arXiv subjects

Bradley E. Schaefer

Publications and source records attributed to Bradley E. Schaefer.

103 records · Page 6Linked to original sources

arXiv1999

Gamma Ray Burst Host Galaxies Have `Normal' Luminosities

The galactic environment of Gamma Ray Bursts can provide good evidence about the nature of the progenitor system, with two old arguments implying that the burst host galaxies are significantly subluminous. New data and new analysis have now reversed this picture: (A) Even though the first two known host galaxies are indeed greatly subluminous, the next eight hosts have absolute magnitudes typical for a population of field galaxies. A detailed analysis of the 16 known hosts (ten with red shifts) shows them to be consistent with a Schechter luminosity function with $R^{*} = -21.8 \pm 1.0$ as expected for normal galaxies. (B) Bright bursts from the Interplanetary Network are typically 18 times brighter than the faint bursts with red shifts, however the bright bursts do not have galaxies inside their error boxes to limits deeper than expected based on the luminosities for the two samples being identical. A new solution to this dilemma is that a broad burst luminosity function along with a burst number density varying as the star formation rate will require the average luminosity of the bright sample ($>$$6 \times 10^{58} ph \cdot s^{-1}$ or $>$$1.7 \times 10^{52} \cdot erg \cdot s^{-1}$) to be much greater than the average luminosity of the faint sample ($\sim 10^{58} ph \cdot s^{-1}$ or $\sim 3 \times 10^{51} erg \cdot s^{-1}$). This places the bright bursts at distances for which host galaxies with a normal luminosity will not violate the observed limits. In conclusion, all current evidence points to GRB host galaxies being normal in luminosity.

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arXiv1999

Superflares on Ordinary Solar-Type Stars

Short duration flares are well known to occur on cool main-sequence stars as well as on many types of `exotic' stars. Ordinary main-sequence stars are usually pictured as being static on time scales of millions or billions of years. Our sun has occasional flares involving up to $\sim 10^{31}$ ergs which produce optical brightenings too small in amplitude to be detected in disk-integrated brightness. However, we identify nine cases of superflares involving $10^{33}$ to $10^{38}$ ergs on normal solar-type stars. That is, these stars are on or near the main-sequence, are of spectral class from F8 to G8, are single (or in very wide binaries), are not rapid rotators, and are not exceedingly young in age. This class of stars includes many those recently discovered to have planets as well as our own Sun, and the consequences for any life on surrounding planets could be profound. For the case of the Sun, historical records suggest that no superflares have occurred in the last two millennia.

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arXiv1999

Are superflares on solar analogues caused by extra-solar planets?

Stellar flares with ${10^2-10^7}$ times more energy than the largest solar flare have been detected from 9 normal F and G main sequence stars (Schaefer, King & Deliyannis 1999). These superflares have durations of hours to days and are visible from at least x-ray to optical frequencies. The absence of world-spanning aurorae in historical records and of anomalous extinctions in the geological record indicate that our Sun likely does not suffer superflares. In seeking to explain this new phenomenon, we are struck by its similarity to large stellar flares on RS Canum Venaticorum binary systems, which are caused by magnetic reconnection events associated with the tangling of magnetic fields between the two stars. The superflare stars are certainly not of this class, although we propose a similar flare mechanism. That is, superflares are caused by magnetic reconnection between fields of the primary star and a close-in Jovian planet. Thus, by only invoking known planetary properties and reconnection scenarios, we can explain the energies, durations, and spectra of superflares, as well as explain why our Sun does not have such events.

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arXiv1999

Discovery of the Optical Transient of the Gamma Ray Burst 990308

The optical transient of the faint Gamma Ray Burst 990308 was detected by the QUEST camera on the Venezuelan 1-m Schmidt telescope starting 3.28 hours after the burst. Our photometry gives $V = 18.32 \pm 0.07$, $R = 18.14 \pm 0.06$, $B = 18.65 \pm 0.23$, and $R = 18.22 \pm 0.05$ for times ranging from 3.28 to 3.47 hours after the burst. The colors correspond to a spectral slope of close to $f_ν \propto ν^{1/3}$. Within the standard synchrotron fireball model, this requires that the external medium be less dense than $10^{4} cm^{-3}$, the electrons contain $> 20%$ of the shock energy, and the magnetic field energy must be less than 24% of the energy in the electrons for normal interstellar or circumstellar densities. We also report upper limits of $V > 12.0$ at 132 s (with LOTIS), $V > 13.4$ from 132-1029s (with LOTIS), $V > 15.3$ at 28.2 min (with Super-LOTIS), and a 8.5 GHz flux of $< 114 μJy$ at 110 days (with the Very Large Array). WIYN 3.5-m and Keck 10-m telescopes reveal this location to be empty of any host galaxy to $R > 25.7$ and $K > 23.3$. The lack of a host galaxy likely implies that it is either substantially subluminous or more distant than a red shift of $\sim 1.2$.

