8,441 research outputs found

    A decades-long fast-rise-exponential-decay flare in low-luminosity AGN NGC 7213

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    We analysed the four-decades-long X-ray light curve of the low-luminosity active galactic nucleus (LLAGN) NGC 7213 and discovered a fast-rise-exponential-decay (FRED) pattern, i.e. the X-ray luminosity increased by a factor of β‰ˆ4\approx 4 within 200d, and then decreased exponentially with an ee-folding time β‰ˆ8116\approx 8116d (β‰ˆ22.2\approx 22.2 yr). For the theoretical understanding of the observations, we examined three variability models proposed in the literature: the thermal-viscous disc instability model, the radiation pressure instability model, and the tidal disruption event (TDE) model. We find that a delayed tidal disruption of a main-sequence star is most favourable; either the thermal-viscous disk instability model or radiation pressure instability model fails to explain some key properties observed, thus we argue them unlikely.Comment: Accepted for publication in MNRAS, updated version after proof correction

    Fundamental Plane of Black Hole Activity in Quiescent Regime

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    A correlation among the radio luminosity (LRL_{\rm R}), X-ray luminosity (LXL_{\rm X}), and black hole mass (MBHM_{\rm BH}) in active galactic nuclei (AGNs) and black hole binaries is known to exist and is called the "Fundamental Plane" of black hole activity. Yuan & Cui (2005) predicts that the radio/X-ray correlation index, ΞΎX\xi_{\rm X}, changes from ΞΎXβ‰ˆ0.6\xi_{\rm X}\approx 0.6 to ΞΎXβ‰ˆ1.2βˆ’1.3\xi_{\rm X}\approx 1.2-1.3 when LX/LEddL_{\rm X}/L_{\rm Edd} decreases below a critical value ∼10βˆ’6\sim 10^{-6}. While many works favor such a change, there are also several works claiming the opposite. In this paper, we gather from literature a largest quiescent AGN (defined as LX/LEdd<10βˆ’6L_{\rm X}/L_{\rm Edd} < 10^{-6}) sample to date, consisting of 7575 sources. We find that these quiescent AGNs follow a ΞΎXβ‰ˆ1.23\xi_{\rm X}\approx 1.23 radio/X-ray relationship, in excellent agreement with the Yuan \& Cui prediction. The reason for the discrepancy between the present result and some previous works is that their samples contain not only quiescent sources but also "normal" ones (i.e., LX/LEdd>10βˆ’6L_{\rm X}/L_{\rm Edd} > 10^{-6}). In this case, the quiescent sources will mix up with those normal ones in LRL_{\rm R} and LXL_{\rm X}. The value of ΞΎX\xi_{\rm X} will then be between 0.60.6 and ∼1.3\sim1.3, with the exact value being determined by the sample composition, i.e., the fraction of the quiescent and normal sources. Based on this result, we propose that a more physical way to study the Fundamental Plane is to replace LRL_{\rm R} and LXL_{\rm X} with LR/LEddL_{\rm R}/L_{\rm Edd} and LX/LEddL_{\rm X}/L_{\rm Edd}, respectively.Comment: 11 pages, 7 figures, accepted for publication in The Astrophysical Journa

    A strong negative correlation between radio loudness RUVR_{\rm UV} and optical-to-X-ray spectral index Ξ±ox\alpha_{\rm ox} in low-luminosity AGNs

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    It has been argued for years that the accretion mode changes from bright active galactic nuclei (AGNs) to low-luminosity AGNs (LLAGNs) at a rough dividing point of bolometric Eddington ratio λ∼10βˆ’2\lambda \sim 10^{-2}. In this work, we strengthen this scenario through investigation of the relationship between the radio loudness RUVR_{\rm UV} and the optical-to-X-ray spectral index Ξ±ox\alpha_{\rm ox} in LLAGNs with 10βˆ’6≲λ≲10βˆ’310^{-6} \lesssim \lambda \lesssim 10^{-3}. We compile from literature a sample of 32 LLAGNs, consisting 18 LINERs and 14 low Eddington ratio Seyfert galaxies, and observe a strong negative RUVR_{\rm UV}--Ξ±ox\alpha_{\rm ox} relationship, with large scatter in both RUVR_{\rm UV} and Ξ±ox\alpha_{\rm ox}. We further demonstrate that this negative correlation, and the additional two negative relationships reported in literature (RUVR_{\rm UV}--Ξ»\lambda and Ξ±ox\alpha_{\rm ox}--Ξ»\lambda correlations), can be understood consistently and comprehensively under the truncated accretion--jet model, the model that has been applied successfully applied to LLAGNs. We argue that the scatter in the observations are (mainly) due to the spread in the viscosity parameter Ξ±\alpha of a hot accretion flow, a parameter that potentially can serve as a diagnose of the strength and/or configuration of magnetic fields in accretion flows.Comment: 8 pages, 3 figures, 2 tables. Accepted by MNRA

    Relationship Between the Kinetic Power and Bolometric Luminosity of Jets: limitation from black hole X-ray binaries, active galactic nuclei, and gamma-ray bursts

