widely observed in the Sun, with coronal mass ejections (CMEs) playing a role analogous to the UFO. We derive an upper limit of 30 R _g on the height of the magnetic loop from which the flare originates. Using the UFO measured properties to characterize the magnetic field, we obtain B > 1.3 × 10 ⁴ G for the field annihilated during the flare from total energy considerations, and B ≈ 500 G for the momentary magnetic field during reconnection from a dynamical consideration.

Power-law (PL) distribution functions (DF) are prevalent in highly diverse systems. The systems range in size from nanometer to mega light years, in complexity from dust grains to living organisms, and characterize the distribution of various events in nature and in various human activities. To gain some insight on why PL DF are so prevalent, we explore the conditions leading to the formation of a PL DF in a simple system of colliding hard sphere. We follow the time evolution of the energy DF through direct Monte Carlo simulations. In statistical equilibrium, the DF evolves into the Maxwell-Boltzmann DF. A transition to a PL DF occurs when the following occurs: (1) The system is initially far from equilibrium. For example, a mix of light and heavy particles with the same velocity. (2) The system dynamics is scale free, which holds in the intermediate asymptotic regime, far from the initial and the final equilibrium states. The scale-free dynamics leads to a DF which evolves in a self-similar form. (3) The system is open with a scale-free boundary condition. For example, a constant injection of particles far from equilibrium. The DF PL index is set by the time dependence of the self-similar DF and by the boundary condition. The PL index is independent of the self-similar DF form. Conditions (1)–(3) are common in a great variety of systems, which may explain why PL DF are so prevalent in nature.

Broad Absorption Line Quasars (BALQs) generally exhibit significant outflows that may interact with the surrounding medium, resulting in radio emission. We selected a sample of 13 powerful radio-quiet (RQ) BALQs, where the UV outflow kinetic power is measurable, and detected nine of them with the Very Large Array A configuration at 5.5 and 9.0 GHz. The radio emission is mostly unresolved and is generally constrained within a scale of α _5.5−9.0 is steep (<−0.5) in five objects and is flat or inverted (>−0.5) in four objects. We discuss how the steep-slope emission can be associated with the UV outflows, and how the flat-slope emission can be intrinsically steep but flattened by free–free absorption from the UV outflowing gas. However, we find no correlation between the radio luminosity and the estimated outflow kinetic power, which suggests that the outflows are likely not a major source of the observed radio emission. In addition, the radio-loudness of these RQ BALQs is comparable to that of typical RQ quasars, implying that the UV outflows likely do not produce stronger radio emission compared to non-BALQs. Follow-up radio observations can test the free–free absorption interpretation and can be used as a new probe for outflows in AGN.

We conduct the first systematic survey of a total of 11 optical coronal lines (CLs) in the spectra of a large sample of low-redshift (z < 0.8) Type 1 quasars observed by the Sloan Digital Sky Survey (SDSS). We find that strong CL emission is rare in SDSS even in Type 1 quasars; only 885 out of 19,508 (4.5%) galaxies show at least one CL, with higher ionization potential lines (>100 eV) being even rarer. The [Ne v] λ3426 line, which constitutes the majority of detections, is strongly correlated with the bolometric luminosity. These findings suggest that the optical CLs are significantly suppressed in the majority of local active galactic nuclei (AGNs), possibly as a result of the presence of dust in the emitting regions. We find that the incidence of ionized outflows is significantly higher in CL emitters compared with non-CL emitters, possibly suggesting that dust destruction in outflows enhances CL emission in AGNs. Many CLs show line profiles that are broader than those of narrow lines, and are blueshifted relative to the lower ionization potential lines, suggesting outflows in the highly ionized gas. Given the limited number of detections, we do not find any statistically significant trends in detection statistics or line ratios with black hole mass, Eddington ratio, or AGN bolometric luminosity. The catalog is publicly available and can provide a useful database of the CL properties of low-redshift quasars that can be compared to the growing number of high-z AGNs discovered by JWST.

