Minfeng Gu

The X-Ray Spectral Evolution in X-Ray Binaries and Its Application to Constrain the Black Hole Mass of Ultraluminous X-Ray Sources

We investigate the relationship between the hard X-ray photon index Gamma and the Eddington ratio [ xi = L(X)( 0:5-25 keV)/L(Edd)] in six X-ray binaries (XRBs) with well-constrained black hole masses and distances. We find that different XRBs follow different anticorrelations between Gamma and xi when xi is less than a critical value, while Gamma and xi generally follow the same positive correlation when xi is larger than the critical value. The anticorrelation and the positive correlation may suggest that they are in different accretion modes ( e. g., radiatively inefficient accretion flow [RIAF] and standard disk). We fit both correlations with the linear least-squares method for individual sources, from which the cross point of two fitted lines is obtained. Although the anticorrelation varies from source to source, the cross points of all sources roughly converge to the same point with small scatter (log xi = -2.1 +/- 0.2, Gamma = 1.5 +/- 0.1), which may correspond to the transition point between RIAF and standard accretion disk. Motivated by the observational evidence for the similarity of the X-ray spectral evolution of ultraluminous X-ray sources (ULXs) to that of XRBs, we then constrain the black hole masses for seven ULXs, assuming that their X-ray spectral evolution is similar to that of XRBs and that their Eddington ratios are less than 1. We find that the BH masses of these seven luminous ULXs are around 104 M(circle dot), which are typical for intermediate-mass BHs ( IMBHs). Our results are generally consistent with the BH masses constrained from the timing properties ( e. g., break frequency) or the model fitting with a multicolor disk.

The Jet Power, Radio Loudness, and Black Hole Mass in Radio-loud Active Galactic Nuclei

Jet formation is thought to be closely connected with the mass of the central supermassive black hole in active galactic nuclei. The radio luminosity commonly used in investigating this issue is merely an indirect measure of the energy transported through the jets from the central engine and is severely Doppler boosted in core-dominated radio quasars. In this work, we investigate the relationship between the jet power and the black hole mass, by estimating the jet power using extrapolated extended 151 MHz flux density from the VLA 5 GHz extended radio emission, for a sample of 146 radio-loud quasars compiled from the literature. After removing the effect of relativistic beaming in the radio and optical emission, we find a significant intrinsic correlation between the jet power and the black hole mass. It strongly implies that the jet power, like jet formation, is closely connected with the black hole mass. To eliminate the beaming effect in the conventional radio loudness, we define a new radio loudness as the ratio of the radio extended luminosity to the optical luminosity estimated from the broad-line luminosity. In a tentatively combined sample of radio-quiet with our radio-loud quasars, the apparent gap around the conventional radio loudness R = 10 is not prominent for the new-defined radio loudness. In this combined sample, we find a significant correlation between the black hole mass and new-defined radio loudness.

Multi-colour optical monitoring of eight red blazars

Context. We present the observational results of multi-colour optical monitoring of eight red blazars from 2003 September to 2004 February. Aims. The aim of our monitoring is to investigate the spectral variability as well as the flux variations at short and long time sc ales. Methods. The observations were carried out using the 1.0 m robotic telescope of Mt. Lemmon Optical Astronomy Observatory, in Arizona, USA, the 0.6 m telescope of Sobaeksan Optical Astronomy Observatory and the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory, in the Republic of Korea. Results. During the observations, all sources show strong flux variat ions with amplitudes of larger than 0.5 mag. Variations with amplitudes of over 1 mag are found in four sources. Intraday variations with amplitudes larger than 0.15 mag, and a rapid brightness increase with a rate of ∼ 0.2 mag per day in four days, are detected in S5 0716+71. We investigate the relationship between the colour index and source brightness for each source. We find that two out of three FSRQs tend to be redde r when they are brighter, and, conversely, all BL Lac objects tend to be bluer. In particular, we find a significant anti-correlation betwee n the V− I colour index and R magnitude for 3C 454.3. This implies that the spectrum became steeper when the source was brighter, which is opposite to the common trend for blazars. In contrast, significant p ositive correlations are found in 3C 66A, S5 0716+71, and BL Lac. However, there are only very weak correlations for PKS 0735+17 and OJ 287. Conclusions. We propose that the different relative contributions of the thermal versus non-the rmal radiation to the optical emission may be responsible for the different trends of the colour index with brightness in FSRQs and BL Lac objects.

