27 research outputs found
Systematic Distortion in Cosmic Microwave Background Maps
To minimize instrumentally induced systematic errors, cosmic microwave
background (CMB) anisotropy experiments measure temperature differences across
the sky using paires of horn antennas, temperature map is recovered from
temperature differences obtained in sky survey through a map-making procedure.
To inspect and calibrate residual systematic errors in recovered temperature
maps is important as most previous studies of cosmology are based on these
maps. By analyzing pixel-ring couping and latitude dependence of CMB
temperatures, we find notable systematic deviation from CMB Gaussianity in
released Wilkinson Microwave Anisotropy Probe (WMAP) maps. The detected
deviation is hard to explain by any process in the early universe and can not
be ignored for a precision cosmology study.Comment: accepted for publication in Sci China G-Phy Mech Astro
Pseudo-Dipole Signal Removal from WMAP Data
It is discovered in our previous work that different observational
systematics, e.g., errors of antenna pointing directions, asynchronous between
the attitude and science data, can generate pseudo-dipole signal in full-sky
maps of the cosmic microwave background (CMB) anisotropy published by The
Wilkinson Microwave Anisotropy Probe (WMAP) team. Now the antenna sidelobe
response to the Doppler signal is found to be able to produce similar effect as
well. In this work, independent to the sources, we uniformly model the
pseudo-dipole signal and remove it from published WMAP7 CMB maps by model
fitting. The result demonstrates that most of the released WMAP CMB quadrupole
is artificial.Comment: V3: using WMAP7 dat
Optical and Near-Infrared Observations of the Highly Reddened, Rapidly Expanding Type Ia Supernova 2006X in M100
We present extensive optical (UBVRI), near-infrared (JK) light curves and
optical spectroscopy of the Type Ia supernova (SN) 2006X in the nearby galaxy
NGC 4321 (M100). Our observations suggest that either SN 2006X has an
intrinsically peculiar color evolution, or it is highly reddened [E(B -
V)_{host} = 1.42+/-0.04 mag] with R_V = 1.48+/-0.06, much lower than the
canonical value of 3.1 for the average Galactic dust. SN 2006X also has one of
the highest expansion velocities ever published for a SN Ia. Compared with the
other SNe Ia we analyzed, SN 2006X has a broader light curve in the U band, a
more prominent bump/shoulder feature in the V and R bands, a more pronounced
secondary maximum in the I and near-infrared bands, and a remarkably smaller
late-time decline rate in the B band. The B - V color evolution shows an
obvious deviation from the Lira-Phillips relation at 1 to 3 months after
maximum brightness. At early times, optical spectra of SN 2006X displayed
strong, high-velocity features of both intermediate-mass elements (Si, Ca, and
S) and iron-peak elements, while at late times they showed a relatively blue
continuum, consistent with the blue U-B and B-V colors at similar epochs. A
light echo and/or the interaction of the SN ejecta and its circumstellar
material may provide a plausible explanation for its late-time photometric and
spectroscopic behavior. Using the Cepheid distance of M100, we derive a Hubble
constant of 72.7+/-8.2 km s^{-1} Mpc^{-1}(statistical) from the normalized
dereddened luminosity of SN 2006X. We briefly discuss whether abnormal dust is
a universal signature for all SNe Ia, and whether the most rapidly expanding
objects form a subclass with distinct photometric and spectroscopic properties.Comment: 48 pages, 20 figures and 11 tables. Accepted Version (ApJ, 2008,
March issue
The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients
Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs)
show a positive correlation between the X-ray flux at which the
low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the
following HS state. By analyzing the data from the All Sky Monitor (ASM)
onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux
after the source reaches its HS flux peak still correlates with the transition
flux during soft X-ray transient (SXT) outbursts. By studying large outbursts
or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the
correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition,
respectively. These time scales correspond to the viscous time scale in a
standard accretion disk around a stellar mass black hole or a neutron star at a
radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion
flow either correlate over a large range of radii at a given time or correlate
over a long period of time at a given radius. If the accretion geometry is a
two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow
in the LH state, the disk flow with a radius up to ~105 Rg would have
contributed to the nearly instantaneous non-thermal radiation directly or
indirectly, and therefore affects the time when the state transition occurs.Comment: 7 pages, 3 figure