arXiv subjects

D. H. Hartmann

Publications and source records attributed to D. H. Hartmann.

At least 109 records · Page 6Linked to original sources

arXiv2008

POET: POlarimeters for Energetic Transients

POET (Polarimeters for Energetic Transients) is a Small Explorer mission concept proposed to NASA in January 2008. The principal scientific goal of POET is to measure GRB polarization between 2 and 500 keV. The payload consists of two wide FoV instruments: a Low Energy Polarimeter (LEP) capable of polarization measurements in the energy range from 2-15 keV and a high energy polarimeter (Gamma-Ray Polarimeter Experiment -- GRAPE) that will measure polarization in the 60-500 keV energy range. Spectra will be measured from 2 keV up to 1 MeV. The POET spacecraft provides a zenith-pointed platform for maximizing the exposure to deep space. Spacecraft rotation will provide a means of effectively dealing with systematics in the polarization response. POET will provide sufficient sensitivity and sky coverage to measure statistically significant polarization for up to 100 GRBs in a two-year mission. Polarization data will also be obtained for solar flares, pulsars and other sources of astronomical interest.

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arXiv2008

GRB 080913 at redshift 6.7

We report on the detection by Swift of GRB 080913, and subsequent optical/near-infrared follow-up observations by GROND which led to the discovery of its optical/NIR afterglow and the recognition of its high-z nature via the detection of a spectral break between the i' and z' bands. Spectroscopy obtained at the ESO-VLT revealed a continuum extending down to lambda = 9400 A, and zero flux for 7500 A < lambda<9400 A, which we interpret as the onset of a Gunn-Peterson trough at z=6.695+-0.025 (95.5% conf. level), making GRB 080913 the highest redshift GRB to date, and more distant than the highest-redshift QSO. We note that many redshift indicators which are based on promptly available burst or afterglow properties have failed for GRB 080913. We report on our follow-up campaign and compare the properties of GRB 080913 with bursts at lower redshift. In particular, since the afterglow of this burst is fainter than typical for GRBs, we show that 2m-class telescopes can identify most high-redshift GRBs.

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arXiv2008

Neutrino-Nucleus Reaction Cross Sections for Light Element Synthesis in Supernova Explosions

The neutrino-nucleus reaction cross sections of 4He and 12C are evaluated using new shell model Hamiltonians. Branching ratios of various decay channels are calculated to evaluate the yields of Li, Be, and B produced through the nu-process in supernova explosions. The new cross sections enhance the yields of 7Li and 11B produced during the supernova explosion of a 16.2 M_odot star model compared to the case using the conventional cross sections by about 10%. On the other hand, the yield of 10B decreases by a factor of two. The yields of 6Li, 9Be, and the radioactive nucleus 10Be are found at a level of 10^{-11} M_odot. The temperature of nu_{mu,tau}- and bar{nu}_{mu,tau}-neutrinos inferred from the supernova contribution of 11B in Galactic chemical evolution models is constrained to the 4.3-6.5 MeV range. The increase in the 7Li and 11B yields due to neutrino oscillations is demonstrated with the new cross sections.

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arXiv2008

A photometric redshift of $z=1.8^{+0.4}_{-0.3}$ for the \agile GRB 080514B

Aims: The AGILE gamma-ray burst GRB 080514B is the first burst with detected emission above 30 MeV and an optical afterglow. However, no spectroscopic redshift for this burst is known. Methods: We compiled ground-based photometric optical/NIR and millimeter data from several observatories, including the multi-channel imager GROND, as well as ultraviolet \swift UVOT and X-ray XRT observations. The spectral energy distribution of the optical/NIR afterglow shows a sharp drop in the \swift UVOT UV filters that can be utilized for the estimation of a redshift. Results: Fitting the SED from the \swift UVOT $uvw2$ band to the $H$ band, we estimate a photometric redshift of $z=1.8^{+0.4}_{-0.3}$, consistent with the pseudo redshift reported by Pelangeon & Atteia (2008) based on the gamma-ray data. Conclusions: The afterglow properties of GRB 080514B do not differ from those exhibited by the global sample of long bursts, supporting the view that afterglow properties are basically independent of prompt emission properties.

