A. B. Sanin

An exceptionally bright flare from SGR 1806-20 and the origins of short-duration γ-ray bursts

Soft-γ-ray repeaters (SGRs) are galactic X-ray stars that emit numerous short-duration (about 0.1u2009s) bursts of hard X-rays during sporadic active periods. They are thought to be magnetars: strongly magnetized neutron stars with emissions powered by the dissipation of magnetic energy. Here we report the detection of a long (380u2009s) giant flare from SGR 1806–20, which was much more luminous than any previous transient event observed in our Galaxy. (In the first 0.2u2009s, the flare released as much energy as the Sun radiates in a quarter of a million years.) Its power can be explained by a catastrophic instability involving global crust failure and magnetic reconnection on a magnetar, with possible large-scale untwisting of magnetic field lines outside the star. From a great distance this event would appear to be a short-duration, hard-spectrum cosmic γ-ray burst. At least a significant fraction of the mysterious short-duration γ-ray bursts may therefore come from extragalactic magnetars.

Hydrogen mapping of the lunar south pole using the LRO neutron detector experiment LEND.

Watering the Moon About a year ago, a spent upper stage of an Atlas rocket was deliberately crashed into a crater at the south pole of the Moon, ejecting a plume of debris, dust, and vapor. The goal of this event, the Lunar Crater Observation and Sensing Satellite (LCROSS) experiment, was to search for water and other volatiles in the soil of one of the coldest places on the Moon: the permanently shadowed region within the Cabeus crater. Using ultraviolet, visible, and near-infrared spectroscopy data from accompanying craft, Colaprete et al. (p. 463; see the news story by Kerr; see the cover) found evidence for the presence of water and other volatiles within the ejecta cloud. Schultz et al. (p. 468) monitored the different stages of the impact and the resulting plume. Gladstone et al. (p. 472), using an ultraviolet spectrograph onboard the Lunar Reconnaissance Orbiter (LRO), detected H2, CO, Ca, Hg, and Mg in the impact plume, and Hayne et al. (p. 477) measured the thermal signature of the impact and discovered that it had heated a 30 to 200 square-meter region from ∼40 kelvin to at least 950 kelvin. Paige et al. (p. 479) mapped cryogenic zones predictive of volatile entrapment, and Mitrofanov et al. (p. 483) used LRO instruments to confirm that surface temperatures in the south polar region persist even in sunlight. In all, about 155 kilograms of water vapor was emitted during the impact; meanwhile, the LRO continues to orbit the Moon, sending back a stream of data to help us understand the evolution of its complex surface structures. A controlled spacecraft impact into a crater in the lunar south pole plunged through the lunar soil, revealing water and other volatiles. Hydrogen has been inferred to occur in enhanced concentrations within permanently shadowed regions and, hence, the coldest areas of the lunar poles. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to detect hydrogen-bearing volatiles directly. Neutron flux measurements of the Moon’s south polar region from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) spacecraft were used to select the optimal impact site for LCROSS. LEND data show several regions where the epithermal neutron flux from the surface is suppressed, which is indicative of enhanced hydrogen content. These regions are not spatially coincident with permanently shadowed regions of the Moon. The LCROSS impact site inside the Cabeus crater demonstrates the highest hydrogen concentration in the lunar south polar region, corresponding to an estimated content of 0.5 to 4.0% water ice by weight, depending on the thickness of any overlying dry regolith layer. The distribution of hydrogen across the region is consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources.

