The high quality light curves of Kepler space telescope make it possible to analyze the optical variability of AGNs with an unprecedented time resolution. Studying the asymmetry in variations could give independent constraints on the physical models
for AGN variability. In this paper, we use Kepler observations of 19 sources to perform analyses on the variability asymmetry of AGNs. We apply smoothing-correction to light curves to deduct the bias to high frequency variability asymmetry, caused by long term variations which are poorly sampled due to the limited length of light curves. A parameter $beta$ based on structure functions is introduced to quantitively describe the asymmetry and its uncertainty is measured using extensive Monte-Carlo simulations. Individual sources show no evidence of asymmetry at timescales of $1sim20$ days and there is not a general trend toward positive or negative asymmetry over the whole sample. Stacking data of all 19 AGNs, we derive averaged $bar{beta}$ of 0.00$pm$0.03 and -0.02$pm$0.04 over timescales of 1$sim$5 days and 5$sim$20 days, respectively, statistically consistent with zero. Quasars and Seyfert galaxies show similar asymmetry parameters. Our results indicate that short term optical variations in AGNs are highly symmetric.
In this paper we present a temporal and spectral analysis of X-ray data from the XMM and Chandra observations of the ultrasoft and variable Seyfert galaxy RX J1301.9+2747. In both observations the source clearly displays two distinct states in the X-
ray band, a long quiescent state and a short flare (or eruptive) state which differs in count rates by a factor of 5--7. The transition from quiescent to flare state occurs in 1--2 ks. We have observed that the quiescent state spectrum is unprecedentedly steep with a photon index Gamma~7.1, and the spectrum of the flare state is flatter with Gamma~4.4. X-rays above 2 keV were not significantly detected in either state. In the quiescent state, the spectrum appears to be dominated by a black body component of temperature about ~30--40 eV, which is comparable to the expected maximum effective temperature from the inner accretion disk. The quiescent state however, requires an additional steep power-law, presumably arising from the Comptonization by transient heated electrons. Optical spectrum from the Sloan Digital Sky Survey shows Seyfert-like narrow lines for RX J1301.9+2747, while the HST imaging reveals a central point source for the object. In order to precisely determine the hard X-ray component, future longer X-ray observations are required. This will help constrain the accretion disk model for RX J1301.9+2747, and shed new light into the characteristics of the corona and accretion flows around black holes.
We present the result of the Chandra high-resolution observation of the Seyfert~2 galaxy NGC 7590. This object was reported to show no X-ray absorption in the low-spatial resolution ASCA data. The XMM observations show that the X-ray emission of NGC
7590 is dominated by an off-nuclear ultra-luminous X-ray source (ULX) and an extended emission from the host galaxy, and the nucleus is rather weak, likely hosting a Compton-thick AGN. Our recent Chandra observation of NGC 7590 enables to remove the X-ray contamination from the ULX and the extended component effectively. The nuclear source remains undetected at ~4x10^{-15} erg/s/cm^-2 flux level. Although not detected, Chandra data gives a 2--10 keV flux upper limit of ~6.1x10^{-15} erg/s/cm^-2 (at 3 sigma level), a factor of 3 less than the XMM value, strongly supporting the Compton-thick nature of the nucleus. In addition, we detected five off-nuclear X-ray point sources within the galaxy D25 ellipse, all with 2 -- 10 keV luminosity above 2x10^{38} erg/s (assuming the distance of NGC 7590). Particularly, the ULX previously identified by ROSAT data was resolved by Chandra into two distinct X-ray sources. Our analysis highlights the importance of high spatial resolution images in discovering and studying ULXs.
To investigate the validity of the Wiedemann-Franz (WF) law in disordered but metallic cuprates, the low-temperature charge and heat transport properties are carefully studied for a series of impurity-substituted and carrier-overdoped La_{1.8}Sr_{0.2
}Cu_{1-z}M_zO_4 (M = Zn or Mg) single crystals. With moderate impurity substitution concentrations of z = 0.049 and 0.082 (M = Zn), the resistivity shows a clear metallic behavior at low temperature and the WF law is confirmed to be valid. With increasing impurity concentration to z = 0.13 (M = Zn) or 0.15 (M = Mg), the resistivity shows a low-T upturn but its temperature dependence indicates a finite conductivity in the T to 0 limit. In this weakly-localized metallic state that is intentionally achieved in the overdoped regime, a {it negative} departure from the WF law is found, which is opposite to the theoretical expectation.