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arXiv1999

The Late Time Light Curve of SN 1998bw Associated with GRB980425

We report 139 photometric observations through the B, V, and I filters of the supernova SN 1998bw, an object which is associated with the Gamma-Ray Burst GRB 980425. Detailed light curves of this unique supernova can be compared to theoretical models, so we report here our light curve for 123 days between 27 June 1998 and 28 October 1998. The light curve of SN 1988bw is consistent with those of the Type Ic class. We find that the magnitude-versus-time relation for this supernova is linear to within 0.05 mags in all colors over the entire duration of our study. Our measured uniform decline rates are $0.0141 \pm 0.0002$, $0.0184 \pm 0.0003$, and $0.0181 \pm 0.0003$ magnitudes per day in the B, V, and I bands. The linear decline and the rate of that decline suggest that late time light curve is powered by the radioactive decay of cobalt with some leakage of the gamma rays.

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arXiv1999

The Spectrum of GRB 930131 (``Superbowl Burst'') from 20 keV to 200 MeV

We have constructed a broad-band spectrum for GRB 930131 (the ``Superbowl Burst''), ranging from 20 keV to 200 MeV, by combining spectral information from the Gamma Ray Observatory's BATSE, COMPTEL and EGRET instruments. We present general methods for combining spectra from different time intervals obtained by the same instrument as well as for combining spectra from the same time interval taken by different instruments. The resulting spectrum is remarkably flat (in nu F_nu-space) up to high energies. We find that the spectral shape can be successfully fitted by the shocked synchrotron emission model of Tavani. We present evidence that the flatness of the spectrum at high energies is not due to spectral time-variability.

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arXiv1998

Severe Limits on Variations of the Speed of Light with Frequency

Explosive astrophysical events at high red shift can be used to place severe limits on the fractional variation in the speed of light ($Δc/c$), the photon mass ($m_γ$), and the energy scale of quantum gravity ($E_{QG}$). I find $Δc/c < 6.3 \times 10^{-21}$ based on the simultaneous arrival of a flare in GRB 930229 with a rise time of $220 \pm 30 μs$ for photons of 30 keV and 200 keV. The limit on $m_γ$ is $4.2 \times 10^{-44} g$ for GRB 980703 from radio to gamma ray observations. The limit on $E_{QG}$ is $8.3 \times 10^{16}$ GeV for GRB 930131 from 30 keV to 80 MeV photons.

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arXiv1998

Severe New Limits on the Host Galaxies of Gamma Ray Bursts

The nature of Gamma Ray Bursts (GRBs) remains a complete mystery, despite the recent breakthrough discovery of low energy counterparts, although it is now generally believed that at least most GRBs are at cosmological distances. Virtually all proposed cosmological models require bursters to reside in ordinary galaxies. This can be tested by looking inside the smallest GRB error boxes to see if ordinary galaxies appear at the expected brightness levels. This letter reports on an analysis of the contents of 26 of the smallest regions, many from the brightest bursts. These events will have $z < 0.4$ and small uncertainties about luminosity functions, K corrections and galaxy evolutions; whereas the recent events with optical transients are much fainter and hence have high redshifts and grave difficulties in interpretation. This analysis strongly rejects the many models with peak luminosities of $10^{57} photons \cdot s^{-1}$ as deduced from the $LogN-LogP$ curve with no evolution. Indeed, the lower limit on acceptable luminosities is $6 \times 10^{58} photons \cdot s^{-1}$. The only possible solution is to either place GRBs at unexpectedly large distances (with $z > 5.9$ for the faint BATSE bursts) or to require bursters to be far outside any normal host galaxy.

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arXiv1998

Gamma Ray Bursts Have Millisecond Variability

We have performed searches for isolated flares and for steady flickering in Gamma Ray Burst light curves on the microsecond to millisecond time scales. Two bursts out of our sample of 20 revealed four isolated flares with time scales from $256 μs$ to $2048 μs$. A wavelet analysis for our sample showed low level flickering for all bursts on time scales from $256 μs$ to 33 ms, with the majority of bursts containing rise times faster than four milliseconds and 30% having rise times faster than one millisecond. These results show that millisecond variability is common in classical bursts and not some exceptional activity by a possibly separate class of bursts. These fast rise times can be used to place severe limits on burst models: (1) The characteristic thickness of the energy generation region must be less than 1200 km along the line of sight. (2) The angular size of the gamma ray emission region as subtended from the central source must be less than 42 arc-seconds. (3) The expanding ejecta must have a range of Lorentz factors along a radius line with a dispersion of less than roughly 2%. (4) Within the external shock scenario, the characteristic dimension of the impacted cloud must be smaller than 16 Astronomical Units on average. (5) Within the collimated jet scenario, the collimation angle must be smaller than 42 arc-seconds.