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arXiv2008

Multiwavelength analysis of the intriguing GRB 061126: the reverse shock scenario and magnetization

We present a detailed study of the prompt and afterglow emission from Swift GRB 061126 using BAT, XRT, UVOT data and multi-color optical imaging from ten ground-based telescopes. GRB 061126 was a long burst (T_90=191 s) with four overlapping peaks in its gamma-ray light curve. The X-ray afterglow, observed from 26 min to 20 days after the burst, shows a simple power-law decay with alpha_X=1.290 \pm 0.008. Optical observations presented here cover the time range from 258 s (Faulkes Telescope North) to 15 days (Gemini North) after the burst; the decay rate of the optical afterglow shows a steep-to-shallow transition (from alpha_1=1.48 \pm 0.06 to alpha_2=0.88 \pm 0.03) approximately 13 min after the burst. We suggest the early, steep component is due to a reverse shock and show that the magnetic energy density in the ejecta, expressed as a fraction of the equipartion value, is a few ten times larger than in the forward shock in the early afterglow phase. The ejecta might be endowed with primordial magnetic fields at the central engine. The optical light curve implies a late-time break at about 1.5 days after the burst, while there is no evidence of the simultaneous break in the X-ray light curve. We model the broad band emission and show that some afterglow characteristics (the steeper decay in X-ray and the shallow spectral index from optical to X-ray) are difficult to explain in the framework of the standard fireball model. This might imply that the X-ray afterglow is due to an additional emission process, such as late time central engine activity rather than blast-wave shock emission. The possible chromatic break at 1.5 days after the burst would give support to the additional emission scenario.

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arXiv2008

Three X-ray transients in M31 observed with Swift

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disk with innermost temperature of 250-600 eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M31 to estimate their rate, and we find a lower limit of 9/yr.

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arXiv2008

The Robotic Super-LOTIS Telescope: Results & Future Plans

We provide an overview of the robotic Super-LOTIS (Livermore Optical Transient Imaging System) telescope and present results from gamma-ray burst (GRB) afterglow observations using Super-LOTIS and other Steward Observatory telescopes. The 0.6-m Super-LOTIS telescope is a fully robotic system dedicated to the measurement of prompt and early time optical emission from GRBs. The system began routine operations from its Steward Observatory site atop Kitt Peak in April 2000 and currently operates every clear night. The telescope is instrumented with an optical CCD camera and a four position filter wheel. It is capable of observing Swift Burst Alert Telescope (BAT) error boxes as early or earlier than the Swift UV/Optical Telescope (UVOT). Super-LOTIS complements the UVOT observations by providing early R- and I-band imaging. We also use the suite of Steward Observatory telescopes including the 1.6-m Kuiper, the 2.3-m Bok, the 6.5-m MMT, and the 8.4-m Large Binocular Telescope to perform follow-up optical and near infrared observations of GRB afterglows. These follow-up observations have traditionally required human intervention but we are currently working to automate the 1.6-m Kuiper telescope to minimize its response time.

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arXiv2008

Relativistic Particle-In-Cell Simulation Studies of Prompt and Early Afterglows from GRBs

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks e.g. gamma-ray bursts (GRBs) active galactic nuclei (AGNs) and microquasars commonly exhibit power-law emission spectra. Recent PIC simulations of relativistic electron-ion (or electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In collisionless relativistic shocks particle (electron, positron and ion) acceleration is due to plasma waves and their associated instabilities (e.g. the Weibel (filamentation) instability) created in the shock region. The simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform small-scale magnetic fields. These fields contribute to the electron's transverse deflection behind the jet head. The resulting ``jitter'' radiation from deflected electrons has different properties compared to synchrotron radiation which assumes a uniform magnetic field. Jitter radiation may be important for understanding the complex time evolution and/or spectra in gamma-ray bursts, relativistic jets in general and supernova remnants.

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arXiv2007

EXIST's Gamma-Ray Burst Sensitivity

We use semi-analytic techniques to evaluate the burst sensitivity of designs for the EXIST hard X-ray survey mission. Applying these techniques to the mission design proposed for the Beyond Einstein program, we find that with its very large field-of-view and faint gamma-ray burst detection threshold, EXIST will detect and localize approximately two bursts per day, a large fraction of which may be at high redshift. We estimate that EXIST's maximum sensitivity will be ~4 times greater than that of Swift's Burst Alert Telescope. Bursts will be localized to better than 40 arcsec at threshold, with a burst position as good as a few arcsec for strong bursts. EXIST's combination of three different detector systems will provide spectra from 3 keV to more than 10 MeV. Thus, EXIST will enable a major leap in the understanding of bursts, their evolution, environment, and utility as cosmological probes.

