arXiv subjects

R. J. Nemiroff

Publications and source records attributed to R. J. Nemiroff.

15 recordsLinked to original sources

arXiv2024

Search for Dyson rings around pulsars: unexpected light curves

Finding Dyson rings around distant pulsars may involve identifying light curve features that have not been previously identified. Previous studies covered the detection of a ring structure uniformly brightened by the central pulsar, mostly in infrared light. Here, more complex light curves are explored, which arise inherently from the pulsar beam spot's commonly predicted superluminal speed. These speeds may cause multiple images of the pulsar's spot on the Dyson ring to appear simultaneously to a distant observer, and so feature bright creation and annihilation events. Therefore, it is possible that even if Dyson ring structures had been observed previously, they might have remained unnoticed. Similar light curve features may appear on naturally occurring dust rings around pulsars that reflect detectable pulsar radiation.

astro-ph.IM↗

arXiv2011

The Possible Impact of GRB Detector Thresholds on Cosmological Standard Candles

GRB satellites are relatively inefficient detectors of dim hard bursts because they trigger on photon counts, which are number-biased against hard photons. Therefore, for example, given two bursts of identical peak luminosity near the detection threshold, a dim soft burst will be preferentially detected over a dim hard burst. This detector bias can create or skew an apparent correlation where increasingly hard GRBs appear increasingly bright. Although such correlations may be obfuscated by a middle step where GRBs need to be bright enough to have their actual redshifts determined, it is found that the bias is generally pervasive. This result is derived here through simulations convolving a wide variety of possible GRB brightnesses and spectra with the BATSE Large Area Detectors (LAD) detection thresholds. The presented analyses indicate that the rest-frame $νF_ν$ spectrum peak energy of long-duration GRBs, $\epi$, is not a good cosmological standard candle without significant corrections for selection effects. Therefore, the appearance of $\epi$ in seeming correlations such as the Amati ($E_{iso}-\epi$), Ghirlanda ($E_γ-\epi$), and $L_{iso}-\epi$ relations is statistically real but strongly influenced by so far uncalibrated GRB detector thresholds.

astro-ph.HE↗

arXiv2010

Continuous Monitoring of Comet Holmes from Before the 2007 Outburst

The outburst and subsequent brightness evolution of Comet 17P/Holmes has been observed using the MMT Observatory's All-Sky Camera (Pickering 2006) on Mt. Hopkins near Tucson, Arizona, USA. The comet was picked up at the limiting visual magnitude of 5.5 on October 24.38 and tracked by the camera continuously until sunrise four hours later. During this time the comet brightened to visual magnitude 3.5. Comet Holmes was next observed just after sunset on October 25.23 at visual magnitude 2.5 where it remained approximately constant over the next three days. The comet then began to dim slowly and was followed into the early months of 2008 with periods of dense time coverage.

astro-ph.EP↗

arXiv1998

Gravitationally Lensed Gamma-Ray Bursts as Probes of Dark Compact Objects

If dark matter in the form of compact objects comprises a large fraction of the mass of the universe, then gravitational lensing effects on gamma-ray bursts are expected. We utilize BATSE and Ulysses data to search for lenses of different mass ranges, which cause lensing in the milli, pico, and femto regimes. Null results are used to set weak limits on the cosmological abundance of compact objects in mass ranges from 10$^{-16}$ to 10$^{-9}$ $M_{\odot} $. A stronger limit is found for a much discussed $Ω= 0.15$ universe dominated by black holes of masses $\sim 10^{6.5} M_{\odot}$, which is ruled out at the $\sim$ 90% confidence level.

astro-ph↗

arXiv1998

On Source Density Evolution of Gamma-ray Bursts

Recent optical afterglow observations of gamma-ray bursts indicate a setting and distance scale that many relate to star-formation regions. In this paper, we use $ $ and a set of artificial trigger thresholds to probe several potential GRB source density evolutionary scenarios. In particular, we compare a uniform subset of BATSE 4B data to cosmological scenarios where GRBs evolve as the comoving density, the star formation rate, the QSO rate, and the SN Type Ic rate. Standard candle bursts with power-law spectra and a universe without vacuum energy were assumed. Our results significantly favor a comoving density model, implying that GRB source density evolution is weaker than expected in these evolutionary scenarios. GRB density might still follow star-formation rates given proper concurrent GRB luminosity evolution, significant beaming, significant error in standard candle assumptions, or were a significant modification of star formation rate estimates to occur.