On the masses of black holes in radio-loud quasars

The central black hole masses of a sample of radio-loud quasars are estimated by using the data of H beta linewidth and the optical continuum luminosity. The vast majority of the quasars in this sample have black hole masses larger than 10(8) M(circle dot), while a few quasars may contain relatively smaller black holes. We found a significant anti-correlation between the radio loudness and the central black hole mass. It might imply that the jet formation is governed by the black hole mass.

The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei

We find a significant anticorrelation between the hard X-ray photon indexand the Eddington ratio Lbol/LEdd for a sample of low-ionization nuclear emission-line regions and local Seyfert galaxies, compiled from literature with Chandra or XMM-Newton observations. This result is in contrast with the positive correlation found in luminous active galactic nuclei (AGN), while it is similar to that of X-ray binaries (XRBs) in the low/hard state. Our result is qualitatively consistent with the spectra produced from advection-dominated accretion flows (ADAFs). It implies that the X-ray emission of low-luminosity active galactic nuclei (LLAGN) may originate from the Comptonization process in ADAF, and the accretion process in LLAGN may be similar to that of XRBs in the low/hard state, which is different from that in luminous

Coordinated Multiwavelength Observation of 3C 66A during the WEBT Campaign of 2003-2004*

The BL Lac object 3C 66A was the target of an extensive multiwavelength monitoring campaign from 2003 July through 2004 April (with a core campaign from 2003 September to 2003 December) involving observations throughout the electromagnetic spectrum. Radio, infrared, and optical observations were carried out by the WEBT-ENIGMA collaboration. At higher energies, 3C 66A was observed in X-rays (RXTE), and at very high energy (VHE) in γ-rays (STACEE, VERITAS). In addition, the source has been observed with the VLBA at nine epochs throughout the period 2003 September to 2004 December, including three epochs contemporaneous with the core campaign. A gradual brightening of the source over the course of the campaign was observed at all optical frequencies, culminating in a very bright maximum around 2004 February 18. The WEBT campaign revealed microvariability with flux changes of ~5% on timescales as short as ~2 hr. The source was in a relatively bright state, with several bright flares on timescales of several days. The spectral energy distribution (SED) indicates a νFν peak in the optical regime. A weak trend of optical spectral hysteresis with a trend of spectral softening throughout both the rising and decaying phases has been found. On longer timescales, there appears to be a weak indication of a positive hardness-intensity correlation for low optical fluxes, which does not persist at higher flux levels. The 3-10 keV X-ray flux of 3C 66A during the core campaign was historically high and its spectrum very soft, indicating that the low-frequency component of the broadband SED extends beyond ~10 keV. No significant X-ray flux and/or spectral variability was detected. STACEE and Whipple observations provided upper flux limits at >150 and >390 GeV, respectively. The 22 and 43 GHz data from the three VLBA epochs made between 2003 September and 2004 January indicate a rather smooth jet with only very moderate internal structure. Evidence for superluminal motion (8.5 ± 5.6 h-1 c) was found in only one of six components, while the apparent velocities of all other components are consistent with 0. The radial radio brightness profile suggests a magnetic field decay ∝r-1 and, thus, a predominantly perpendicular magnetic field orientation.

The bulk kinetic power of radio jets in active galactic nuclei

Based on the Konigls inhomogeneous jet model, we estimate the jet parameters, such as bulk Lorentz factor Gamma, viewing angle theta and electron number density n(e) from radio very long-baseline interferometry and X-ray data for a sample of active galactic nuclei (AGNs) assuming that the X-rays are from the jet rather than the intracluster gas. The bulk kinetic power of jets is then calculated using the derived jet parameters. We find a strong correlation between the total luminosity of broad emission lines and the bulk kinetic power of the jets. This result supports the scenario that the accretion process is tightly linked with the radio jets, though how the disc and jet are coupled is not revealed by present correlation analysis. Moreover, we find a significant correlation between the bulk kinetic power and radio extended luminosity. This implies that the emission from the radio lobes is closely related with the energy flux transported through jets from the central part of AGNs.