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arXiv2007

Constraints on an optical afterglow and on supernova light following the short burst GRB 050813

We report early follow-up observations of the error box of the short burst 050813 using the telescopes at Calar Alto and at Observatorio Sierra Nevada (OSN), followed by deep VLT/FORS2 I-band observations obtained under very good seeing conditions 5.7 and 11.7 days after the event. Neither a fading afterglow, nor a rising SN component was found, so the potential GRB host galaxy has not been identified based on a comparison of the two VLT images taken at different epoches. We discuss if any of the galaxies present in the original 10 arcsec XRT error circle could be the host. In any case, the optical afterglow of GRB 050813 was of very low luminosity. We conclude that all these properties are consistent with the binary compact merger hypothesis for the progenitor of GRB 050813.

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arXiv2006

The GRB 060218/SN 2006aj event in the context of other Gamma-Ray Burst Supernovae

The supernova SN 2006aj associated with GRB 060218 is the second-closest GRB-SN observed to date (z=0.033). We present Very Large Telescope, Liverpool Telescope, and Katzman Automatic Imaging Telescope multi-color photometry of SN 2006aj. This supernova is found to be subluminous and rapidly evolving. Its early light curve includes an additional wavelength-dependent component, which can be interpreted as shock break-out. We compare the photometric evolution of multi-band light curves with the corresponding properties of the present sample of more than 10 GRB-SNe with precisely known redshifts. Using host-galaxy extinction measurements, we derive extinction-corrected GRB-SN luminosities and place SN 2006aj in the context of this GRB-selected supernova sample as well as in the context of local stripped-envelope supernovae.

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arXiv2006

Early-Time Chromatic Variations in the Wind-Swept Medium of GRB 021211 and the Faintness of its Afterglow

We present Follow-Up Network for Gamma-Ray Bursts (FUN GRB) Collaboration observations of the optical afterglow of GRB 021211 made between 143 seconds and 102 days after the burst. Our unique data set includes the earliest filtered detections and color information for an afterglow in the pre-Swift era. We find that the afterglow is best described by (1) propagation through a wind-swept medium, (2) a cooling break that is blueward of the observed optical frequencies, and (3) a hard electron energy distribution. However, superimposed on this "standard model" behavior we find significant chromatic variations within the first few hours after the burst. We consider possible reasons for these variations, including the possibility that they are due to a dust echo. Finally, we constrain physical parameters that describe the afterglow and surrounding medium for a variety of scenarios and find that GRB 021211's afterglow is faint for a combination of 3-4 reasons: (1) a low fraction of energy in relativistic electrons, (2) a low density for the wind-swept medium, implying either a low mass-loss rate and/or a high wind velocity for the progenitor, (3) a wide opening/viewing angle for the jet, and possibly (4) moderate source frame extinction. The jet appears to be significantly far from equipartition and magnetically dominated. More extreme versions of this might explain the darkness of many afterglows in the Swift era.

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arXiv2006

Neutrino Oscillation Effects on Supernova Light Element Synthesis

Neutrino oscillations affect light element synthesis through the neutrino-process in supernova explosions. The 7Li and 11B yields produced in a supernova explosion of a 16.2 solar-mass star model increase by factors of 1.9 and 1.3 in the case of large mixing angle solution with normal mass hierarchy and sin^{2}2theta_{13} > 0.002 compared with those without the oscillations. In the case of inverted mass hierarchy or nonadiabatic 13-mixing resonance, the increment of their yields is much smaller. Neutrino oscillations raise the reaction rates of charged-current neutrino-process reactions in the region outside oxygen-rich layers. The number ratio of 7Li/11B could be a tracer of normal mass hierarchy and relatively large theta_{13}, still satisfying sin^{2}2theta_{13} < 0.1, through future precise observations in stars having strong supernova component.

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arXiv2006

Supernova Neutrino Nucleosynthesis of Light Elements with Neutrino Oscillations

Light element synthesis in supernovae through neutrino-nucleus interactions, i.e., the nu-process, is affected by neutrino oscillations in the supernova environment. There is a resonance of 13-mixing in the O/C layer, which increases the rates of charged-current nu-process reactions in the outer He-rich layer. The yields of 7Li and 11B increase by about a factor of 1.9 and 1.3, respectively, for a normal mass hierarchy and an adiabatic 13-mixing resonance, compared to those without neutrino oscillations. In the case of an inverted mass hierarchy and a non-adiabatic 13-mixing resonance, the increase in the 7Li and 11B yields is much smaller. Observations of the 7Li/11B ratio in stars showing signs of supernova enrichment could thus provide a unique test of neutrino oscillations and constrain their parameters and the mass hierarchy.

