Engineering topics

Schaefer, Bradley E.

Publications and source records attributed to Schaefer, Bradley E..

At least 19 records

DOE OSTI2022

Discovery of extreme, roughly daily superflares on the recurrent nova V2487 Oph

ABSTRACT V2487 Oph is a recurrent nova with detected eruptions in 1900 and 1998. Startlingly, V2487 Oph shows flares, called ‘Superflares’, with up to 1.10 mag amplitude, fast rises of under one-minute, always with an initial impulsive spike followed by a roughly exponential tail, typically one-hour durations, and with random event times averaging once-per-day. The typical flare energy E is over 1038 erg, while the yearly energy budget is 1041 erg. V2487 Oph Superflares obey three relations; the number distribution of flare energies scales as E−2.34 ± 0.35, the waiting time from one flare to the next is proportional to E of the first event, and flare durations scale as E0.44 ± 0.03. Scenarios involving gravitational energy and nuclear energy fail to satisfy the three relations. The magnetic energy scenario, however, can explain all three relations. This scenario has magnetic field lines above the disc being twisted and amplified by the motions of their footprints, with magnetic reconnection releasing energy that comes out as Superflare light. This exact mechanism is already well known to occur in white light solar flares, in ordinary M-type flare stars, and in the many Superflare stars observed all across the H-R diagram. Superflares on Superflare stars have rise times, light-curve shapes, and durations that are very similar to those on V2487 Oph. So we conclude that the V2487 Oph Superflares are caused by large-scale magnetic reconnection. V2487 Oph is now the most extreme Superflare star, exhibiting the largest known flare energy (1.6 × 1039 erg) and the fastest occurrence rate.

Schaefer, Bradley E.↗

DOE OSTI2015

PHOTOMETRY OF THE STINGRAY NEBULA (V839 ARA) FROM 1889 TO 2015 ACROSS THE IONIZATION OF ITS PLANETARY NEBULA

Up until around 1980, the Stingray was an ordinary B1 post-AGB star, but then it suddenly sprouted bright emission lines like in a planetary nebula (PN), and soon after this the Hubble Space Telescope (HST) discovered a small PN around the star, so apparently we have caught a star in the act of ionizing a PN. We report here on a well-sampled light curve from 1889 to 2015, with unique coverage of the prior century plus the entire duration of the PN formation plus three decades of its aftermath. Surprisingly, the star anticipated the 1980s ionization event by declining from B = 10.30 in 1889 to B = 10.76 in 1980. Starting in 1980, the central star faded fast, at a rate of 0.20 mag year{sup −1}, reaching B = 14.64 in 1996. This fast fading is apparently caused by the central star shrinking in size. From 1994 to 2015, the V-band light curve is almost entirely from the flux of two bright [O iii] emission lines from the unresolved nebula, and it shows a consistent decline at a rate of 0.090 mag year{sup −1}. This steady fading (also seen in the radio and infrared) has a timescale equal to that expected for ordinary recombination within the nebula, immediately after a short-duration ionizing event in the 1980s. We are providing the first direct measure of the rapidly changing luminosity of the central star on both sides of a presumed thermal pulse in 1980, with this providing a strong and critical set of constraints, and these are found to sharply disagree with theoretical models of PN evolution.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

The 2010 eruption of the recurrent nova U Scorpii: The multi-wavelength light curve

The recurrent nova U Scorpii most recently erupted in 2010. Our collaboration observed the eruption in bands ranging from the Swift XRT and UVOT w2 (193 nm) to K-band (2200 nm), with a few serendipitous observations stretching down to WISE W2 (4600 nm). Considering the time and wavelength coverage, this is the most comprehensively observed nova eruption to date. We present here the resulting multi-wavelength light curve covering the two months of the eruption as well as a few months into quiescence. For the first time, a U Sco eruption has been followed all the way back to quiescence, leading to the discovery of new features in the light curve, including a second, as-yet-unexplained, plateau in the optical and near-infrared. Using this light curve we show that U Sco nearly fits the broken power law decline predicted by Hachisu and Kato, with decline indices of −1.71 ± 0.02 and −3.36 ± 0.14. With our unprecedented multi-wavelength coverage, we construct daily spectral energy distributions and then calculate the total radiated energy of the eruption, E{sub rad}=6.99{sub −0.57}{sup +0.83}×10{sup 44} erg. From that, we estimate the total amount of mass ejected by the eruption to be m{sub ej}=2.10{sub −0.17}{sup +0.24}×10{sup −6}M{sub ⊙}. We compare this to the total amount of mass accreted by U Sco before the eruption, to determine whether the white dwarf undergoes a net mass loss or gain, but find that the values for the amount of mass accreted are not precise enough to make a useful comparison.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