Discovery of GRB 020405 and Its Late Red Bump

We present the discovery of GRB 020405 made with the Interplanetary Network (IPN). With a duration of 60 s, the burst appears to be a typical long-duration event. We observed the 75 arcmin2 IPN error region with the Mount Stromlo Observatorys 50 inch robotic telescope and discovered a transient source that subsequently decayed and was also associated with a variable radio source. We identify this source as the afterglow of GRB 020405. Subsequent observations by other groups found varying polarized flux and established a redshift of 0.690 to the host galaxy. Motivated by the low redshift, we triggered observations with WFPC2 on board the Hubble Space Telescope (HST). Modeling the early ground-based data with a jet model, we find a clear red excess over the decaying optical light curves that is present between day 10 and day 141 (the last HST epoch). This bump has the spectral and temporal features expected of an underlying supernova (SN). In particular, the red color of the putative SN is similar to that of the SN associated with GRB 011121 at late time. Restricting the sample of GRBs to those with z < 0.7, a total of five bursts, red bumps at late times are found in GRB 970228, GRB 011121, and GRB 020405. It is possible that the simplest idea, namely, that all long-duration γ-ray bursts have underlying SNe with a modest dispersion in their properties (especially peak luminosity), is sufficient to explain the nondetections.

INVERSE COMPTON X-RAY EMISSION FROM SUPERNOVAE WITH COMPACT PROGENITORS: APPLICATION TO SN2011fe

We present a generalized analytic formalism for the inverse Compton X-ray emission from hydrogen-poor supernovae and apply this framework to SNxa02011fe using Swift X-Ray Telescope (XRT), UVOT, and Chandra observations. We characterize the optical properties of SNxa02011fe in the Swift bands and find them to be broadly consistent with a normal SN Ia, however, no X-ray source is detected by either XRT or Chandra. We constrain the progenitor system mass-loss rate (3σ c.l.) for wind velocity vw = 100 km s–1. Our result rules out symbiotic binary progenitors for SN 2011fe and argues against Roche lobe overflowing subgiants and main-sequence secondary stars if 1% of the transferred mass is lost at the Lagrangian points. Regardless of the density profile, the X-ray non-detections are suggestive of a clean environment (n CSM < 150 cm–3) for 2 × 1015 R 5 × 1016 cm around the progenitor site. This is either consistent with the bulk of material being confined within the binary system or with a significant delay between mass loss and supernova explosion. We furthermore combine X-ray and radio limits from Chomiuk et al. to constrain the post-shock energy density in magnetic fields. Finally, we searched for the shock breakout pulse using gamma-ray observations from the Interplanetary Network and find no compelling evidence for a supernova-associated burst. Based on the compact radius of the progenitor star we estimate that the shock breakout pulse was likely not detectable by current satellites.

The Dynamic Albedo of Neutrons (DAN) Experiment for NASA's 2009 Mars Science Laboratory

We present a summary of the physical principles and design of the Dynamic Albedo of Neutrons (DAN) instrument onboard NASAs 2009 Mars Science Laboratory (MSL) mission. The DAN instrument will use the method of neutron-neutron activation analysis in a space application to study the abundance and depth distribution of water in the martian subsurface along the path of the MSL rover.

Testing polar spots of water-rich permafrost on the Moon: LEND observations onboard LRO

[1]xa0Results are presented for the LEND instrument onboard LRO for the detection of local spots of suppression and excess of epithermal neutron emission at the lunar poles. Twelve local Neutron Suppression Regions (NSRs) and Neutron Excess Regions (NERs) are detected. It is shown using the data from the LOLA and Diviner instruments that six NSRs have the empirical property “less local irradiation and lower temperature – fewer local neutrons.” These NSRs may be identified with spots of water-ice rich permafrost on the Moon. It is shown that detected NSRs are include in both permanently shadowed and illuminated areas, and they are not coincident with Permanently Shadowed Regions (PSRs) at the bottom of polar craters, as has been commonly expected before LEND presented neutron data with high spatial resolution.

Experiment LEND of the NASA Lunar Reconnaissance Orbiter for high-resolution mapping of neutron emission of the Moon.

