1,187 research outputs found
Different kinds of long-term variability from Cygnus X-1
We present a study of the long-term variability of Cyg X-1 using data from
the RXTE/ASM and the RXTE/PCA during the time between the two soft states of
1996 and 2001/2002. This period has been characterized by many short ASM
flaring episodes which we have identified as "failed state transitions". The
150 d period which has been seen before and shortly after the 1996 soft state
is not obviously present in the ASM rate during most of this time. Applying
selection criteria from our pointed RXTE/PCA observations to exclude the
flaring episodes we show that the 150 d period can indeed still be
significantly detected in the hard state. Furthermore, while the ~420 d
timescale associated with the flaring is reduced in the selected hard state
count rate, it is still pronounced in the temporal evolution of the
corresponding hardness ratios. The Ryle radio flux is also consistent with the
150 d period being present but distorted during this time.Comment: 4 pages, 6 figures, to appear in Proceedings of "X-ray Timing 2003:
Rossi and Beyond", ed. P. Kaaret, F.K. Lamb, & J.H. Swan
New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations
One aspect of the quantum nature of spacetime is its "foaminess" at very
small scales. Many models for spacetime foam are defined by the accumulation
power , which parameterizes the rate at which Planck-scale spatial
uncertainties (and thephase shifts they produce) may accumulate over large
path-lengths. Here is defined by theexpression for the path-length
fluctuations, , of a source at distance , wherein , with being the Planck
length. We reassess previous proposals to use astronomical observations
ofdistant quasars and AGN to test models of spacetime foam. We show explicitly
how wavefront distortions on small scales cause the image intensity to decay to
the point where distant objects become undetectable when the path-length
fluctuations become comparable to the wavelength of the radiation. We use X-ray
observations from {\em Chandra} to set the constraint ,
which rules out the random walk model (with ). Much firmer
constraints canbe set utilizing detections of quasars at GeV energies with {\em
Fermi}, and at TeV energies with ground-based Cherenkovtelescopes: and , respectively. These limits on
seem to rule out , the model of some physical interest.Comment: 11 pages, 9 figures, ApJ, in pres
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