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arXiv1998

A Gamma Ray Burst with a 220 Microsecond Rise Time and a Sharp Spectral Cutoff

The Gamma Ray Burst GRB920229 has four extreme and unprecedented properties; a rise in brightness with an e-folding time scale of $220 \pm 30 μs$, a fall in brightness with an e-folding time scale of $400 \pm 100 μs$, a large change in spectral shape over a time of $768 μs$, and a sharp spectral cutoff to high energies with $ΔE/E = 18 %$. The rapid changes occur during a spike in the light curve which was seen 0.164 s after the start of the burst. The spectrum has a peak $νF_ν$ at 200 keV with no significant flux above 239 keV, although the cutoff energy shifts to less than 100 keV during the decay of the spike. These numbers can be used to place severe limits on fireball models of bursts. The thickness of the energy production region must be smaller than $\sim 66 km$, ejected shells must have a dispersion of the Lorentz factor of less than roughly 1% along a particular radius, and the angular size of the radiation emission region is of order 1 arc-minute as viewed from the burst center. The physical mechanism that caused the sharp spectral cutoff has not been determined.

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arXiv1998

The Peak Brightness of SN1974G in NGC4414 and the Hubble Constant

The light curve of the Type Ia supernova SN1974G (in NGC4414) is important because the Hubble Space Telescope has measured the distance to the host galaxy by means of Cepheid variables and thus the Hubble Constant can be derived. Light curves from the secondary literature are inadequate since the majority of data is misreported, the majority of the published data is overlooked, and the majority of all data is unpublished, while comparison star sequences have offsets of over half a magnitude. I have recovered and validated all data, remeasured the comparison stars, and performed light curve template fits. I find the observed peak B and V magnitudes to be $12.48 \pm 0.05$ and $12.30 \pm 0.05$, with a decline rate of $Δm_{15} = 1.11 \pm 0.06$. For $E(B-V) = 0.16 \pm 0.07$, the unabsorbed peak magnitudes are $B = 11.82 \pm 0.29$ and $V = 11.80 \pm 0.22$. With the distance modulus to NGC4414 as $μ= 31.41 \pm 0.23$, I find $H_{0} = 55 \pm 8 km \cdot s^{-1} \cdot Mpc^{-1}$.

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arXiv1998

Time Dilation in the Peak-to-Peak Time Scale of GRBs

We present strong evidence of time stretching in the peak-to-peak time scales in the light curves of BATSE Gamma Ray Bursts (GRBs). Extensive tests are performed on artificially dilated bursts to verify that the procedure for extracting the peak-to-peak time scales correctly recovers the stretching of bursts. The resulting robust algorithm is then applied to the 4B GRB database. We derive a stretching factor of $1.92 \pm 0.13$ between the brightest burst group ($P > 7.7 photon \cdot cm^{-2} \cdot s^{-1}$) and the dimmest burst group ($P = 1.0 \sim 1.4 photon \cdot cm^{-2} \cdot s^{-1}$) with several independent peak-to-peak time scale definitions and they agree within uncertainties. Such an agreement strongly supports the interpretation of the observed time stretching as time dilation caused by the cosmological expansion, rather than physical selection effects. We fit the result to cosmological models with $Λ= 0$, $Ω_{0}$ from 0.2 to 1.0, and contrained the standard candle luminosity to be $L_{0}= 7.0 \pm 2.0 \pm 2.7 \times 10^{56} photons \cdot s^{-1}$. Our luminosity value is fully consistent with the value from the combined PVO and BATSE LogN-LogP curve with the BATSE bright bursts at low redshifts of $z_{bright} = 0.11 \pm 0.02 \pm 0.025$. This luminosity fit is definitely inconsistent with the the larger distance scale implied from associating burst density with star formation rates.

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arXiv1997

Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data

The surest solution of the Gamma Ray Burst (GRB) mystery is to find an unambiguous low-energy quiescent counterpart. However, to date no reasonable candidates have been identified in the x-ray, optical, infrared, or radio ranges. The Hubble Space Telescope (HST) has now allowed for the first deep ultraviolet searches for quiescent counterparts. This paper reports on multiepoch ultraviolet searches of five GRB positions with HST. We found no sources with significant ultraviolet excesses, variability, parallax, or proper motion in any of the burst error regions. In particular, we see no sources similar to that proposed as a counterpart to the GRB970228. While this negative result is disappointing, it still has good utility for its strict limits on the no-host-galaxy problem in cosmological models of GRBs. For most cosmological models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes for some of the brightest bursts provide the most critical test, and our limits are a severe problem for all published cosmological burst models.

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