Spectral Models for Low-luminosity Active Galactic Nuclei in LINERs: The Role of Advection-dominated Accretion and Jets

We perform an exploratory study of the physical properties of accretion flows and jets in low-luminosity active galactic nuclei (LLAGNs) by modeling the spectral energy distributions (SEDs) of 12 LLAGNs in low-ionization nuclear emission-line regions (LINERs). These SEDs we constructed from high-resolution radio, X-ray and optical/UV observations of the immediate vicinity of the black hole. We adopt a coupled accretion-jet model comprising an inner advection-dominated accretion flow (ADAF) and an outer standard thin disk. We present best-fit models in which either the ADAF or the jet dominate the X-ray emission. Six sources in our sample display an optical-UV excess with respect to ADAF and jet models; this excess can be explained as emission from the truncated disk with transition radii 30-225 Rs in four of them. In almost all sources the optical emission can also be attributed to unresolved, old stellar clusters with masses ~1E7-1E8 Msun. We find evidence for a correlation between the accretion rate and jet power and an anti-correlation between the radio-loudness and the accretion rate. We confirm previous findings that the radio emission is severely underpredicted by ADAF models and explained by the relativistic jet. We find evidence for a nonlinear relation between the X-ray and bolometric luminosities and a slight IR excess in the average model SED compared to that of quasars. We suggest that the hardness of the X-ray spectrum can be used to identify the X-ray emission mechanism and discuss directions for progress in understanding the origin of the X-rays.Comment: Accepted for publication in MNRAS. 27 pages, 22 figures. arXiv admin note: substantial text overlap with arXiv:1112.464

A correlation between the highest energy cosmic rays and nearby active galactic nuclei detected by Fermi

We analyze the correlation of the positions of gamma-ray sources in the Fermi Large Area Telescope First Source Catalog (1FGL) and the First LAT Active Galactic Nuclei (AGN) Catalog (1LAC) with the arrival directions of ultra-high-energy cosmic rays (UHECRs) observed with the Pierre Auger Observatory, in order to investigate the origin of UHECRs. We find that Galactic sources and blazars identified in the 1FGL are not significantly correlated with UHECRs, while the 1LAC sources display a mild correlation (2.6 sigma level) on a ~2.4 degree angular scale. When selecting only the 1LAC AGNs closer than 200 Mpc, we find a strong association (5.4 sigma) between their positions and the directions of UHECRs on a ~17 degree angular scale; the probability of the observed configuration being due to an isotropic flux of cosmic rays is 5x10^{-8}. There is also a 5 sigma correlation with nearby 1LAC sources on a 6.5 degree scale. We identify 7 "gamma-ray loud" AGNs which are associated with UHECRs within ~17 degree and are likely candidates for the production sites of UHECRs: Centaurus A, NGC 4945, ESO 323-G77, 4C+04.77, NGC 1218, RX J0008.0+1450 and NGC 253. We interpret these results as providing additional support to the hypothesis of the origin of UHECRs in nearby extragalactic objects. As the angular scales of the correlations are large, we discuss the possibility that intervening magnetic fields might be considerably deflecting the trajectories of the particles on their way to Earth.Comment: 23 pages, 7 figures, accepted for publication in Ap

Hard X-ray Emission from the M87 AGN Detected with NuSTAR

M87 hosts a 3-6 billion solar mass black hole with a remarkable relativistic jet that has been regularly monitored in radio to TeV bands. However, hard X-ray emission \gtrsim 10keV, which would be expected to primarily come from the jet or the accretion flow, had never been detected from its unresolved X-ray core. We report NuSTAR detection up to 40 keV from the the central regions of M87. Together with simultaneous Chandra observations, we have constrained the dominant hard X-ray emission to be from its unresolved X-ray core, presumably in its quiescent state. The core spectrum is well fitted by a power law with photon index Gamma=2.11 (+0.15 -0.11). The measured flux density at 40 keV is consistent with a jet origin, although emission from the advection-dominated accretion flow cannot be completely ruled out. The detected hard X-ray emission is significantly lower than that predicted by synchrotron self-Compton models introduced to explain emission above a GeV.Comment: 5 pages, 4 figures, updated to better match the published version in the Astrophysical Journal Letters. A minor typo in the published version (angular scale should be 1 arcsec = 78 pc instead, no result of the paper is affected) is fixed her

