H. T. Liu

Absorption of 10-200 GeV Gamma Rays by Radiation from Broad-Line Regions in Blazars

In this paper, we study the photon-photon pair production optical depth for gamma rays with energies from 10 to 200 GeV emitted by powerful blazars due to the diffuse radiation field of the broad-line region (BLR). There are four key parameters in the BLR model employed to determine the gamma-gamma attenuation optical depth of these gamma rays. They are the gamma-ray emitting radius R-gamma, the BLR luminosity L-BLR, the BLR half thickness h, and the ratio tau(BLR)/f(cov) of the Thomson optical depth to the covering factor of BLR. For FSRQs, on average, it is impossible for gamma rays with energies from 10 to 200 GeV to escape from the diffuse radiation field of the BLR. If GLAST could detect these gamma rays for most of FSRQs, the gamma-ray emitting region is likely to be outside the cavity formed by the BLR. Otherwise, the emitting region is likely to be inside the BLR cavity. As examples, we estimate the photon-photon absorption optical depth of gamma rays with energies from 10 to 200 GeV for two powerful blazars, HFSRQ PKS 0405-123 and FSRQ 3C 279. Comparing our results with GLAST observations in the future could test whether the model employed and the relevant assumptions in this paper are reliable and reasonable, and then limit constraints on the position of the gamma-ray emitting region relative to the BLR and the properties of the BLR.

ABSORPTION OF 10 GeV-1 TeV GAMMA RAYS BY RADIATION FROM THE BROAD-LINE REGION IN 3C 279

We study the photon-photon pair production optical depth for 10 GeV-1 TeV gamma rays from 3C 279 due to the diffuse radiation of the broad-line region (BLR). Assuming a power-law spectrum a(1)E(gamma)(2)(-a) for the photon intensity of very high energy gamma rays, a(1) greater than or similar to 405 and a(2) greater than or similar to 6.4 are inferred from the integrated photon fluxes measured by MAGIC and H. E. S. S. Based on this power-law spectrum, the preabsorbed spectra are calculated by correcting for photon-photon absorption on the diffuse photons of the BLR (internal absorption) and the extragalactic background light (external absorption). The position of the gamma-ray-emitting region, R-gamma, determines the relative contributions of these two diffuse radiation components to the total absorption for 10 GeV-1 TeV gamma rays. The internal absorption could make the spectral shape of the gamma rays more complex than if they were corrected only for external absorption and could lead to the formation of arbitrarily softening and hardening gamma-ray spectra. It should be requisite for the internal absorption to be considered in studying 10 GeV-1 TeV gamma rays from powerful blazars. The energy of the annihilated gamma-ray photons due to the internal absorption is likely to be mainly reradiated around GeV energies. Our results indicate that R-gamma may lie between the inner and outer radii of the BLR for 3C 279. This implies for powerful blazars that R-gamma might be neither inside the BLR cavity nor outside the BLR, but within the BLR shell. Observations by GLAST, MAGIC, H. E. S. S., and VERITAS in the near future could provide more constraints on the position of the gamma-ray-emitting region relative to the BLR.

Tests for Standard Accretion Disk Models by Variability in Active Galactic Nuclei

In this paper, standard accretion disk models of active galactic nuclei (AGNs) are tested using light curves of 26 objects that have been well observed using reverberation mapping. Timescales of variations are estimated by the most common definition of the variability timescale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The timescales of variations measured by the two methods are consistent with each other. If the typical value of the viscosity parameter alpha similar to 0.1 is adopted, the measured optical variability timescales are closest to the thermal timescales of the standard disks. If alpha is allowed to range from similar to 0.03 to similar to 0.2, the measured timescales are consistent with the thermal timescales of the standard disks. There is a linear relation between the measured variability timescales and black hole masses; this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the z-transformed discrete correlation function (ZDCF) between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with, or are likely to be conditionally in favor of, the standard accretion disk models of AGNs.

