arXiv subjects

D. Xu

Publications and source records attributed to D. Xu.

98 records · Page 6Linked to original sources

arXiv2006

Radio-loud Narrow-Line Type 1 Quasars

We present the first systematic study of (non-radio-selected) radio-loud narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the `Catalogue of Quasars and Active Nuclei' with several radio and optical catalogues led to the identification of 11 radio-loud NLS1 candidates including 4 previously known ones. Most of the radio-loud NLS1s are compact, steep spectrum sources accreting close to, or above, the Eddington limit. The radio-loud NLS1s of our sample are remarkable in that they occupy a previously rarely populated regime in NLS1 multi-wavelength parameter space. While their [OIII]/H_beta and FeII/H_beta intensity ratios almost cover the whole range observed in NLS1 galaxies, their radio properties extend the range of radio-loud objects to those with small widths of the broad Balmer lines. Among the radio-detected NLS1 galaxies, the radio index R distributes quite smoothly up to the critical value of R ~ 10 and covers about 4 orders of magnitude in total. Statistics show that ~7% of the NLS1 galaxies are formally radio-loud while only 2.5% exceed a radio index R > 100. Several mechanisms are considered as explanations for the radio loudness of the NLS1 galaxies and for the lower frequency of radio-louds among NLS1s than quasars. While properties of most sources (with 2-3 exceptions) generally do not favor relativistic beaming, the combination of accretion mode and spin may explain the observations. (abbreviated)

astro-ph↗

arXiv2005

The radio loud narrow-line quasar SDSSJ172206.03+565451.6

We report identification of the radio loud narrow-line quasar SDSS J172206.03+565451.6 which we found in the course of a search for radio loud narrow-line Active Galactic Nuclei (AGN). SDSSJ172206.03+565451.6 is only the ~4th securely identified radio loud narrow-line quasar and the second-most radio loudest with a radio index R_1.4 ~ 100-700. Its black hole mass, M_BH = (2-3) 10^7 M_sun, is unusually small given its radio loudness, and the combination of mass and radio index puts SDSSJ172206.03+565451.6 in a scarcely populated region of M_BH-R diagrams. SDSSJ172206.03+565451.6 is a classical Narrow-Line Seyfert1-type object with FWHM_Hbeta = 1490 km/s, an intensity ratio of [OIII]/Hbeta = 0.7 and FeII emission complexes with FeII4570/Hbeta = 0.7. The ionization parameter of its narrow-line region, estimated from the line ratio [OII]/[OIII], is similar to Seyferts and its high ratio of [NeV]/[NeIII] indicates a strong EUV to soft-X-ray excess. We advertise the combined usage of [OII]/[OIII] and [NeV]/[NeIII] diagrams as a useful diagnostic tool to estimate ionization parameters and to constrain the EUV continuum shape relatively independent from other parameters.

astro-ph↗

arXiv2005

A New Relation between GRB Rest-Frame Spectra and Energetics and Its Utility on Cosmology

We investigate the well-measured spectral and energetic properties of 20 gamma-ray bursts (GRBs) in their cosmological rest frames. We find a tight relation between the isotropic-equivalent gamma-ray energy Eiso, the local peak energy E'p of the νF_ν spectrum, and the local break time t'b of the GRB afterglow light curve, which reads Eiso*t'b\propto E'p^{1.95\pm0.08} (χ^2_ν=1.40; Omega_M=0.27, Omega_Lambda=0.73). Such a power-law relation can be understood via the high-energy radiation processes for the GRB prompt emission accompanying the beaming effects. We then consider this relation as an intrinsic one for the observed GRB sample, and obtain a constraint on the mass density Omega_M=0.24^{+0.16}_{-0.12} (1σ) for a flat Lambda-CDM universe, and a χ^2_{dof}=1.33 for Omega_M\approx0.3 and Omega_Lambda\approx0.7. Ongoing GRB observations in the Swift era are expected to confirm this relation and make its cosmological utility progress much.

astro-ph↗

arXiv2005

Can Gamma-Ray Bursts Be Used to Measure Cosmology? A Further Analysis

Three different methods of measuring cosmology with gamma-ray bursts (GRBs) have been proposed since a relation between the $γ$-ray energy $E_γ $ of a GRB jet and the peak energy $E_p$ of the $νF_ν$ spectrum in the burst frame was reported by Ghirlanda and coauthors. In Method I, to calculate the probability for a favored cosmology, only the contribution of the $E_γ-E_p$ relation that is already best fitted for this cosmology is considered. We apply this method to a sample of 17 GRBs, and obtain the mass density $Ω_M=0.15^{+0.45}_{-0.13}$ ($1σ$) for a flat $Λ$CDM universe. In Method II, to calculate the probability for some certain cosmology, contributions of all the possible $E_γ-E_p$ relations that are best fitted for their corresponding cosmologies are taken into account. With this method, we find a constraint on the mass density $0.14<Ω_M<0.69$ ($1 σ$) for a flat universe. In Method III, to obtain the probability for some cosmology, contributions of all the possible $E_γ-E_p$ relations associated with their unequal weights are considered. With this method, we obtain an inspiring constraint on the mass density $0.16<Ω_M<0.45$ ($1 σ$) for a flat universe, and a $χ^2_{dof}=19.08/15=1.27$ for the concordance model of $Ω_M=0.27$. (abridged)