HST images flash ionization of old ejecta by the 2011 eruption of recurrent Nova T PYXIDIS

T Pyxidis is the only recurrent nova known to be surrounded by knots of material ejected in previous outbursts. Following the eruption that began on 2011 April 14.29, we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space Telescope narrowband Hα images of T Pyx. The ionizing flash of radiation from the nova event had no discernible effect on the surrounding ejecta until at least 55 days after the eruption began. Photoionization of hydrogen located north and south of the central star was seen 132 days after the beginning of the eruption. That photoionized hydrogen recombined in the following 51 days, allowing us to determine a hydrogen atom density of at least 7×10{sup 5} cm{sup −3}—at least an order of magnitude denser than the previously detected, unresolved [N ii] knots surrounding T Pyx. Material to the northwest and southeast was photoionized, and became bright between 132 and 183 days after the eruption began. Ninety-nine days later that northwest and southeast hydrogen had recombined. Both then (282 days after outburst) and 101 days later, we detected almost no trace of hydrogen emission around T Pyx. We determine that there is a large reservoir of previously unseen, cold diffuse hydrogen overlapping the previously detected, [N ii]-emitting knots of T Pyx ejecta. The mass of this newly detected hydrogen is model-dependent, but is is probably an order of magnitude larger than that of the [N ii] knots. We also determine that there is no significant reservoir of undetected hydrogen-rich ejecta, with density comparable to the flash-ionized ejecta we have detected, from the outer boundaries of the previously detected ejecta out to about twice that distance. The lack of distant ejecta is consistent with the Schaefer et al. scenario for T Pyx, in which the star underwent its first eruption within five years of 1866 after many millennia of quiescence, followed by the six observed recurrent nova eruptions since 1890. The lack of distant ejecta, demonstrated by our observations, is not consistent with scenarios in which T Pyx has been erupting continuously as a recurrent nova for many centuries or millennia.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2015

INVESTIGATION OF THE PROGENITORS OF THE TYPE Ia SUPERNOVAE ASSOCIATED WITH THE LMC SUPERNOVA REMNANTS 0505-67.9 AND 0509-68.7

Although Type Ia supernovae have been heavily scrutinized due to their use in making cosmological distance estimates, we are still unable to definitively identify the progenitors for the entire population. While answers have been presented for certain specific systems, a complete solution remains elusive. We present observations of two supernova remnants (SNRs) in the Large Magellanic Cloud, SNR 0505-67.9 and SNR 0509-68.7, for which we have identified the center of the remnant and the 99.73% containment central region in which any companion star left over after the supernova must be located. Both remnants have a number of potential ex-companion stars near their centers; all possible single and double degenerate progenitor models remain viable for these two supernovae. Future observations may be able to identify the true ex-companions for both remnants.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

NOVA AQUILAE 1918 (V603 Aql) FADED BY 0.44 MAG PER CENTURY FROM 1938 TO 2013

We present the light curve of the old nova V603 Aql (Nova Aql 1918) from 1898-1918 and 1934-2013 using 22,721 archival magnitudes. All of our magnitudes are either in, or accurately transformed into, the Johnson B and V magnitude systems. This is vital because offsets in old sequences and the visual-to-V transformation can cause errors of 0.1-1.0 mag if not corrected. Our V603 Aql light curve is the first time that this has been done for any nova. Our goal was to see the evolution of the mass accretion rate on a century timescale, and to test the long-standing prediction of the Hibernation model that old novae should be fading significantly in the century after their eruption is over. The 1918 nova eruption was completely finished by 1938 when the nova decline stopped, and when the star had faded to fainter than its pre-nova brightness of B = 11.43 ± 0.03 mag. We find that the nova light from 1938 to 2013 was significantly fading, with this being seen consistently in three independent data sets (the Sonneberg plates in B, the American Association of Variable Star Observers (AAVSO) V light curve, and the non-AAVSO V light curve). We find that V603 Aql has been declining in brightness at an average rate of 0.44 ± 0.04 mag per century since 1938. This work provides remarkable confirmation of an important prediction of the Hibernation model.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2005

Error Analysis of Remotely-Acquired Mossbauer Spectra

On the Mars Exploration Rovers, Mossbauer spectroscopy has recently been called upon to assist in the task of mineral identification, a job for which it is rarely used in terrestrial studies. For example, Mossbauer data were used to support the presence of olivine in Martian soil at Gusev and jarosite in the outcrop at Meridiani. The strength (and uniqueness) of these interpretations lies in the assumption that peak positions can be determined with high degrees of both accuracy and precision. We summarize here what we believe to be the major sources of error associated with peak positions in remotely-acquired spectra, and speculate on their magnitudes. Our discussion here is largely qualitative because necessary background information on MER calibration sources, geometries, etc., have not yet been released to the PDS; we anticipate that a more quantitative discussion can be presented by March 2005.