We present the parsec-scale radio spectra of a representative sample of 13 Palomar–Green radio-quiet quasars, based on our new Very Long Baseline Array (VLBA) observations at 8.4 and 23.6 GHz and our earlier VLBA studies at 1.5 and 5.0 GHz. The radio core emission generally exhibits a flat spectrum at 1.5–5.0 GHz, which indicates a compact optically thick synchrotron source on a scale smaller than the broad-line region (BLR) radius R _BLR ∼ 0.01–0.1 pc. The 8.4–23.6 GHz spectral slope remains flat in four objects indicating the inner radius of the radio source R _in < 0.1R _BLR, and becomes steep in four other objects indicating R _in ∼ 0.5 R _BLR. The flat 8.4–23.6 GHz slope sources may be associated with a continuous ejection starting at the accretion disk corona. The steep 8.4–23.6 GHz slope sources may be produced by an interaction of an active-galactic-nucleus-driven wind with the BLR gas or a low-power jet extending to the BLR scale. Seven of these eight objects, which have a flat or steep 8.4–23.6 GHz slope, reside at the Eddington ratios L/L _Edd < 0.3, and four of the remaining five objects, where the 8.4–23.6 GHz fluxes are too faint to significantly constrain the slope, reside at L/L _Edd > 0.3. The 8.4–23.6 GHz radio emission in the high-L/L _Edd objects may be weak due to more extended emission from a radiation-pressure-driven wind. Future submillimeter observations can further constrain the inward radial extent of the radio emission down to the coronal scale.

We present multifrequency (5–345 GHz) and multiresolution radio observations of 1ES 1927+654, widely considered one of the most unusual and extreme changing-look active galactic nuclei (CL-AGNs). The source was first designated a CL-AGN after an optical outburst in late 2017 and has since displayed considerable changes in X-ray emission, including the destruction and rebuilding of the X-ray corona in 2019–2020. Radio observations prior to 2023 show a faint and compact radio source typical of a radio-quiet AGN. Starting in 2023 February, 1ES 1927+654 began exhibiting a radio flare with a steep exponential rise, reaching a peak 60 times previous flux levels, and has maintained this higher level of radio emission for over a year to date. The 5–23 GHz spectrum is broadly similar to gigahertz-peaked radio sources, which are understood to be young radio jets less than ∼1000 yr old. Recent high-resolution Very Long Baseline Array observations at 23.5 GHz now show resolved extensions on either side of the core, with a separation of ∼0.15 pc, consistent with a new and mildly relativistic bipolar outflow. A steady increase in the soft X-ray band (0.3–2 keV) concurrent with the radio may be consistent with jet-driven shocked gas, though further observations are needed to test alternate scenarios. This source joins a growing number of CL-AGNs and tidal disruption events that show late-time radio activity, years after the initial outburst.

Forbidden collisionally excited optical atomic transitions from high-ionization-potential (IP ≥ 54.8 eV) ions, such as Ca⁴⁺, Ne⁴⁺, Fe⁶⁺, Fe¹⁰⁺, Fe¹³⁺, Ar⁹⁺, and S¹¹⁺, are known as optical coronal lines (CLs). The spectral energy distributions (SEDs) of active galactic nuclei (AGNs) typically extend to hundreds of electron volts and above, which should be able to produce such highly ionized gas. However, optical CLs are often not detected in AGNs. Here we use photoionization calculations with the cloudy spectral synthesis code to determine possible reasons for the rarity of these optical CLs. We calculate CL luminosities and equivalent widths from radiation-pressure-confined photoionized gas slabs exposed to an AGN continuum. We consider the role of dust, metallicity, and ionizing SED in the formation of optical CLs. We find that (i) dust reduces the strength of most CLs by ∼3 orders of magnitude, primarily as a result of depletion of metals onto the dust grains; (ii) in contrast to the CLs, the more widely observed lower-IP optical lines such as [O iii] 5007 Å are less affected by depletion, and some are actually enhanced in dusty gas; and (iii) many optical CLs become detectable in dustless gas, and are particularly strong for a hard ionizing SED. This implies that prominent CL emission likely originates in dustless gas. Our calculations also suggest optical CL emission is enhanced in galaxies with low-mass black holes characterized by a harder radiation field and a low dust-to-metals ratio. The fact that optical CLs are not widely observed in the early Universe with JWST may point to rapid dust formation at high redshift.

