1,110 research outputs found
HI Epoch of Reionization Arrays
There are few data available with which to constrain the thermal history of
the intergalactic medium (IGM) following global recombination. Thus far, most
constraints flow from analyses of the Cosmic Microwave Background and optical
spectroscopy along a few lines of sight. However, direct study of the IGM in
emission or absorption against the CMB via the 1S hyperfine transition of
Hydrogen would enable broad characterization thermal history and source
populations. New generations of radio arrays are in development to measure this
line signature. Bright foreground emission and the complexity of instrument
calibration models are significant hurdles. How to optimize these is uncertain,
resulting in a diversity in approaches. We discuss recent limits on line
brightness, array efforts including the new Large Aperture Experiment to Detect
the Dark Ages (LEDA), and the next generation Hydrogen Reionization Array
(HERA) concept.Comment: 8 pages, 4 figures, 1 table. Invited review to the 11th Asian-Pacific
Regional IAU Meeting 2011, NARIT Conference Series, Vol. 1 eds. S.
Komonjinda, Y. Kovalev, and D. Ruffolo (2012
Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State
The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsar
GX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from an
intensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recovered
towards the end of the observation. This extended minimum lasted ~40,000
seconds. Phase folded light curves at a barycentric rotation period of
124.36568 +/- 0.00020 seconds show that near the center of the extended minimum
the source stopped pulsing in the traditional sense but retained a weak dip
feature at the rotation period. Away from the extended minimum the dips are
progressively narrower at higher energies and may be interpreted as
obscurations or eclipses of the hot spot by the accretion column. The pulse
profile changed from leading-edge bright before the extended minimum to
trailing-edge bright after it. Data from the Burst and Transient Source
Experiment (BATSE) show that a torque reversal occurred <10 days after our
observation. Our data indicate that the observed rotation departs from a
constant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigma
significance. We infer that we may have serendipitously obtained data, with
high sensitivity and temporal resolution about the time of an accretion disk
spin reversal. We also observed a rapid flare which had some precursor
activity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical
Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Angular Momentum Transfer in the Binary X-ray Pulsar GX 1+4
We describe three presentations relating to the X-ray pulsar GX 1+4 at a
workshop on magnetic fields and accretion at the Astrophysical Theory Centre,
Australian National University on 1998, November 12-13. Optical and X-ray
spectroscopy indicate that GX 1+4 is seen through a cloud of gravitationaly
bound matter. We discuss an unstable negative feedback mechanism (originally
proposed by Kotani et al, 1999), based on X-ray heating of this matter which
controls the accretion rate when the source is in a low X-ray luminosity state.
A deep minimum lasting ~6 hours occurred during observations with the RXTE
satellite over 1996, July 19-21. The shape of the X-ray pulses changed
remarkably from before to after the minimum. These changes may be related to
the transition from neutron star spin-down to spin-up which occurred at about
the same time. Smoothed particle hydrodynamic simulations of the effect of
adding matter with opposite angular momentum to an existing disc, show that it
is possible for a number of concentric rings with alternating senses of
rotation to co-exist in a disc. This could provide an explanation for the
step-like changes in Pdot which are observed in GX 1+4. Changes at the inner
boundary of the disc occur at the same timescale as that imposed at the outer
boundary. Reversals of material torque on the neutron star occur at a minimum
in L_X.Comment: 10 pages, 5 figures; accepted for publication by PAS
Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4-3114
Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX
J0812.4-3114 and A 0535+26) have previously been suggested to arise from
partial eclipses of the emission region by the accretion column occurring once
each rotation period. We present pulse-phase spectroscopy from Rossi X-ray
Timing Explorer satellite observations of GX 1+4 and RX J0812.4-3114 which for
the first time confirms this interpretation. The dip phase corresponds to the
closest approach of the column axis to the line of sight, and the additional
optical depth for photons escaping from the column in this direction gives rise
to both the decrease in flux and increase in the fitted optical depth measured
at this phase. Analysis of the arrival time of individual dips in GX~1+4
provides the first measurement of azimuthal wandering of a neutron star
accretion column. The column longitude varies stochastically with standard
deviation 2-6 degrees depending on the source luminosity. Measurements of the
phase width of the dip both from mean pulse profiles and individual eclipses
demonstrates that the dip width is proportional to the flux. The variation is
consistent with that expected if the azimuthal extent of the accretion column
depends only upon the Keplerian velocity at the inner disc radius, which varies
as a consequence of the accretion rate Mdot.Comment: 7 pages, 5 figures, accepted by MNRAS. Included reference
Spectral variation in the X-ray pulsar GX 1+4 during a low-flux episode
The X-ray pulsar GX 1+4 was observed with the RXTE satellite for a total of
51ks between 1996 July 19 - 21. During this period the flux decreased smoothly
from an initial mean level of ~ 6 X 10^36 erg/s to a minimum of ~ 4 X 10^35
erg/s (2-60 keV, assuming a source distance of 10 kpc) before partially
recovering towards the initial level at the end of the observation.
