313 research outputs found
Frequency Limits on Naked-Eye Optical Transients Lasting from Minutes to Years
How often do bright optical transients occur on the sky but go unreported? To
constrain the bright end of the astronomical transient function, a systematic
search for transients that become bright enough to be noticed by the unaided
eye was conducted using the all-sky monitors of the Night Sky Live network. Two
fisheye continuous cameras (CONCAMs) operating over three years created a data
base that was searched for transients that appeared in time-contiguous CCD
frames. Although a single candidate transient was found (Nemiroff and Shamir
2006), the lack of more transients is used here to deduce upper limits to the
general frequency of bright transients. To be detected, a transient must have
increased by over three visual magnitudes to become brighter than visual
magnitude 5.5 on the time scale of minutes to years. It is concluded that, on
the average, fewer than 0.0040 ( seconds) transients with
duration between minutes and hours, occur anywhere on the sky at any
one time. For transients on the order of months to years, fewer than 160
( year) occur, while for transients on the order of years to
millennia, fewer than 50 ( year) occur.Comment: Accepted for publication in A
Towards Locating the Brightest Microlensing Events on the Sky
It is estimated that a star brighter than visual magnitude 17 is undergoing a
detectable gravitational microlensing event, somewhere on the sky, at any given
time. It is assumed that both lenses and sources are normal stars drawn from a
standard Bahcall-Soneira model of our Galaxy. Furthermore, over the time scale
of a year, a star 15th magnitude or brighter should undergo a detectable
gravitational lens amplification. Detecting and studying the microlensing event
rate among the brightest 10 stars could yield a better understanding of
Galactic stellar and dark matter distributions. Diligent tracking of bright
microlensing events with even small telescopes might detect planets orbiting
these stellar lenses.Comment: 19 pages, 4 figures, accepted by Ap
Visual Distortions Near a Neutron Star and Black Hole
The visual distortion effects visible to an observer traveling around and
descending to the surface of an extremely compact star are described.
Specifically, trips to a ``normal" neutron star, a black hole, and an
ultracompact neutron star with extremely high surface gravity, are described.
Concepts such as multiple imaging, red- and blue-shifting, conservation of
surface brightness, the photon sphere, and the existence of multiple Einstein
rings are discussed in terms of what the viewer would see. Computer generated,
general relativistically accurate illustrations highlighting the distortion
effects are presented and discussed. A short movie (VHS) depicting many of
these effects is available to those interested free of charge.Comment: 23 pages, Plain TeX (v. 3.0), figures in American Journal of Physics,
61, 619, 1993, video available upon written (hard copy) request onl
Superluminal Spot Pair Events in Astronomical Settings: Sweeping Beams
Sweeping beams of light can cast spots moving with superluminal speeds across
scattering surfaces. Such faster-than-light speeds are well-known phenomena
that do not violate special relativity. It is shown here that under certain
circumstances, superluminal spot pair creation and annihilation events can
occur that provide unique information to observers. These spot pair events are
{\it not} particle pair events -- they are the sudden creation or annihilation
of a pair of relatively illuminated spots on a scattering surface. Real spot
pair illumination events occur unambiguously on the scattering surface when
spot speeds diverge, while virtual spot pair events are observer dependent and
perceived only when real spot radial speeds cross the speed of light.
Specifically, a virtual spot pair creation event will be observed when a real
spot's speed toward the observer drops below , while a virtual spot pair
annihilation event will be observed when a real spot's radial speed away from
the observer rises above . Superluminal spot pair events might be found
angularly, photometrically, or polarimetrically, and might carry useful
geometry or distance information. Two example scenarios are briefly considered.
The first is a beam swept across a scattering spherical object, exemplified by
spots of light moving across Earth's Moon and pulsar companions. The second is
a beam swept across a scattering planar wall or linear filament, exemplified by
spots of light moving across variable nebulae including Hubble's Variable
Nebula. In local cases where the sweeping beam can be controlled and repeated,
a three-dimensional map of a target object can be constructed. Used
tomographically, this imaging technique is fundamentally different from lens
photography, radar, and conventional lidar.Comment: 30 pages, 8 figures, accepted for publication in PAS
Extension of an Exponential Light Curve GRB Pulse Model Across Energy Bands
A simple mathematical model of GRB pulses in time, suggested in Norris et al.
(2005), is extended across energy. For a class of isolated pulses, two of those
parameters appear effectively independent of energy. Specifically, statistical
fits indicate that pulse amplitude and pulse width are energy
dependent, while pulse start time and pulse shape are effectively energy
independent. These results bolster the Pulse Start and Pulse Scale conjectures
of Nemiroff (2000) and add a new Pulse Shape conjecture which states that a
class of pulses all have the same shape. The simple resulting pulse counts
model is , where is the
time since the start of the pulse. This pulse model is found to be an
acceptable statistical fit to many of the fluent separable BATSE pulses listed
in Norris et al. (2005). Even without theoretical interpretation, this
cross-energy extension may be immediately useful for fitting prompt emission
from GRB pulses across energy channels with a minimal number of free
parameters.Comment: 11 pages, 5 figures. Accepted by MNRA
Tile or Stare? Cadence and Sky Monitoring Observing Strategies that Maximize the Number of Discovered Transients
To maximize the number of transients discovered on the sky, should
sky-monitoring projects stare at one location or continually jump from location
to location, tiling the sky? If tiling is preferred, what cadence maximizes the
discovery rate? As sky monitoring is a growing part of astronomical observing,
utilized to find such phenomena as supernovae, microlensing, and planet
transits, well thought out answers to these questions are increasingly
important. Answers are sky, source, and telescope dependent and should include
information about the source luminosity distribution near the observation
limit, the duration of variability, the nature of the dominant noise, and the
magnitude of down and slew times. Usually, a critical slope of the effective
cumulative transient apparent luminosity distribution (Log N - Log S) at the
limiting magnitude will define when "tile" or "stare" is superior. For
shallower slopes, when "tile" is superior, optimal cadences and pointing
algorithms are discussed. For transients discovered on a single exposure or
time-contiguous series of exposures, when down and slew times are small and the
character of the noise is unchanged, the most productive cadence for isotropic
power-law luminosity distributions is the duration of the transient -- faster
cadences waste time re-discovering known transients, while slower cadences
neglect transients occurring in other fields. A "cadence creep" strategy might
find an optimal discovery cadence experimentally when one is not uniquely
predetermined theoretically. Guest investigator programs might diversify
previously dedicated sky monitoring telescopes by implementing bandpasses and
cadences chosen to optimize the discovery of different types of transients.
Example analyses are given for SuperMACHO, LSST, and GLAST.Comment: 28 pages, 4 figures. Accepted to Astronomical Journal. Mission
specific correspondence welcome (to [email protected]
Attributes of Gravitational Lensing Parallax
The density of stars and MACHOs in the universe could theoretically be
determined or limited by simultaneous measurements of compact sources by well
separated observers. A gravitational lens effect would be expected to create a
slight differential amplification between the observers detectable with
sufficiently sensitive relative photometry: "lensing parallax." When applied to
expanding fireballs such as those from GRBs and supernovae, the mass of the
lens can be indicated by the end of lensing parallax, when the angular size of
the source becomes much greater than the angular size of the Einstein ring of
the lens.Comment: 7 pages, to be published in Astrophysics and Space Scienc
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