ABSORPTION OF 10 GeV-1 TeV GAMMA RAYS FROM 3C 279

In this paper, we revisit gamma-ray-emitting region for 10 GeV-1 TeV gamma rays from 3C 279 through studying the photon-photon absorption optical depth due to the diffuse radiation of the broad-line region (BLR) and the extragalactic background light (EBL). Based on the power-law spectrum detected by MAGIC, the preabsorbed spectra are inferred by correcting the photon-photon absorption on the diffuse photons of the BLR (internal absorption) and the EBL (external absorption). Position of gamma-ray-emitting region R(gamma) determines the relative contributions of this two diffuse radiations to the total absorption. Our results indicate that R(gamma) may be within the BLR shell for 3C 279, likely closer to the inner radius, which is consistent with our previous results. This is neither consistent with the suggestions of Bottcher et al., that very high energy (VHE) gamma-ray emission is produced far outside the BLR, nor with the assumptions of Tavecchio and Mazin, that VHE gamma-ray-emitting region is inside the BLR cavity. R(gamma) is a key physical quantity that could set some constraints on emission mechanisms that produce the VHE gamma rays from 3C 279. Observations of Fermi-Large Area Telescope, MAGIC, H. E. S. S., and VERITAS in the near future could give more constraints on the position of gamma-ray-emitting region relative to the BLR.

Constraining broad-line regions from time lags of broad emission lines relative to radio emission

In this paper, a new method is proposed to estimate the broad-line region sizes of ultraviolet (UV) lines RuvBLR. It is applied to 3C 273. First, we derive the time lags of radio emission relative to broad emission lines Lya and C iv by the z-transformed discrete correlation function (ZDCF) method. The broad lines lag the 5-, 8-, 15-, 22- and 37-GHz emission. The measured lags tuvob are of the order of years. For a given line, tuvob decreases as the radio frequency increases. This trend results from the radiative cooling of relativistic electrons. Both UV lines have a lag of tuvob=-2.74+0.06- 0.25 yr relative to the 37-GHz emission. These results are consistent with those derived from the Balmer lines in Paper I. Secondly, we derive the time lags of the lines Lya, C iv, H, H beta and Ha relative to the 37-GHz emission by the flux randomization/random subset selection (FR/RSS) Monte Carlo method. The measured lags are tob=-3.40+0.31- 0.05, -3.40+0.41- 0.14, -2.06+0.36- 0.92, -3.40+1.15- 0.20 and -3.56+0.35- 0.18 yr for the lines Lya, C iv, H, H beta and Ha, respectively. These estimated lags are consistent with those derived by the ZDCF method within the uncertainties. Based on the new method, we derive RuvBLR= 2.54+0.71- 0.35 to 4.01+0.90- 1.16 and 2.54+0.80- 0.43 to 4.01+0.98- 1.24 light-years for the Lya and C iv lines, respectively. Considering the uncertainties, these estimated sizes are consistent with those obtained in the classical reverberation mapping for the UV and the Balmer lines. This indicates that their emitting regions are not separated so large as in the classical mapping of the UV and optical lines. These results seem to depart from the stratified ionization structures obtained in the classical mapping.

Search for Intra-day Optical Variability in Mrk 501

We present our observations of the optical intra-day variability (IDV) in

CONSTRAINTS ON BLACK HOLE MASSES WITH TIMESCALES OF VARIATIONS IN BLAZARS

gamma

A new method to measure the virial factors in the reverberation mapping of active galactic nuclei

-ray BL Lac object Mrk 501. The observations were run with the 1.02 m and 2.4 m optical telescopes at Yunnan Observatories from 2005 April to 2012 May. The light curve at the

Hints of Correlation between Broad-line and Radio Variations for 3C 120

R

Revisiting Correlations Between Broad-Line and Jet Emission Variations for AGNs: 3C 120 and 3C 273

band on 2010 May 15 passes both variability tests (the