astro-ph↗

arXiv1996

On Suggestive Correlations Between GRBs and Clusters of Galaxies

Recent claims of angular correlations between gamma-ray bursts (GRBs) and clusters of galaxies are evaluated in light of existing but previously uncorrelated GRB positional data. Additional GRB data sets we use include sub-samples of soft BATSE 3B bursts, bursts located by the Interplanetary Network (IPN), and GRBs localized by COMPTEL. We confirm a previously reported excess by Rood and Struble (1996) of the 185 rich, nearby clusters of galaxies (Abell, Corwin, and Olowin 1989, ACO) in the 1-$σ$ error circles of 74 BATSE 3B positions, but find a typical correlation strength of only $~$2.5-$σ$ for typical sub-samples. However, none of the 185 ACO clusters occur in the 1-$σ$ error boxes of 40 IPN GRBs or 18 COMPTEL GRBs. When all ACO clusters are correlated with BATSE 3B GRBs however, we find an increasingly strong correlation for GRBs with decreasingly small error boxes, reaching above the 3.5-$σ$ level. We also find a slight excess of {\it soft} BATSE GRBs near the positions of 185 rich, nearby ACO clusters, but the significance of the correlation averages only $~$2.5-$σ$ for sub-samples delineated by softness. We caution that the statistical significance of all these correlations is marginal, and so conclude that the excess is at best only suggestive of a physical association. Statistical fluke is still a strong possibility. BATSE could confirm or refute such correlations in a 10-year lifetime.

astro-ph↗

arXiv1996

Time-Dilation, Log N - Log P, and Cosmology

We investigate whether a simple cosmology can fit GRB results in both time dilation and Log N - Log P simultaneously. Simplifying assumptions include: all GRBs are spectrally identical to BATSE trigger 143, $Ω=1$ universe, and no luminosity and number density evolution. Observational data used includes: the BATSE 3B peak brightness distribution (64-ms time scale), the Pioneer Venus Orbiter (PVO) brightness distribution, and the Norris et al. time dilation results for peak aligned profiles presented at this meeting. We find acceptable cosmological fits to the brightness distributions when placing BATSE trigger 143 at a redshift of 0.15 $\pm$ 0.10. This translates into a $(1 + z_{dim}) / (1 + z_{bright})$ factor of about 1.50 $\pm$ 0.50 between selected brightness extremes of the Norris et al. sample. Norris et al. estimate, however, that $(1 + z_{dim}) / (1 + z_{bright})$ $\approx$ 2.0 $\pm$ 0.5 when considering duration tests. The difference is marginal and could be accounted for by evolution. We therefore find that evolution of GRBs is preferred but not demanded.

astro-ph↗

arXiv1996

Calibration of Tests for Time Dilation in GRB Pulse Structures

Two tests for cosmological time dilation in $γ$-ray bursts -- the peak alignment and auto-correlation statistics -- involve averaging information near the times of peak intensity. Both tests require width corrections, assuming cosmological origin for bursts, since narrower temporal structure from higher energy would be redshifted into the band of observation, and since intervals between pulse structures are included in the averaging procedures. We analyze long ($>$ 2 s) BATSE bursts and estimate total width corrections for trial time-dilation factors (TDF = [1+$z_{\rm dim}$]/[1+$z_{\rm brt}$]) by time-dilating and redshifting bright bursts. Both tests reveal significant trends of increasing TDF with decreasing peak flux, but neither provides sufficient discriminatory power to distinguish between actual TDFs in the range 2--3.

astro-ph↗

arXiv1996

Test for Time Dilation of Intervals Between Pulse Structures in GRBs

If $γ$-ray bursts are at cosmological distances, then not only their constituent pulses but also the intervals between pulses should be time-dilated. Unlike time-dilation measures of pulse emission, intervals would appear to require negligible correction for redshift of narrower temporal structure from higher energy into the band of observation. However, stretching of pulse intervals is inherently difficult to measure without incurring a timescale-dependent bias since, as time profiles are stretched, more structure can appear near the limit of resolution. This problem is compounded in dimmer bursts because identification of significant structures becomes more problematic. We attempt to minimize brightness bias by equalizing signal-to-noise (s/n) level of all bursts. We analyze wavelet-denoised burst profiles binned to several resolutions, identifying significant fluctuations between pulse structures and interjacent valleys. When bursts are ranked by peak flux, an interval time-dilation signature is evident, but its magnitude and significance are dependent upon temporal resolution and s/n level.

astro-ph↗

arXiv1995

Gamma-Ray Burst Peak Duration as a Function of Energy

Gamma-ray burst time histories often consist of many peaks. These peaks tend to be narrower at higher energy. If gamma-ray bursts are cosmological, the energy dependence of gamma-ray burst time scales must be understood in order to correct the time scale dependence due to the expansion of the universe. By using the average autocorrelation function and the average pulse width, we show that the narrowing with energy follows, quite well, a power law. The power law index is about -0.4. This is the first quantitative relationship between temporal and spectral structure in gamma-ray bursts. It is unclear what physics causes this relationship. The average autocorrelation has a universal shape such that one energy range scales linearly with time into all other energy ranges. This shape is approximately the sum of two exponential.