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arXiv2006

Are Ti44-Producing Supernovae Exceptional?

According to standard models supernovae produce radioactive $^{44}$Ti, which should be visible in gamma-rays following decay to $^{44}$Ca for a few centuries. $^{44}Ti production is believed to be the source of cosmic $^{44}$Ca, whose abundance is well established. Yet, gamma-ray telescopes have not seen the expected young remnants of core collapse events. The $^{44}$Ti mean life of $τ\simeq$ 89 y and the Galactic supernova rate of $\simeq$ 3/100 y imply $\simeq$ several detectable $^{44}Ti gamma-ray sources, but only one is clearly seen, the 340-year-old Cas A SNR. Furthermore, supernovae which produce much $^{44}Ti are expected to occur primarily in the inner part of the Galaxy, where young massive stars are most abundant. Because the Galaxy is transparent to gamma-rays, this should be the dominant location of expected gamma-ray sources. Yet the Cas A SNR as the only one source is located far from the inner Galaxy (at longitude 112 degree). We evaluate the surprising absence of detectable supernovae from the past three centuries. We discuss whether our understanding of SN explosions, their $^{44}Ti yields, their spatial distributions, and statistical arguments can be stretched so that this apparent disagreement may be accommodated within reasonable expectations, or if we have to revise some or all of the above aspects to bring expectations in agreement with the observations. We conclude that either core collapse supernovae have been improbably rare in the Galaxy during the past few centuries, or $^{44}Ti-producing supernovae are atypical supernovae. We also present a new argument based on $^{44}$Ca/$^{40}$Ca ratios in mainstream SiC stardust grains that may cast doubt on massive-He-cap Type I supernovae as the source of most galactic $^{44}$Ca.

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arXiv2006

Probing Cosmic Chemical Evolution with Gamma-Ray Bursts: GRB060206 at z=4.048

Aim: We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution. Method: Absorption line study of the GRB afterglow spectrum. Results: We determine the redshift of the GRB to be z=4.04795+/-0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated Sii lines we derive a metallicity of [S/H] =-0.84+/-0.10. This is one of the highest metallicities measured from absorption lines at z~4. From the very high column densities for the forbidden Siii*, Oi*, and Oi** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H$_2$ molecules with logN(H_2) ~ 17.0, translating into a molecular fraction of logf \~ -3.5 with f=2N(H_2)/(2N(H_2)+ N(Hi)). Even if GRBs are only formed by single massive stars with metallicities below ~0.3Z(solar), they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

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arXiv2005

Evidence for Supernova light in all Gamma-Ray Burst afterglows

We present an update of our systematic analysis of all Gamma-Ray Burst (GRB) afterglow data, now published through the end of 2004, in an attempt to detect the predicted supernova light component. We fit the observed photometric light curves as the sum of an afterglow, an underlying host galaxy, and a supernova component. The latter is modeled using published UBVRI light curves of SN 1998bw as a template. The total sample of afterglows with established redshifts contains now 29 bursts (GRB 970228 - GRB 041006). For 13 of them a weak supernova excess (scaled to SN 1998bw) was found. In agreement with our earlier result (Zeh et al. 2004) we find that also in the updated sample all bursts with redshift < 0.7 show a supernova excess in their afterglow light curves. The general lack of a detection of a supernova component at larger redshifts can be explained with selection effects. These results strongly support our previous conclusion based on all afterglow data of the years 1997 to 2002 that in fact all afterglows of long-duration GRBs contain light from an associated supernova.

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arXiv2004

GRB 040403: a faint X-ray rich Gamma-ray Burst discovered by INTEGRAL

GRB 040403 is one of the faintest gamma-ray bursts for which a rapid and accurate localization has been obtained. Here we report on the gamma-ray properties of this burst, based on observations with the IBIS instrument aboard INTEGRAL, and the results of searches for its optical afterglow. The steep spectrum (power law photon index = 1.9 in the 20-200 keV range) implies that GRB 040403 is most likely an X-ray rich burst. Our optical limit of R > 24.2 at 16.5 hours after the burst, indicates a rather faint afterglow, similar to those seen in other relatively soft and faint bursts.

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