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 ($t_{dur} / 60$ seconds) transients with duration $t_{dur}$ 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 ($t_{dur} / 1$ year) occur, while for transients on the order of years to millennia, fewer than 50 ($t_{dur}/1$ year)$^2$ occur.Comment: Accepted for publication in A

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 $A$ and pulse width $\tau$ 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 $P(t,E) = A(E) \ {\rm exp} (-t/\tau(E) - \tau(E)/t)$, where $t$ 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

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

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

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]

X-ray study of low-temperature annealed arsenic-implanted silicon

Low-temperature anneals (500–650 °C) of 2, 4, and 8×10^15 cm^−2 As+ implanted in silicon at 50 keV were studied by x-ray double crystal diffraction. The rocking curves were analyzed by a kinematical model. Two regions of strain were found in the solid-phase epitaxially regrown layer. One layer was uniform and positively strained. The other was nonuniform and negatively strained. By comparing rocking curves of repeatedly etched layers it was found that the surface layer is negatively strained, corresponding largely to the substitutional As in the regrown layer. The positively strained region lies at the interface between the implanted layer and the undamaged silicon substrate

Probing For Machos of Mass 10−15M⊙10^{-15}M_\odot-10−7M⊙10^{-7}M_\odot with Gamma-Ray Burst Parallax Spacecraft

Two spacecraft separated by \sim 1\,\au and equipped with gamma-ray burst (GRB) detectors could detect or rule out a cosmological density of Massive Compact Halo Objects (MACHOs) in the mass range 10^{-15} M_{\odot}\lsim M \lsim 10^{-7} M_{\odot} provided that GRBs prove to be cosmological. Previously devised methods for detecting MACHOs have spanned the mass range 10^{-16} M_{\odot}\lsim M \lsim 10^{7} M_{\odot}, but with a gap of several orders of magnitude near $10^{-9} M_{\odot}$. For MACHOs and sources both at a cosmological distance, the Einstein radius is \sim 1\,\au\,(M/10^{-7} M_\odot)^{1/2}. Hence, if a GRB lies within the Einstein ring of a MACHO of mass M\lsim 10^{-7}M_\odot as seen by one detector, it will not lie in the Einstein ring as seen by a second detector \sim 1\,\au away. This implies that if GRBs are measured to have significantly different fluxes by the two detectors, this would signal the presence of a MACHO \lsim 10^{-7}M_\odot. By the same token, if the two detectors measured similar fluxes for several hundred events a cosmological abundance of such low-mass MACHOs would be ruled out. The lower limit of sensitivity, M\lsim 10^{-15}M_\odot is set by the finite size of the source. If low-mass MACHOs are detected, there are tests which can discriminate among events generated by MACHOs in the three mass ranges M\lsim 10^{-12}\,M_\odot, 10^{-12}\,M_\odot\lsim M\lsim 10^{-7}\,M_\odot, and M\gsim 10^{-7}\ M_\odot. Further experiments would then be required to make more accurate mass measurements.Comment: 8 pages, uuencoded postscript, no figure