Does a broad-line region (BLR) wind in radio-quiet (RQ) active galactic nuclei (AGN) extend to parsec scales and produce radio emission? We explore the correlations between a parsec-scale radio wind and the BLR wind in a sample of 19 RQ Palomar–Green quasars. The radio wind is defined based on the spectral slope and the compactness of the emission at 1.5–5 GHz, and the BLR wind is defined by the excess blue wing in the C iv emission line profile. The five objects with both radio and BLR wind indicators are found at high Eddington ratios, L/L _Edd (≥0.66), and eight of the nine objects with neither radio nor BLR winds reside at low L/L _Edd (≤0.28). This suggests that the BLR wind and the radio wind in RQ AGN are related to a radiation-pressure-driven wind. Evidence for free–free absorption by AGN photoionized gas, which flattens the spectral slope, is found in two objects. Radio outflows in three low-L/L _Edd (0.05–0.12) objects are likely from a low-power jet, as suggested by additional evidence. The presence of a mild equatorial BLR wind in four intermediate-L/L _Edd (0.2–0.4) objects can be tested with future spectropolarimetry.

We present new 1.5–8.5 GHz Very Long Baseline Array (VLBA) observations and 0.32–1.26 GHz Giant Meterwave Radio Telescope (GMRT) observations of J0354−1340, which is the only known radio-quiet (RQ) or radio-intermediate (RI) narrow-line Seyfert 1 galaxy with a 100 kpc, two-sided radio jet. A parsec-scale, one-sided jet in the southeastern direction from the core emission is found in the VLBA observations, while the kiloparsec-scale jet observed with the Karl G. Jansky Very Large Array (VLA) and GMRT is in the south–north direction. Core spectra on parsec and kiloparsec scales are presented in combination with archival VLA Sky Survey observations at 3.0 GHz and VLA C-configuration observations at 5.5 GHz. The parsec-scale emission dominates the kiloparsec-scale emission above ∼5 GHz, and the spectrum is inverted due to synchrotron self-absorption. This indicates a compact synchrotron source with a size of ∼0.04 pc, which is associated with either the jet base or the corona. A subkiloparsec-scale jet, which is unresolved on scales of ∼3″, probably dominates the emission below ∼5 GHz. Future radio observations can explore the jet structure between the parsec and 100 kpc scales, the origin of their direction mismatch, and the parsec-scale jet proper motion. It remains to be explored how common such large-scale jets are in RQ or RI active galactic nuclei.

Recent studies have proposed that the nuclear millimeter continuum emission observed in nearby active galactic nuclei (AGNs) could be created by the same population of electrons that gives rise to the X-ray emission that is ubiquitously observed in accreting black holes. We present the results of a dedicated high-spatial-resolution (∼60–100 mas) Atacama Large Millimeter/submillimeter Array (ALMA) campaign on a volume-limited (10 keV) selected radio-quiet AGNs. We find an extremely high detection rate (25/26 or 94 − 6 + 3 % ), which shows that nuclear emission at millimeter wavelengths is nearly ubiquitous in accreting SMBHs. Our high-resolution observations show a tight correlation between the nuclear (1–23 pc) 100 GHz and the intrinsic X-ray emission (1σ scatter of 0.22 dex). The ratio between the 100 GHz continuum and the X-ray emission does not show any correlation with column density, black hole mass, Eddington ratio, or star formation rate, which suggests that the 100 GHz emission can be used as a proxy of SMBH accretion over a very broad range of these parameters. The strong correlation between 100 GHz and X-ray emission in radio-quiet AGNs could be used to estimate the column density based on the ratio between the observed 2–10 keV ( F 2 – 10 keV obs ) and 100 GHz (F _{100 GHz}) fluxes. Specifically, a ratio log ( F 2 – 10 keV obs / F 100 GHz ) ≤ 3.5 strongly suggests that a source is heavily obscured ( log ( N H / cm − 2 ) ≳ 23.8 ). Our work shows the potential of ALMA continuum observations to detect heavily obscured AGNs (up to an optical depth of one at 100 GHz, i.e., N _H ≃ 10²⁷ cm⁻²), and to identify binary SMBHs with separations <100 pc, which cannot be probed by current X-ray facilities.