Short timescale variations of the H{\alpha} double-peaked profile of the nucleus of NGC 1097

The broad (FWHM ~ 10,000 km/s) double-peaked H{\alpha} profile from the LINER/Seyfert 1 nucleus of NGC 1097 was discovered in 1991, and monitored for the following 11 years. The profile showed variations attributed to the rotation of gas in a non-axisymmetric Keplerian accretion disk, ionized by a varying radiatively inefficient accretion flow (RIAF) located in the inner parts of the disk. We present and model 11 new spectroscopic observations of the double-peaked profile taken between 2010 March and 2011 March. This series of observations was motivated by the finding that in 2010 March the flux in the double-peaked line was again strong, becoming, in 2010 December, even stronger than in the observations of a decade ago. We also discovered shorter timescale variations than in the previous observations: (1) the first, of ~7 days, is interpreted as due to "reverberation" of the variation of the ionizing source luminosity, and the timescale of 7 days as the light crossing time between the source and the accretion disk; this new timescale and its interpretation provides a distance between the emitting gas and the supermassive black hole and as such introduces a new constraint on its mass; (2) the second, of approximately 5 months, was attributed to the rotation of a spiral arm in the disk, which was found to occur on the dynamical timescale. We use two accretion disk models to fit theoretical profiles to the new data, both having non-axisymmetric emissivities produced by the presence of an one-armed spiral. Our modeling constrains the rotation period for the spiral to be approximately 18 months. This work supports our previous conclusion that the broad double-peaked Balmer emission lines in NGC 1097, and probably also in other low-luminosity active nuclei, originate from an accretion disk ionized by a central RIAF.Comment: Published in ApJ (2012 March). 13 pages, 11 figure

Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097

We present a model for the accretion flow around the supermassive black hole in the LINER nucleus of NGC 1097 which fits the optical to X-ray spectral energy distribution (SED). The X-ray segment of the SED is based on observations with the Chandra X-Ray Observatory, which are reported here for the first time. The inner part of the flow is modeled as a radiatively inefficient accretion flow (RIAF) and the outer part as a standard thin disk. The value of the transition radius (~225 Schwarzschild radii) between the RIAF and outer thin disk was obtained from our previous fitting of the double-peaked Balmer emission line profile, which originates in the thin disk. The black hole mass was inferred from measurements of the stellar velocity dispersion in the host galaxy. When these parameters are used in the accretion flow model, the SED can be successfully reproduced, which shows that the line profile model and the accretion flow model are consistent with each other. A small remaining excess in the near-UV is accounted by the contribution of an obscured starburst located within 9 pc from the nucleus, as we reported in an earlier paper. The radio flux is consistent with synchrotron emission of a relativistic jet modeled by means of the internal shock scenario. In an appendix we also analyze the Chandra X-ray observations of the ~1 kpc circumnuclear star-forming ring and of an ultraluminous compact X-ray source located outside the ring.Comment: 10 pages and 6 figures formatted with emulateapj, accepted for publication in Ap

Advection-dominated accretion, jets and the spectral energy distribution of LINERs

