109 research outputs found
Ly Leaks in the Absorption Spectra of High Redshift QSOs
Spectra of high redshift QSOs show deep Gunn-Peterson absorptions on the blue
sides of the \Lya emissions lines. They can be decomposed into components
called \Lya leaks, defined to be emissive regions in complementary to otherwise
zero-fluxed absorption gaps. Just like \Lya absorption forests at low
redshifts, \Lya leaks are both easy to find in observations and containing rich
sets of statistical properties that can be used to study the early evolution of
the IGM. Among all properties of a leak profile, we investigate its equivalent
width in this paper, since it is weakly affected by instrumental resolution and
noise. Using 10 Keck QSO spectra at , we have measured the number
density distribution function , defined to be the number of leaks per
equivalent width and per redshift , in the redshift range .
These new observational statistics, in both the differential and cumulative
forms, fit well to hydro numerical simulations of uniform ionizing background
in the CDM cosmology. In this model, Ly leaks are mainly due
to low density voids. It supports the early studies that the IGM at
would still be in a highly ionized state with neutral hydrogen fraction . Measurements of at would be effective to probe the
reionization of the IGM.Comment: 3 figs, accepted by ApJ
X-ray fluorescent lines from the Compton-thick AGN in M51
The cold disc/torus gas surrounding active galactic nuclei (AGN) emits fluorescent lines when irradiated by hard X-ray photons. The fluorescent lines of elements other than Fe and Ni are rarely detected due to their relative faintness. We report the detection of Kα lines of neutral Si, S, Ar, Ca, Cr, and Mn, along with the prominent Fe Kα, Fe Kβ, and Ni Kα lines, from the deep Chandra observation of the low-luminosity Compton-thick AGN in M51. The Si Kα line at 1.74 keV is detected at ∼3σ, the other fluorescent lines have a significance between 2 and 2.5 σ, while the Cr line has a significance of ∼1.5σ. These faint fluorescent lines are made observable due to the heavy obscuration of the intrinsic spectrum of M51, which is revealed by NuSTAR observation above 10 keV. The hard X-ray continuum of M51 from Chandra and NuSTAR can be fitted with a power-law spectrum with an index of 1.8, reprocessed by a torus with an equatorial column density of NH ∼ 7 × 10^(24) cm^(−2) and an inclination angle of 74°. This confirms the Compton-thick nature of the nucleus of M51. The relative element abundances inferred from the fluxes of the fluorescent lines are similar to their solar values, except for Mn, which is about 10 times overabundant. It indicates that Mn is likely enhanced by the nuclear spallation of Fe
A Unified Fitting of HI and HeII Ly\alpha Transmitted Flux of QSO HE2347 with LCDM Hydrodynamic Simulations
Using cosmological hydrodynamic simulations of the LCDM model, we present a
comparison between the simulation sample and real data sample of HI and HeII
Ly\alpha transmitted flux in the absorption spectra of the QSO HE2347-4342. The
LCDM model is successful in simultaneously explaining the statistical features
of both HI and HeII Ly\alpha transmitted flux. It includes: 1.) the power
spectra of the transmitted flux of HI and HeII can be well fitted on all scales
> 0.28h^{-1} Mpc for H, and > 1.1h^{-1} Mpc for He; 2.) the Doppler parameters
of absorption features of HeII and HI are found to be turbulent-broadening; 3.)
the ratio of HeII to HI optical depths are substantially scattered, due to the
significant effect of noise. A large part of the \eta-scatter is due to the
noise in the HeII flux. However, the real data contain more low-\eta events
than simulation sample. This discrepancy may indicate that the mechanism
leading extra fluctuations upon the simulation data, such as a fluctuating UV
radiation background, is needed. Yet, models of these extra fluctuations should
satisfy the constraints: 1.) if the fluctuations are Gaussian, they should be
limited by the power spectra of observed HI and HeII flux; 2.) if the
fluctuations are non-Gaussian, they should be limited by the observed
non-Gaussian features of the HI and HeII flux.Comment: 21 pages, 7 figs, ApJ in pres
The spin-down accretion regime of Galactic ultra-luminous X-ray pulsar Swift J0243.6+6124
The relative high fluxes of the Galactic ultra-luminous X-ray pulsar Swift
J0243 allow a detailed study of its spin-down regime in quiescence state, for a
first time. After the 2017 giant outburst, its spin frequencies show a linear
decreasing trend with some variations due to minor outbursts. The linear
spin-down rate is Hz/s during the period of lowest
luminosity, from which one can infer a dipole field G.
The relation during the spin-down regime is complex, and the
is close to 0 when the luminosity reaches both the high end
() and the lowest value (). The luminosity of
zero-torque is different for the giant outburst and other minor outbursts. It
is likely due to different accretion flows for different types of outburst, as
evidenced by the differences of the spectra and pulse profiles at a similar
luminosity for different types of outburst (giant or not). The pulse profile
changes from double peaks in the spin-up state to a single broad peak in the
low spin-down regime, indicating the emission beam/region is larger in the low
spin-down regime. These results show that accretion is still ongoing in the low
spin-down regime for which the neutron star is supposed to be in a propeller
state.Comment: 7 pages, 7 figs, to appear in ApJ, comments welcom
- …