A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, would not only enable explorations of a large class of many-body physics phenomena, but could also be used for quantum information processing. We report on the creation of an ultracold dense gas of 40 K 87 Rb polar molecules. Using a single step of STIRAP (STImulated Raman Adiabatic Passage) via two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 10 12 cm −3 , and an expansion-determined translational temperature of 350 nK. The polar molecules have a permanent electric 1 arXiv:0808.2963v2 [quant-ph]

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single powerlaw with photon index -1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of ∼ 290 keV. We model the photospheric emission with a multicolor blackbody and its shape indicates that the photospheric radius increases at higher latitudes. We derive the averaged photospheric radius R ph = (1.1 ± 0.3) × 10 12 Y 1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of two and has a maximal value of Γ = 750 Y 1/4 . Here Y is the ratio between the total fireball energy and the energy emitted in the gamma-rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We interpret the broad band emission as synchrotron emission at R ∼ 4 × 10 15 cm. Our analysis emphasize the importance of having high temporal resolution when performing spectral analysis on GRBs, since there is strong spectral evolution.

A thermal radiative component is likely to accompany the first stages of the prompt emission of gamma-ray bursts (GRBs) and X-ray flashes (XRFs). We analyze the effect of such a component on the observable spectrum, assuming that the observable effects are due to a dissipation process occurring below or near the thermal photosphere. We consider both the internal shock model and a ''slow heating'' model as possible dissipation mechanisms. For comparable energy densities in the thermal and leptonic components, the dominant emission mechanism is Compton scattering. This leads to a nearly flat energy spectrum ( F / 0 ) above the thermal peak at %10-100 keV and below 10-100 MeV, for a wide range of optical depths 0:03 P e P 100, regardless of the details of the dissipation mechanism or the strength of the magnetic field. At lower energies steep slopes are expected, while above 100 MeV the spectrum depends on the details of the dissipation process. For higher values of the optical depth, a Wien peak is formed at 100 keV-1 MeV, and no higher energy component exists. For any value of e , the number of pairs produced does not exceed the baryon-related electrons by a factor of larger than a few. We conclude that dissipation near the thermal photosphere can naturally explain both the steep slopes observed at low energies and a flat spectrum above 10 keV, thus providing an alternative scenario to the optically thin synchrotron-SSC model.

We perform time-resolved spectroscopy on the prompt emission in gammaray bursts (GRBs) and identify a thermal, photospheric component peaking at a temperature of a few hundreds keV. This peak does not necessarily coincide with the broad band (keV-GeV) power peak. We show that this thermal component exhibits a characteristic temporal behavior. We study a sample of 56 long bursts, all strong enough to allow time-resolved spectroscopy. We analyze the evolution of both the temperature and flux of the thermal component in 49 individual time-resolved pulses, for which the temporal coverage is sufficient, and find that the temperature is nearly constant during the first few seconds, after which it decays as a power law with a sample-averaged index of −0.68. The thermal flux first rises with an averaged power-law index of 0.63 after which it decays with an averaged index of −2. The break times are the same to within errors. We find that the ratio of the observed to the emergent thermal flux typically exhibits a monotoneous power-law increase during the entire pulse as well as during complex bursts. Thermal photons carry a significant fraction (∼ 30% to more than 50%) of the prompt emission energy (in the observed 25-1900 keV energy band), thereby significantly contributing to the high radiative efficiency. Finally, we show here that the thermal emission can be used to study the properties of the photosphere, hence the physical parameters of the GRB fireball.

The composition of gamma-ray burst (GRB) ejecta is still a mystery. The standard model invokes an initially hot "fireball" composed of baryonic matter. Here we analyze the broad band spectra of GRB 080916C detected by the Fermi satellite. The featureless Band-spectrum of all five epochs as well as the detections of > ∼ 10 GeV photons in this burst place a strong constraint on the prompt emission radius R γ , which is typically > ∼ 10 15 cm, independent on the details of the emission process. The lack of detection of a thermal component as predicted by the baryonic models strongly suggests that a significant fraction of the outflow energy is initially not in the "fireball" form, but is likely in a Poynting flux entrained with the baryonic matter. The ratio between the Poynting and baryonic fluxes is at least ∼ (15 − 20) at the photosphere radius, if the Poynting flux is not directly converted to kinetic energy below the photosphere.

In recent years, increasing evidence has emerged for a thermal component in the g-and X-ray spectrum of the prompt emission phase in gamma-ray bursts. The temperature and flux of the thermal component show a characteristic break in the temporal behavior after a few seconds. We show here that measurements of the temperature and flux of the thermal component at early times (before the break) allow the determination of the values of two of the least restricted fireball model parameters: the size at the base of the flow and the outflow bulk Lorentz factor. Relying on the thermal emission component only, this measurement is insensitive to the inherent uncertainties of previous estimates of the bulk motion Lorentz factor. We give specific examples of the use of this method: for GRB 970828 at redshift , we show that the physical size at the base of the z p 0.9578 flow is cm and the Lorentz factor of the flow is , and for GRB between the total fireball energy and the energy emitted in g-rays.

Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle dependent velocity profiles, Γ = Γ(r, θ, φ), where Γ is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Γ ≈ Γ 0 /[(θ/θ j ) p + 1], where θ j is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, θ v . We show that for narrow jets (θ j Γ 0 f ew), the obtained low energy photon index is α ≈ −1 (dN/dE ∝ E α ), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at θ v ≈ θ j . This result is surprisingly similar to the average low energy photon index seen in gamma-ray bursts. For wide jets (θ j Γ 0 f ew) observed at θ v θ j , a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

We present a comparative study of the observed properties of the optical and X-ray afterglows of short-and long-duration γ -ray bursts (GRBs). Using a large sample of 37 short and 421 long GRBs, we find a strong correlation between the afterglow brightness measured after 11 hr and the observed fluence of the prompt emission. Both the optical (R band) and X-ray flux densities (F R and F X ) scale with the γ -ray fluence, F γ . For bursts with a known redshift, a tight correlation exists between the afterglow flux densities at 11 hr (rest frame) and the total isotropic γ -ray energy, E γ,ISO : F R,X ∝ E γ,ISO α , with α 1. The constant of proportionality is nearly identical for long and short bursts, when E γ,ISO is obtained from the Swift data. Additionally, we find that for short busts with F γ 10 −7 erg cm −2 , optical afterglows are nearly always detected by reasonably deep early observations. Finally, we show that the ratio F R /F X has very similar values for short and long bursts. These results are difficult to explain in the framework of the standard scenario, since they require that either (1) the number density of the surrounding medium of short bursts is typically comparable to, or even larger than the number density of long bursts; (2) short bursts explode into a density profile, n(r) ∝ r −2 ; or (3) the prompt γ -ray fluence depends on the density of the external medium. We therefore find it likely that either basic assumptions on the properties of the circumburst environment of short GRBs or else the standard models of GRB emission must be re-examined. We believe that the most likely solution is that the ambient density surrounding typical short bursts is higher than has generally been expected: a typical value of ∼1 cm −3 is indicated. We discuss recent modifications to the standard binary merger model for short bursts which may be able to explain the implied density.