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    The correlation between the kinetic power PjetP_{\rm jet} and intrinsic bolometric luminosity LjetL_{\rm jet} of jets may reveal the underlying jet physics in various black hole systems. Based on the recent work by Nemmen et al. (2012), we re-investigate this correlation with additional sources of black-hole X-ray binaries (BXBs) in hard/quiescent states and low-luminosity active galactic nuclei (LLAGNs). The new sample includes 29 sets of data from 7 BXBs and 20 LLAGNs, with PjetP_{\rm jet} and LjetL_{\rm jet} being derived from spectral modeling of the quasi-simultaneous multi-band spectra under the accretion-jet scenario. Compared to previous works, the range of luminosity is now enlarged to more than 2020 decades, i.e. from 1031erg/s10^{31}{\rm erg/s} to 1052erg/s10^{52}{\rm erg/s}, which allows for better constraining of the correlation. One notable result is that the jets in BXBs and LLAGNs almost follow the same Pjetβˆ’LjetP_{\rm jet} - L_{\rm jet} correlation that was obtained from blazars and gamma-ray bursts (GRBs). The slope indices we derived are 1.03Β±0.011.03\pm0.01 for the whole sample, 0.85Β±0.060.85\pm0.06 for BXB subsample, 0.71Β±0.110.71\pm0.11 for LLAGN subsample, and 1.01Β±0.051.01\pm0.05 for the LLAGN-blazar subsample, respectively. The correlation index around unit implies the independence of jet efficiency on the luminosity or kinetic power. Our results may further support the hypothesis that similar physical processes exist in the jets of various black hole systems.Comment: 5 pages, 3 figures, to appear in ApJL, updated versio

    X-ray spectra of hot accretion flows

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    We study radiative properties of hot accretion flows in a general relativistic model with an exact treatment of global Comptonization, developed in our recent works. We note a strong dependence of electron temperature on the strength of magnetic field and we clarify that the underlying mechanism involves the change of the flow structure, with more strongly magnetised flows approaching the slab geometry more closely. We find that the model with thermal synchrotron radiation being the main source of seed photons agrees with the spectral index vs Eddington ratio relation observed in black hole transients below 1 per cent of the Eddington luminosity, LEdd, and models with a weak direct heating of electrons (small delta) are more consistent with observations. Models with large delta predict slightly too soft spectra, furthermore, they strongly overpredict electron temperatures at ~0.01 LEdd. The low-luminosity spectra, at <0.001 LEdd, deviate from a power-law shape in the soft X-ray range and we note that the first-scattering bump often resembles a thermal like component, with the temperature of a few hundred eV, superimposed on a power-law spectrum. The model with thermal Comptonization of thermal synchrotron radiation does not agree with well studied AGNs observed below ~0.01 LEdd, for which there is a substantial evidence for the lack of an inner cold disc. This indicates that the model of hot flows powering AGNs should be revised, possibly by taking into account an additional (but internal to the flow) source of seed photons.Comment: 16 pages, 10 figures, accepted in MNRA

    Radiative heating in the kinetic mode of AGN feedback

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    AGN feedback is now widely believed to play a crucial role in the co-evolution between the central black hole and its host galaxy. Two feedback modes have been identified, namely the radiative and kinetic modes, which correspond to the luminous AGNs and low-luminosity AGNs (LLAGNs), respectively. In this paper, we investigate the radiative heating in the kinetic mode. This process is potentially important because: 1) the radiation power of LLAGNs is higher than the jet power over a wide parameter range, 2) the spectral energy distribution of LLAGNs is such that the radiative heating is more effective compared to that of luminous AGNs with the same luminosity, and 3) most of the time in the lifecycle of an AGN is spent in the LLAGNs phase. In this paper, adopting the characteristic broad-band spectral energy distributions of LLAGNs, we calculate the value of "Compton temperature" (TcT_{\rm c}), which determines the radiative heating by Compton scattering. We find that Tc∼(5βˆ’15)Γ—107T_{\rm c}\sim (5-15)\times 10^7 K, depending on the spectrum of individual LLAGN and at which distance from the black hole we evaluate the heating. We also compare this heating process with other radiative heating and cooling processes such as photoionization/recombination. Our result can be used for an accurate calculation of the radiative heating in the study of AGN feedback.Comment: 9 pages, 3 figures, 3 tables. ApJ accepte

    Jet-dominated quiescent states in black hole X-ray binaries: the case of V404 Cyg

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    The dynamical and radiative properties of the quiescent state (X-ray luminosity <1034Β ergsβˆ’1cmβˆ’2<10^{34}\ {\rm erg s^{-1} cm^{-2}}) of black hole X-ray transients (BHXTs) remains unclear, mainly because of low-luminosity and poor data quantity. We demonstrate that, the simultaneous multi-wavelength (including radio, optical, ultraviolet and X-ray bands) spectrum of V404 Cyg in its bright quiescent state can be well described by the radiation from the companion star and more importantly, the compact jet. Neither the outer thin disc nor the inner hot accretion flow is important in the total spectrum. Together with several additional recent findings, i.e. the power-law X-ray spectrum and the constant X-ray spectral shape (or constant photon index) in contrast to the dramatic change in the X-ray luminosity, we argue the quiescent state spectrum of BHXTs is actually jet-dominated. Observational features consistent with this jet model are also discussed as supporting evidences.Comment: accepted for the publication in MNRAS Letters, 5 pages, 2 figure
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