astro-ph↗

arXiv1994

Duration Distributions of Bright and Dim BATSE Gamma-Ray Bursts

We have measured the T90 and T50 durations of bright and dim gamma-ray bursts (GRBs) detected by the Compton Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE). The T90 (T50) duration is defined as the interval over which 5\% (25\%) to 95\% (75\%) of the burst counts accumulate. Out of 775 bursts observed by BATSE, 159 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. A Kolmogorov-Smirnov test yields a probability of 6 x $10^{-5}$ that the T50 durations of the dim and bright samples are drawn from the same parent population. We find that the centroid and extent of the duration distribution for the dim sample are scaled by approximately a factor of two relative to those of the bright sample. The measured time dilation factor is not sensitive to choice of energy band. These results are quantitatively consistent with previous tests for time dilation in a smaller sample of BATSE bursts. The dimmer bursts, if cosmological, would lie at redshifts of order two.

astro-ph↗

arXiv1994

Gross Spectral Differences between Bright and Dim Gamma-Ray Bursts

We find that dim gamma-ray bursts (GRBs) are softer than bright GRBs, as indicated on average by data from the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory. We show that this correlation is statistically significant with respect to variations due to random differences between GRBs. This effect is discernable using a variety of methods and data sets, including public domain data. We analyze several types of systematic errors and selection effects in the BATSE data and conclude that the observed effect is not dominated by any of them. We therefore assert that this dim/soft effect is a real property of GRBs. It is possible that this correlation is a consequence of the time dilation detected by Norris et al. (1994) and that this is additional evidence that burst sources are located at cosmological distances.

astro-ph↗

arXiv1994

Null Result in Gamma-Ray Burst Lensed Echo Search

We have searched for gravitational-lens induced echoes between gamma-ray bursts (GRBs) in NASA's orbiting {\it Compton} Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) data. The search was conducted in two phases. In the first phase we compared all GRBs in a brightness complete sample of the first 260 GRBs with recorded angular positions having at least 5 \% chance of being coincident from their combined positional error. In the second phase, we compared all GRB light curves of the first 611 GRBs with recorded angular positions having at least 55 \% chance of being coincident from their combined positional error. No unambiguous gravitational lens candidate pairs were found in either phase, although a ``library of close calls" was accumulated for future reference. This result neither excludes nor significantly constrains a cosmological origin for GRBs.

astro-ph↗

arXiv1993

Detection of Signature Consistent with Cosmological Time Dilation in Gamma-Ray Bursts

If gamma-ray bursters are at cosmological distances - as suggested by their isotropic distribution on the sky and by their number-intensity relation - then the burst profiles will be stretched in time, by an amount proportional to the redshift, 1 + $z$. We have tested data from the {\it Compton} Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) for such time dilation. Our measures of time scale are constructed to avoid selection effects arising from intensity differences by rescale all bursts to fiducial levels of peak intensity and noise bias. The three tests involved total count rate above background, wavelet decomposition, and alignment of the highest peaks. In all three tests, the dim bursts are stretched by a factor of about two relative to the bright ones, over seven octaves of time scale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts of BATSE's peak-flux completeness limit being at cosmological distances corresponding to $z \sim 1$, and thus with independent cosmological interpretations of the BATSE number-intensity relation.

astro-ph↗

arXiv1993

Gamma-Ray Bursts are Time-Asymmetric

A simple test for time-asymmetry is devised and carried out on the brightest gamma-ray bursts detected by the Burst and Transient Source Experiment (BATSE) on board the {\it Compton} Gamma Ray Observatory. We show evidence that individual bursts are time-asymmetric on all time scales tested, from a time scale shorter than that of pulses which compose GRBs to a time scale similar to a greater envelope that contains these pulses. We also find bursts which manifest significant asymmetry only on time scales comparable to the duration of burst, and bursts for which no clear asymmetry on any time scale is present. The sense of the asymmetry is that bursts and/or component structures rise in a shorter time than they decay. We also find that our whole sample of bursts taken together is time- asymmetric, in that there are significantly more bursts and pulses where the rise is more rapid than the decay, on all time scales tested and for all energy bands tested. When our whole GRB sample is binned at 64-ms and integrated over all BATSE energies, the statistical significance is at the 6 $σ$ level. Models that predict time-symmetry are therefore excluded.

astro-ph↗