Schaefer, Martha W.↗

NASA NTRS1994

BATSE gamma-ray burst line search. 1: Search for narrow lines in spectroscopy detector data

Analysis of data from the Spectroscopy Detectors (SDs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (GRO) has found no convincing line features in the spectra of gamma-ray bursts (GRBs) in almost 3 years of operation, in contrast to expectations based on results from other experiments. In this Letter we discuss the visual search for narrow lines in the SD data. The search has examined 192 bursts, of which approximately 18 were intense enough that lines similar to those seen by instruments on the Ginga satellite would have been visible between approximately 20 and approximately 100 keV. A simplified calculation shows that the BATSE and Ginga results are consistent at the 13% level.

Palmer, David M.↗

NASA NTRS1994

The peak brightness of SN 1937C in IC 4182 and the Hubble constant

The light curve of the Type Ia supernova SN 1937C (in IC 4182) is important because Sandage et al. have measured a distance to the host galaxy by means of Cepheid variables and thus have derived the Hubble constant. However, the peak brightness of SN 1937C has only been derived with the relatively poor original comparison star brightnesses and without regard to a large body of data in the literature. In this paper, I will correct these and other procedural difficulties. I find that the late time photographic light curve appears to have a broken exponential decay with equivalent half-lives of 46 and 58 days with the break near 300 days after maximum. I also find that the peak B-magnitude was 8.71 +/- 0.14 on JD 2428770.0 +/- 1.0 at which time the B-V was -0.03 +/- 0.13. With these improved peak brightnesses, the distance modulus of Sandage et al., and peak absolute magnitudes in the center of the range of modern estimates, I derive the Hubble constant to be 50 km/s Mpc.

Schaefer, Bradley E.↗

NASA NTRS1994

Spectral evolution of a subclass of gamma-ray bursts observed by batse

Among the gamma-ray bursts (GRBs) observed by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory we define a subclass of bursts based on similar morphology: a sharp rise followed by a longer decay time. About 7% of all the gamma-ray bursts observed by BATSE fall into this subclass. We study the spectral evolution of these bursts by fitting models to time-segmented burst spectra and find no clear distinction between the spectral evolutionary properties of this subclass and those of other bursts. Further, we study the high time resolution spectral evolution of this subclass of GRBs using their spectral hardness ratios. A majority of the bursts show hardness ratio leading the counting rate and also display a continuous hard to soft evolution. The time lag between the counting rate and the hardness ratio is found to be directly correlated with the rise time of the counting rate profile. We also find, for the first time, evidence for spectral variation in a timescale of 64 ms.

Bhat, P. N.↗

NASA NTRS1994

BATSE spectroscopy catalog of bright gamma-ray bursts

This paper presents comprehensive results on the spectra of 30 bright gamma ray bursts (GRBs) as observed by the Spectroscopy Detectors (SDs) of the Burst And Transient Source Experiment (BATSE). The data selection was strict in including only spectra that are of high reliability for continuum shape studies. This BATSE Spectroscopy Catalog presents fluences, model fits (for five spectral models for three energy ranges), and photon spectra in a standard manner for each burst. Complete information is provided to describe the data selection and analysis procedures. The catalog results are also presented in electronic format (from the Compton Observatory Science Support Center) and CD-ROM format (AAS CD-ROM series, Vol. 2). These electronic formats also present the count spectra and detector response matrices so as to allow for independent study and fitting by researchers outside the BATSE Team. This BATSE Spectroscopy Catalog complements the catalog from BATSE Large Area Detector (LAD) data by Fishman et al. (1994).

Schaefer, Bradley E.↗

NASA NTRS1994

Rapid searches for counterparts of GRB 930131

A fading counterpart to a gamma-ray burst (GRB) would appear as a point source inside a GRB error region soon after the burst which dims on a timescale from minutes to days. The favorable circumstances of the burst GRB 930131 allowed for an international campaign to search for fading counterparts starting 6.8 hr after the burst. We report observations from many optical sites, two radio telescopes, and archival ROSAT data, including deep Schmidt exposures 35, 44, and 64 hr after the burst. No fading counterparts were detected with our observations.