Low-luminosity active galactic nuclei (LLAGNs) represent the bulk of the AGN population in the present-day universe and they trace low-level accreting supermassive black holes. The observational properties of LLAGNs suggest that their central engines are intrinsically different from those of more luminous AGNs. It has been suggested that accretion in LLAGNs occurs via an advection-dominated accretion flow (ADAF) associated with strong jets. In order to probe the accretion physics in LLAGNs as a class, we model the multiwavelength spectral energy distributions (SEDs) of 24 LINERs (taken from a recent compilation by Eracleous et al.) with a coupled accretion-jet model. The accretion flow is modeled as an inner ADAF outside of which there is a truncated standard thin disk. These SEDs include radio, near-IR to near-UV HST data, and Chandra X-ray data. We find that the radio emission is severely underpredicted by ADAF models but can be explained by the relativistic jet. The origin of the X-ray radiation in most sources can be explained by three distinct scenarios: the X-rays can be dominated by emission from the ADAF, the jet, or from both components contributing at similar levels. From the model fits, we estimate important parameters of the central engine of LINERs, such as the mass accretion rate -- relevant for studies of the feeding of AGNs -- and the mass-loss rate in the jet and the jet power -- relevant for studies of the kinetic feedback from jets.Comment: 6 pages, 3 figures, to be published in the Proceedings of the IAU Symposium no. 267, "Co-Evolution of Central Black Holes and Galaxies: Feeding and Feedback", eds. B.M. Peterson, R.S. Somerville and T. Storchi-Bergmann, in pres

A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems

Black holes generate collimated, relativistic jets which have been observed in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies (active galactic nuclei; AGN). How jet physics scales from stellar black holes in GRBs to the supermassive ones in AGNs is still unknown. Here we show that jets produced by AGNs and GRBs exhibit the same correlation between the kinetic power carried by accelerated particles and the gamma-ray luminosity, with AGNs and GRBs lying at the low and high-luminosity ends, respectively, of the correlation. This result implies that the efficiency of energy dissipation in jets produced in black hole systems is similar over 10 orders of magnitude in jet power, establishing a physical analogy between AGN and GRBs.Comment: Published in Science, 338, 1445 (2012), DOI: 10.1126/science.1227416. This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. Corrected typo in equation 4 of the supplementary materia

Short-Timescale monitoring of the X-ray, UV and broad double-peak emission line of the nucleus of NGC 1097

Recent studies have suggested that the short-timescale ($\lesssim7$ days) variability of the broad ($\sim$10,000 km s$^{-1}$) double-peaked H$\alpha$ profile of the LINER nucleus of NGC1097 could be driven by a variable X-ray emission from a central radiatively inefficient accretion flow (RIAF). To test this scenario, we have monitored the NGC1097 nucleus in X-ray and UV continuum with Swift and the H$\alpha$ flux and profile in the optical spectrum using SOAR and Gemini-South from 2012 August to 2013 February. During the monitoring campaign, the H$\alpha$ flux remained at a very low level --- 3 times lower than the maximum flux observed in previous campaigns and showing only limited ($\sim 20\%$) variability. The X-ray variations were small, only $\sim 13\%$ throughout the campaign, while the UV did not show significant variations. We concluded that the timescale of the H$\alpha$ profile variation is close to the sampling interval of the optical observations, which results in only marginal correlation between the X-ray and H$\alpha$ fluxes. We have caught the AGN in NGC1097 in a very low activity state, in which the ionizing source was very weak and capable of ionizing just the innermost part of the gas in the disk. Nonetheless, the data presented here still support the picture in which the gas that emits the broad double-peaked Balmer lines is illuminated/ionized by a source of high-energy photons which is located interior to the inner radius of the line-emitting part of the disk.Comment: The paper contains 14 pages, 7 figures and is accepted for publication at the Astrophysical Journa

A nonthermal bomb explains the near-infrared superflare of Sgr A*

The Galactic center supermassive black hole, Sgr A*, has experienced a strong, unprecedented flare in May 2019 when its near-infrared luminosity reached much brighter levels than ever measured. We argue that an explosive event of particle acceleration to nonthermal energies in the innermost parts of the accretion flow---a nonthermal bomb---explains the near-IR light curve. We discuss potential mechanisms that could explain this event such as magnetic reconnection and relativistic turbulence acceleration. Multiwavelength monitoring of such superflares in radio, infrared and X-rays should allow a concrete test of the nonthermal bomb model and put better constraints on the mechanism that triggered the bomb.Instituto Argentino de Radioastronomí