astro-ph↗

arXiv2005

Discovery of a massive SCUBA core with both inflow and outflow motions

We report the discovery of a massive SCUBA core with evidence of inflow and outflow motions. This core is detected by SCUBA at both 450 and 850 micro meter. Barely resolved by the telescope beam at 450 micro meter, it has a size of 10 arcsec, corresponding to 0.28 pc at a distance of 5.7 kpc. The dust temperature is estimated to be less than 29 K, the total mass is 820 solar mass and the average density is 1.1e-6 per cube-cm in a region with a radius of 5 arcsec. Follow-up spectral line observations, including HCN (3-2), HCO+ (3-2), H13CO+ (3-2) and C17O (2-1) reveal a typical blue profile which indicates that this core is collapsing. The CO (3-2) line profile is as broad as 38 km/s, indicating outflow motions in this region. This core is approximately 1.5 pc away from the known HII region G25.4NW, but there are no obvious radio, IRAS, MSX or Spitzer sources associated with it. We suggest that this core is at a very early stage of massive star or cluster formation.

astro-ph↗

arXiv2004

Discovery of high-amplitude X-ray variability in the Seyfert-LINER transition galaxy NGC7589

We present the first result of a programme to search for large flux variations in the X-ray sources of the XMM Serendipitous Survey compared to previous ROSAT observations. An increase in X-ray flux by a factor >10 was discovered from the nucleus of the galaxy NGC7589 on a timescale of less than 5 years. The 0.4-10keV XMM spectrum can be approximated by a power-law with photon index of 1.7-1.8, though it seems to flatten above 5keV, suggesting a possible complex model, such as partial covering or disc reflection. A classification based on an analysis of its optical spectrum places NGC7589 in the Seyfert region, but close to the Seyfert-LINER border-line on the AGN diagnostic diagrams. We classify NGC7589 as either Seyfert1.9 or LINERI, in the light of the detection of a broad H_alpha line, which makes NGC7589 an AGN in the low-luminosity regime. We interpret the observed variability in terms of either changes in covering factor of absorbing gas in the AGN, or variability in the intrinsic X-ray luminosity. Should the latter be the case, the inferred Eddington accretion rate increased from the radiatively inefficient accretion dominated regime to a value close to the putative critical value, at which a transition of the accretion mode is supposed to take place. This possibility presents a new prospect of studying accretion physics in the central black holes of external galaxies by direct observing changes of `spectral state', as is common in stellar black hole binary systems.

astro-ph↗

arXiv2004

Constraining $Ω_M$ and Dark Energy with Gamma-Ray Bursts

An $E_{γ,{\rm jet}}\propto {E'_p}^{1.5}$ relationship with a small scatter for current $γ$-ray burst (GRB) data was recently reported, where $E_{γ,{\rm jet}}$ is the beaming-corrected $γ$-ray energy and $E'_p$ is the $νF_ν$ peak energy in the local observer frame. By considering this relationship for a sample of 12 GRBs with known redshift, peak energy, and break time of afterglow light curves, we constrain the mass density of the universe and the nature of dark energy. We find that the mass density $Ω_M=0.35\pm^{0.15}_{0.15}$ (at the $1σ$ confident level) for a flat universe with a cosmological constant, and the $w$ parameter of an assumed static dark-energy equation of state $w=-0.84\pm^{0.57}_{0.83}$ ($1σ$). Our results are consistent with those from type Ia supernovae. A larger sample established by the upcoming {\em Swift} satellite is expected to provide further constraints.

astro-ph↗

arXiv1995

The Nonlinear Meissner Effect in Unconventional Superconductors

We examine the long-wavelength current response in anisotropic superconductors and show how the field-dependence of the Meissner penetration length can be used to detect the structure of the order parameter. Nodes in the excitation gap lead to a nonlinear current-velocity constitutive equation at low temperatures which is distinct for each symmetry class of the order parameter. The effective Meissner penetration length is linear in $H$ and exhibits a characteristic anisotropy for fields in the $ab$-plane that is determined by the positions of the nodes in momentum space. The nonlinear current-velocity relation also leads to an intrinsic magnetic torque for in-plane fields that are not parallel to a nodal or antinodal direction. The torque scales as $H^3$ for $T\rightarrow 0$ and has a characteristic angular dependence. We analyze the effects of thermal excitations, impurity scattering and geometry on the current response of a $d_{x^2-y^2}$ superconductor, and discuss our results in light of recent measurements of the low-temperature penetration length and in-plane magnetization of single-crystals of $YBa_2Cu_3O_{7-δ}$ and $LuBa_2Cu_3O_{7-δ}$.

cond-mat↗