BATSE pulse timing measurements indicate that a torque reversal took place
approximately 10 d after this observation. Both the mean pulse profile and the
photon spectrum varied significantly. The observed variation in the source may
provide important clues as to the mechanism of torque reversals.
The single best-fitting spectral model was based on a component originating
from thermal photons with kT ~ 1 keV Comptonised by a plasma of temperature kT
\~ 7 keV. Both the flux modulation with phase during the brightest interval and
the evolution of the mean spectra over the course of the observation are
consistent with variations in this model component; with, in addition, a
doubling of the column density nH contributing to the mean spectral change.
A strong flare of duration 50 s was observed during the interval of minimum
flux, with the peak flux ~ 20 times the mean level. Although beaming effects
are likely to mask the true variation in Mdot thought to give rise to the
flare, the timing of a modest increase in flux prior to the flare is consistent
with dual episodes of accretion resulting from successive orbits of a locally
dense patch of matter in the accretion disc.Comment: 8 pages, 3 figures, submitted to MNRA
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
Foreground Model and Antenna Calibration Errors in the Measurement of the Sky-Averaged \lambda 21 cm Signal at z~20
The most promising near-term observable of the cosmic dark age prior to
widespread reionization (z~15-200) is the sky-averaged \lambda 21 cm background
arising from hydrogen in the intergalactic medium. Though an individual antenna
could in principle detect the line signature, data analysis must separate
foregrounds that are orders of magnitude brighter than the \lambda 21 cm
background (but that are anticipated to vary monotonically and gradually with
frequency). Using more physically motivated models for foregrounds than in
previous studies, we show that the intrinsic "spectral smoothness" of the
foregrounds is likely not a concern, and that data analysis for an ideal
antenna should be able to detect the \lambda 21 cm signal after deprojecting a
~5th order polynomial in log(\nu). However, we find that the foreground signal
is corrupted by the frequency-dependent response of a real antenna. The
frequency dependence complicates modeling of foregrounds commonly based on the
assumption of spectral smoothness. Much of our study focuses on the
Large-aperture Experiment to detect the Dark Age (LEDA), which combines both
radiometric and interferometric measurements. We show that statistical
uncertainty remaining after fitting antenna gain patterns to interferometric
measurements does not compromise extraction of the \lambda 21 cm signal for a
range of cosmological models after fitting a 7th order polynomial to
radiometric data. Our results generalize to most efforts to measure the
sky-averaged spectrum.Comment: 12 pages, 12 figures, accepted for publication in ApJ. Accepted
version uploade
High Angular Resolution Mid-infrared Imaging of Young Stars in Orion BN/KL
We present Keck LWS images of the Orion BN/KL star forming region obtained in
the first multi-wavelength study to have 0.3-0.5" resolution from 4.7 to 22
microns. The young stellar objects designated infrared source-n and radio
source-I are believed to dominate the BN/KL region. We have detected extended
emission from a probable accretion disk around source-n but infer a stellar
luminosity on the order of only 2000 Lsun. Although source-I is believed to be
more luminous, we do not detect an infrared counterpart even at the longest
wavelengths. However, we resolve the closeby infrared source, IRc2, into an arc
of knots ~1000 AU long at all wavelengths. Although the physical relation of
source-I to IRc2 remains ambiguous, we suggest these sources mark a high
density core (10^7-10^8 pc^-3 over 1000 AU) within the larger BN/KL star
forming cluster. The high density may be a consequence of the core being young
and heavily embedded. We suggest the energetics of the BN/KL region may be
dominated by this cluster core rather than one or two individual sources.Comment: 13 pages including 3 color figures. Accepted to The Astrophysical
Journal Letters pending slight reduction in length. High resolution figures
(jpeg) may be found at
http://cfa-www.harvard.edu/~lincoln/keck.bnkl.midir.ppr
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