S. N. Zhang

The Golden Standard Type Ia Supernova 2005cf: Observations from the Ultraviolet to the Near-Infrared Wavebands

We present extensive photometry at ultraviolet (UV), optical, and near-infrared (NIR) wavelengths, as well as dense sampling of optical spectra, for the normal Type Ia supernova (SN Ia) 2005cf. The optical photometry, performed at eight different telescopes, shows a 1σ scatter of ≾0.03 mag after proper corrections for the instrument responses. From the well-sampled light curves, we find that SN 2005cf reached a B-band maximum at 13.63 ± 0.02 mag, with an observed luminosity decline rate Δm _(15)(B) = 1.05 ± 0.03 mag. The correlations between the decline rate and various color indexes, recalibrated on the basis of an expanded SN Ia sample, yield a consistent estimate for the host-galaxy reddening of SN 2005cf, E(B – V)_(host) = 0.10 ± 0.03 mag. The UV photometry was obtained with the Hubble Space Telescope and the Swift Ultraviolet/Optical Telescope, and the results match each other to within 0.1-0.2 mag. The UV light curves show similar evolution to the broadband U, with an exception in the 2000-2500 A spectral range (corresponding to the F220W/uvm2 filters), where the light curve appears broader and much fainter than that on either side (likely owing to the intrinsic spectral evolution). Combining the UV data with the ground-based optical and NIR data, we establish the generic UV-optical-NIR bolometric light curve for SN 2005cf and derive the bolometric corrections in the absence of UV and/or NIR data. The overall spectral evolution of SN 2005cf is similar to that of a normal SN Ia, but with variety in the strength and profile of the main feature lines. The spectra at early times displayed strong, high-velocity (HV) features in the Ca II H&K doublet and NIR triplet, which were distinctly detached from the photosphere (v ≈ 10,000 km s^(–1)) at a velocity ranging from 20,000 to 25,000 km s^(–1). One interesting feature is the flat-bottomed absorption observed near 6000 A in the earliest spectrum, which rapidly evolved into a triangular shape and then became a normal Si II λ6355 absorption profile at about one week before maximum brightness. This premaximum spectral evolution is perhaps due to the blending of the Si IIλ6355 at photospheric velocity and another HV absorption component (e.g., an Si II shell at a velocity ~18,000 km s^(–1)) in the outer ejecta, and may be common in other normal SNe Ia. The possible origin of the HV absorption features is briefly discussed.

LARGE-SCALE CAVITIES SURROUNDING MICROQUASARS INFERRED FROM EVOLUTION OF THEIR RELATIVISTIC JETS

The black hole (BH) X-ray transient XTE J1550-564 has undergone a strong outburst in 1998 and two relativistic X-ray jets have been detected years later with the Chandra X-ray observatory; the eastern jet was found previously to have decelerated after its first detection. Here, we report a full analysis of the evolution of the western jet; significant deceleration is also detected in the western side. Our analysis indicates that there is a cavity outside the central source and the jets first traveled with constant velocity and then were slowed down by the interactions between the jets and the interstellar medium. The best-fitted radius of the cavity is similar to 0.31 pc on the eastern side and similar to 0.44 pc on the western side, and the densities also show asymmetry, of similar to 0.034 cm(-3) on the east to similar to 0.12 cm(-3) on the west. The best-fitted magnetic fields on both sides are similar to 0.5 mG. Similar analysis is also applied to another microquasar system, H 1743-322, and a large-scale low-density region is also found. Based on these results and the comparison with other microquasar systems, we suggest a generic scenario for microquasar jets, classifying the observed jets into three main categories, with different jet morphologies (and sizes) corresponding to different scales of vacuous environments surrounding them. We also suggest that either continuous jets or accretion disk winds, or both may be responsible for creating these cavities. Therefore X-ray jets from microquasars provide us with a promising method of probing the environment of accreting BHs.

A tidal disruption model for the gamma-ray burst of GRB 060614

The combination of a long duration and the absence of any accompanying supernova clearly shows that GRB 060614 cannot be grouped into the two conventional classes of gamma-ray bursts; i.e., the long/soft bursts deemed to be collapsars and the short/hard bursts deemed to be merging binary compact stars. A new progenitor model is required for this anomalous gamma-ray burst. We propose that GRB 060614 might be produced through the tidal disruption of a star by an intermediate-mass black hole. In this scenario, the long duration and the lack of any associated supernova are naturally expected. The theoretical energy output is also consistent with observations. The observed 9 s periodicity in the gamma-ray light curve of GRB 060614 can also be satisfactorily explained.