Based on density functional theory calculations, we systematically investigate the behaviors of a H atom in Ag-doped ZnO, involving the preference sites, diffusion behaviors, the electronic structures and vibrational properties. We find that a H atom
can migrate to the doped Ag to form a Ag-H complex by overcoming energy barriers of 0.3 - 1.0 eV. The lowest-energy site for H location is the bond center of a Ag-O in the basal plane. Moreover, H can migrate between this site and its equivalent sites with energy cost of less than 0.5 eV. In contrast, dissociation of such a Ag-H complex needs energy of about 1.1 - 1.3 eV. This implies that the Ag-H complexes can commonly exist in the Ag-doped ZnO, which have a negative effect on the desirable p-type carrier concentrations of Ag-doped ZnO. In addition, based on the frozen phonon calculation, the vibrational properties of ZnO with a Ag-H complex are predicted. Some new vibrational modes associated with the Ag-H complex present in the vibrational spectrum of the system.
We present a detailed study of the X-ray and optical spectra of the luminous Seyfert galaxy Mrk 1393, which revealed variable partial obscuration of the active nucleus. The X-ray spectra obtained by XMM-Newton and Swift show moderate absorption with
a column density around 3x10^21 cm^-2, consistent with a dust-reddening interpretation of the steep Balmer decrement seen in recent optical spectra. The X-ray flux in the 0.5 to 2 keV band during the XMM-Newton observation in 2005 and Swift observation in 2006 was a factor 6 brighter than that of the ROSAT All Sky Survey in 1991. In the past 4 years, the broad Halpha line brightened by a factor of 4 accompanied by a decrease in the Balmer decrement. A comparison with literature spectra reveals variations in the dust extinction on time scales of several years, suggesting that the obscuring material is very close to the active nucleus. These observations indicate that a dust-to-gas ratio as high as the Galactic value can be present in moderately thick gas in the vicinity of the central engine within a few parsecs. We suggest that the obscuring material may be debris disrupted from the dusty torus.
Graphene has attracted a great interest in material science due to its novel electronic structrues. Recently, magnetism discovered in graphene based systems opens the possibility of their spintronics application. This paper provides a comprehensive r
eview on the magnetic behaviors and electronic structures of graphene systems, including 2-dimensional graphene, 1-dimensional graphene nanoribbons, and 0-dimensional graphene nanoclusters. Theoretical research suggests that such metal-free magnetism mainly comes from the localized states or edges states. By applying external electric field, or by chemical modification, we can turn the zigzag nanoribbon systems to half metal, thus obtain a perfect spin filter.
The temperature and magnetic-field (H) dependences of thermal conductivity (kappa) of Bi_2Sr_2CaCu_2O_{8+delta} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_
2Sr_2Ca_{1-x}Dy_xCu_2O_{8+delta} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of kappa at temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of kappa, the Dy-doped samples present an additional dip-like term of kappa(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x simeq 0.50, an interesting plateau feature shows up in the low-T kappa(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.
To study the effects of paramagnetic spins on phonons, both the in-plane and the c-axis heat transport of GdBaCo_{2}O_{5+x} (GBCO) single crystals are measured at low temperature down to 0.36 K and in magnetic field up to 16 T. It is found that the p
honon heat transport is very strongly affected by the magnetic field and nearly 5 times increase of the thermal conductivity in several Tesla field is observed at 0.36 K. It appears that phonons are resonantly scattered by paramagnetic spins in zero field and the application of magnetic field removes such strong scattering, but the detailed mechanism is to be elucidated.
Recently, it was proposed that phonons are specularly reflected below about 0.5 K in ordinary single-crystal samples of high-T_c cuprates, and that the low-temperature thermal conductivity should be analyzed by fitting the data up to 0.5 K using an a
rbitrary power law. Such an analysis yields a result different from that obtained from the conventional analysis, in which the fitting is usually restricted to a region below 0.15 K. Here we show that the proposed new analysis is most likely flawed, because the specular phonon reflection means that the phonon mean free path ell gets LONGER than the mean sample width, while the estimated ell is actually much SHORTER than the mean sample width above 0.15 K.