Schaefer, Bradley E.↗

NASA NTRS1994

Lunar crescent visibility

We report the results of five Moonwatches, in which more than 2000 observers throughout North America attempted to sight the thin lunar crescent. For each Moonwatch we were able to determine the position of the Lunar Date Line (LDL), the line along which a normal observer has a 50% probability of spotting the Moon. The observational LDLs were then compared with predicted LDLs derived from crescent visibility prediction algorithms. We find that ancient and medieval rules are higly unreliable. More recent empirical criteria, based on the relative altitude and azimuth of the Moon at the time of sunset, have a reasonable accuracy, with the best specific formulation being due to Yallop. The modern theoretical model by Schaefer (based on the physiology of the human eye and the local observing conditions) is found to have the least systematic error, the least average error, and the least maximum error of all models tested. Analysis of the observations also provided information about atmospheric, optical and human factors that affect the observations. We show that observational lunar calendars have a natural bias to begin early.

Doggett, Leroy E.↗

NASA NTRS1993

Is PSR 1509-58 the remnant of supernova AD 185?

Thorsett has recently advanced the proposal that the counterpart of the supernova recorded by Chinese astronomers in AD 185 is the pulsar PSR 1509-58 in the supernova remnant MSH 15-52. This proposal is attractive since it was claimed to better fit the visibility contraints that plagued earlier identifications and since the spin-down age of the pulsar is 1690 years. I have examined in detail the question of the visibility of a supernova at the proposed position and find that the reported dates of visibility can be matched only if the supernova appeared substantially brighter than Venus at its peak. For the distance and extinction to the pulsar, this implies M(sub V) (max) was brighter than -21.4 mag, whereas the brightest known absolute magnitude of a Type II supernova is -19.65 (H(sub 0) = 75 km/s/Mpc). Thus, the high required luminosity can be used as good evidence against the identification of SN 185 with PSR 1509-58. However, if the identification is retained, then Hubble's Constant would have to have a small value and/or the supernova had to have been a type IIP event with exceptional brilliance. The earlier identification of SN 185 with MSH 14-63 by Clark and Stephenson can also be reconciled with the visibility data.

Schaefer, Bradley E.↗

NASA NTRS1993

Visibility of sunspots

The premier record of long-term solar activity is the sunspot count. After developing a theoretical model of sunspot visibility, this model is tested against observations, and recommendations are made for improved calculation of sunspot counts. The theoretical model's result is a predicted threshold size for sunspot visibility with the unaided (but filtered) eye, direct vision through a telescope, pinhole camera, and telescope projection. Also reported are over 3250 days of sunspot observations from six observers, 38 yr of daily observations by an experienced observer, 1837 days of observations from a network with over 50 experienced observers, observations from 30 inexperienced observers, as well as summaries of results from 102 AAVSO solar observers. The comparison of the observed thresholds with the predicted thresholds reveals agreement to within the uncertainties, so that the model is validated by observation.

Schaefer, Bradley E.↗

NASA NTRS1993

Methodological problems with gamma-ray burst hardness/intensity correlations

The hardness and intensity are easily measured quantities for all gamma-ray bursts (GRBs), and so, many past and current studies have sought correlations between them. This Letter presents many serious methodological problems with the practical definitions for both hardness and intensity. These difficulties are such that significant correlations can be easily introduced as artifacts of the reduction procedure. In particular, cosmological models of GRBs cannot be tested with hardness/intensity correlations with current instrumentation and the time evolution of the hardness in a given burst may be correlated with intensity for reasons that are unrelated to intrinsic change in the spectral shape.

Schaefer, Bradley E.↗

NASA NTRS1993

Microsecond flares in gamma-ray bursts

It has been suggested that gamma-ray burst light curves may consist of many superposed flares with a duration shorter than 30/microsec. If true, the implications for the interpretation of burst data are enormous. With the launch of the Compton Gamma-Ray Observatory, four predictions of Mitrofanov's (1989) suggestion can be tested. Our results which contradict this suggestion are (1) the photon arrival times are not correlated between independent detectors, (2) the spectral hardness and intensity does not depend on the detector area, (3) the bursts seen by detectors which measure photon positions do not see microsecond flares, and (4) burst positions deduced from detectors with different projected areas are close to the positions deduced from time-of-flight differences between separated spacecraft. We conclude, therefore, that gamma-ray bursts are not composed of microsecond flares.

Schaefer, Bradley E.↗

NASA NTRS1993

A ROSAT Wide Field Camera search for XUV bursts

We have searched the ROSAT Wide Field Camera all-sky survey for shorttime-scale (less than 50 s) XUV bursts down to a limiting count of 5 x 10 exp -3 count/sq arcmin, corresponding to an incident flux of about 3 x 10 exp -10 erg/sq cm. In a total observation period of 171 d covering the entire sky, we found no evidence for such events. The present results are used to place limits on the log N-log S relation for gamma-ray bursts and constraints on the spectral shape of the previously reported ultrasoft X-ray transients.

Owens, Alan↗