The scientific objectives of neutron mapping of the Moon are presented as 3 investigation tasks of NASAs Lunar Reconnaissance Orbiter mission. Two tasks focus on mapping hydrogen content over the entire Moon and on testing the presence of water-ice deposits at the bottom of permanently shadowed craters at the lunar poles. The third task corresponds to the determination of neutron contribution to the total radiation dose at an altitude of 50 km above the Moon. We show that the Lunar Exploration Neutron Detector (LEND) will be capable of carrying out all 3 investigations. The design concept of LEND is presented together with results of numerical simulations of the instruments sensitivity for hydrogen detection. The sensitivity of LEND is shown to be characterized by a hydrogen detection limit of about 100 ppm for a polar reference area with a radius of 5 km. If the presence of ice deposits in polar cold traps is confirmed, a unique record of many millions of years of lunar history would be obtained, by which the history of lunar impacts could be discerned from the layers of water ice and dust. Future applications of a LEND-type instrument for Mars orbital observations are also discussed.

Testing lunar permanently shadowed regions for water ice: LEND results from LRO

[1]xa0We use measurements from the Lunar Exploration Neutron Detector (LEND) collimated sensors during more than one year of the mapping phase of NASAs Lunar Reconnaissance Orbiter (LRO) mission to make estimates of the epithermal neutron flux within known large Permanently Shadowed Regions (PSRs). These are compared with the local neutron background measured outside PSRs in sunlit regions. Individual and collective analyses of PSR properties have been performed. Only three large PSRs, Shoemaker and Cabeus in the south and Rozhdestvensky U in the north, have been found to manifest significant neutron suppression. All other PSRs have much smaller suppression, only a few percent, if at all. Some even display an excess of neutron emission in comparison to the sunlit vicinity around them. Testing PSRs collectively, we have not found any average suppression for them. Only the group of 18 large PSRs, with area >200xa0km2, show a marginal effect of small average suppression, ∼2%, with low statistical confidence. A ∼2% suppression corresponds to ∼125 ppm of hydrogen taking into account the global neutron suppression near the lunar poles and assuming a homogeneous H distribution in depth in the regolith. This means that all PSRs, except those in Shoemaker, Cabeus and Rozhdestvensky U craters, do not contain any significant amount of hydrogen in comparison with sunlit areas around them at the same latitude.

Global maps of lunar neutron fluxes from the LEND instrument

[1]xa0The latest neutron spectrometer measurements with the Lunar Exploration Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter (LRO) are presented. It covers more than 1xa0year of mapping phase starting on 15 September 2009. In our analyses we have created global maps showing regional variations in the flux of thermal (energy rangexa0 0.5xa0MeV), and compared these fluxes to variances in soil elemental composition, and with previous results obtained by the Lunar Prospector Neutron Spectrometer (LPNS). We also processed data from LEND collimated detectors and derived a value for the collimated signal of epithermal neutrons based on the comparative analysis with the LEND omnidirectional detectors. Finally, we have compared our final (after the data reduction) global epithermal neutron map with LPNS data.

Water and chlorine content in the Martian soil along the first 1900 m of the Curiosity rover traverse as estimated by the DAN instrument

The presence of hydrated phases in the soil and near-surface bedrock of Gale Crater is thought to be direct evidence for water-rock interaction in the crater in the ancient past. Layered sediments over the Gale Crater floor are thought to have formed in past epochs due to sediment transport, accumulation, and cementation through interaction with fluids, and the observed strata of water-bearing minerals record the history of these episodes. The first data analysis of the Dynamic Albedo of Neutrons (DAN) investigation on board the Curiosity rover is presented for 154 individual points of active mode measurements along 1900 m of the traverse over the first 361 Martian solar days in Gale crater. It is found that a model of constant water content within subsurface should be rejected for practically all tested points, whereas a two-layer model with different water contents in each layer is supported by the data. A so-called direct two-layer model (water content increasing with depth) yields acceptable fits for odometry ranges between 0 and 455 m and beyond 638 m. The mean water (H2O) abundances of the top and bottom layers vary from 1.5 to 1.7u2009wt % and from 2.2 to 3.3u2009wt %, respectively, while at some tested spots the water content is estimated to be as high as ~5u2009wt %. The data for odometry range 455–638 m support an inverse two-layer model (water content decreasing with depth), with an estimated mean water abundance of 2.1u2009±u20090.1u2009wt % and 1.4u2009±u20090.04u2009wt % in the top and bottom layers, respectively.