Probing the Spatial Distribution of the Interstellar Dust Medium by High Angular Resolution X-Ray Halos of Point Sources

We studied the X-ray dust scattering halos around 17 bright X-ray point sources using Chandra data. All sources were observed with the Advanced CCD Imaging Spectrometer and High Energy Transmission Grating in continuous clocking mode (CC mode) or timed exposure mode (TE mode). We use an iterative method to resolve the halos at small scattering angles from the zeroth-order data in CC mode or the first-order data in TE mode, which is not, or less, piled up. Using the interstellar grain models of Weingartner and Draine and Mathis and coworkers to fit the halo profiles, we get the hydrogen column densities and the spatial distributions of the scattering dust grains along the lines of sight (LOSs) to these sources. We find that the scattering dust density very close to these sources is much higher than the normal interstellar medium. For X-ray pulsars GX 301-2 and Vela X-1 with companions of strong stellar winds, the X-ray absorption column densities are much higher than the derived scattering column densities, because the dense media around the X-ray sources produce extremely small angle scatterings that are indistinguishable from point sources, even with Chandras angular resolution. For LMC X-1, most of the scattering and absorption occur in the Large Magellanic Cloud, rather than in the Milky Way. From the obtained X-ray spectra, the cold gas absorption and thus the equivalent hydrogen column are determined. We have obtained the linear correlation between the NH derived from spectral fits and that derived from the grain models of Weingartner and Draine and Mathis and coworkers (except for GX 301-2 and Vela X-1): NH,WD01 = (0.720 ± 0.009)NH,abs + (0.051 ± 0.013) and NN,MRN = (1.156 ± 0.016)NH,abs + (0.062 ± 0.024) in units of 1022 cm-2. High angular resolution X-ray dust scattering halos offer an excellent potential for probing the spatial distributions of the interstellar dust medium.

Multiwavelength observations of the BL lacertae object PKS 2155-304 with XMM-Newton

The optical/UV and X-ray instruments on board XMM-Newton provide an excellent opportunity to perform simultaneous observations of violently variable objects over a broad wavelength range. The UV- and X-ray-bright BL Lac object PKS 2155-304 has been observed repeatedly with XMM-Newton about twice per year. In this paper, we present a detailed analysis of the simultaneous multiwavelength variability of the source from optical to X-rays, based on the currently available XMM-Newton observations. These observations probed the intraday multiwavelength variability at optical/UV and X-ray wavelengths of the source. The UV variability amplitude is substantially smaller than the X-ray one, and the hardness ratios of the UV to X-rays correlates with the X-ray fluxes: the brighter the source, the flatter the UV/X-ray spectra. On 2000 May 30-31, the UV and X-ray light curves were weakly correlated, while the UV variations followed the X-ray ones with no detectable lags on 2000 November 19-21. On 2001 November 30, the source exhibited a major X-ray flare that was not detected in the optical. The intraday UV and X-ray variability presented here is not similar to the interday UV and X-ray variability obtained from the previous coordinated extensive multiwavelength campaigns on the source, indicating that different modes of variability might be operating in PKS 2155-304 on different timescales or from epoch to epoch.

Does the Iron Kα Line of Active Galactic Nuclei Arise from the Cerenkov Line-like Radiation?

When thermal relativistic electrons with isotropic distribution of velocities move in a gas region or impinge upon the surface of a cloud that consists of a dense gas or doped dusts, the Cerenkov effect produces peculiar atomic or ionic emission lines, which is known as the Cerenkov line-like radiation. This newly recognized emission mechanism may find wide applications in high-energy astrophysics. In this paper we tentatively adopt this new line emission mechanism to discuss the origin of the iron Kα feature of active galactic nuclei (AGNs). The motivation of this research is to attempt a solution to a problem encountered by the disk fluorescence line model, i.e., the lack of temporal response of the observed iron Kα line flux to the changes of the X-ray continuum flux. If the Cerenkov line emission is indeed responsible significantly for the iron Kα feature, the conventional scenario around the central supermassive black holes of AGNs would need to be modified to accommodate more energetic, more violent, and much denser environments than previously thought.

HARDNESS RATIO ESTIMATION IN LOW COUNTING X-RAY PHOTOMETRY

Hardness ratios are commonly used in X-ray photometry to indicate spectral properties roughly. It is usually defined as the ratio of counts in two different wavebands. This definition, however, is problematic when the counts are very limited. Here we instead define hardness ratio using the

Maser mechanism of optical pulsations from anomalous X-ray pulsar 4U 0142+61

lambda

An accretion model for the growth of the central black hole associated with ionization instability in quasars

parameter of Poisson processes, and develop an estimation method via Bayesian statistics. Our Monte Carlo simulations show the validity of our method. Based on this new definition, we can estimate the hydrogen column density for the photoelectric absorption of X-ray spectra in the case of low counting statistics.Hardness ratios are commonly used in X-ray photometry to roughly indicate spectral properties. They are usually defined as the ratio of counts in two different wave bands. This definition, however, is problematic when the counts are very limited. Here we instead define the hardness ratio using the lambda-parameter of Poisson processes and develop an estimation method via Bayesian statistics. Our Monte Carlo simulations show the validity of our method. Based on this new definition we can estimate the hydrogen column density for the photoelectric absorption of X-ray spectra in the case of low counting statistics.

A Statistical Analysis of Point-like Sources in the Chandra Galactic Center Survey ∗

A maser curvature emission mechanism in the presence of curvature drift is used to explain the optical pulsations from anomalous X-ray pulsars. For the source of AXP0142+61,the optical pulsation occurs at the radial distance