THE COMPACT RADIO STRUCTURE OF RADIO-LOUD NARROW LINE SEYFERT 1 GALAXIES

We present the compact radio structure of three radio-loud narrow line Seyfert 1 galaxies from the Very Long Baseline Array archive data at 2.3, 5, and 8.4 GHz. In RXS J16290+4007, the radio structure is mostly unresolved. The combination of compact radio structure, high brightness temperature, and inverted spectrum between simultaneous 2.3 and 8.4 GHz strongly favors jet relativistic beaming. Combined with the very long baseline interferometry data at 1.6 and 8.4 GHz from the literature, we argue that RXS J16333+4718 also may harbor a relativistic jet, with resolved core-jet structure in 5 GHz. B3 1702+457 is clearly resolved with a well-defined jet component. The overall radio steep spectrum indicates that B3 1702+457 is likely a source optically defined as NLS1 with radio definition of compact steep spectrum sources. From these three sources, we found that radio loud NLS1s can be either intrinsically radio loud (e.g., B3 1702+457) or apparently radio loud due to jet beaming effects (e.g., RXS J16290+4007 and RXS J16333+4718).

VLBI observations of seven BL Lacertae objects from RGB sample

We present EVN observations of seven BL Lac objects selected from the RGB sample. To investigate the intrinsic radiation property of BL Lac objects, we estimated the Doppler factor with the VLA or MERLIN core and the total 408 MHz luminosity for a sample of 170 BL Lac objects. The intrinsic ( comoving) synchrotron peak frequency was then calculated by using the estimated Doppler factor. Assuming a Lorentz factor of 5, the viewing angle of jets was constrained. The high-resolution VLBI images of seven sources all show a core-jet structure. We estimated the proper motions of three sources with the VLBI archive data, and find that the apparent speed increases with the distance of components to the core for all of them. In our BL Lacs sample, the Doppler factor of LBLs is systematically larger than that of IBLs and HBLs. We find a significant anti-correlation between the total 408 MHz luminosity and the intrinsic synchrotron peak frequency. However, the scatter is much larger than for the blazar sequence. Moreover, we find a significant positive correlation between the viewing angle and the intrinsic synchrotron peak frequency. The BL Lac objects show a continuous distribution on the viewing angle. While LBLs have a smaller viewing angle than that of IBLs and HBLs, IBLs are comparable to HBLs. We conclude that the intrinsic synchrotron peak frequency is not only related to the intrinsic radio power ( though with a large scatter), but also to the viewing angle for the present sample.

What Governs the Bulk Velocity of the Jet Components in Active Galactic Nuclei

We use a sample of radio-loud active galactic nuclei (AGNs) with measured black hole masses to explore the jet formation mechanisms in these sources. Based on Konigls inhomogeneous jet model, the jet parameters, such as the bulk motion Lorentz factor, magnetic field strength, and electron density in the jet, can be estimated with the very long baseline interferometry and X-ray data.. We find a significant correlation between black hole mass and the bulk Lorentz factor of the jet components for this sample, while no significant correlation is present between the bulk Lorentz factor and the Eddington ratio. The massive black holes will be spun up through accretion, as the black holes acquire mass and angular momentum simultaneously through accretion. Recent investigation indeed suggested that most supermassive black holes in elliptical galaxies have on average higher spins than the black holes in spiral galaxies, where random, small accretion episodes (e.g., tidally disrupted stars, accretion of molecular clouds) might have played a more important role. If this is true, then the correlation between black hole mass and the bulk Lorentz factor of the jet components found in this work implies that the motion velocity of the jet components is probably governed by the black hole spin. No correlation is found between the magnetic field strength at 10R S (R S = 2GM/c 2 is the Schwarzschild radius) in the jets and the bulk Lorentz factor of the jet components for this sample. This is consistent with the black hole spin scenario, i.e., the faster moving jets are magnetically accelerated by the magnetic fields threading the horizon of more rapidly rotating black holes. The results imply that the Blandford-Znajek mechanism may dominate over the Blandford-Payne mechanism for the jet acceleration, at least in these radio-loud AGNs.