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At least 919 records · Page 51
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NASA NTRS2021

FRB131104 Swift/BAT Data Revisited: No Evidence of a Gamma-Ray Counterpart

We present a reanalysis of the Swift Burst Alert Telescope (BAT) data around the radio detection of FRB131104. Possible evidence of a gamma-ray counterpart was presented by DeLaunay et al. However, based on our analysis using all the available BAT data, no significant emission is found in either the temporal or the image domain. We place a 5σ fluence upper limit of 3.3 × 10−6 erg cm−2 and 2.7 × 10−6 erg cm−2 (15–150 keV) with an integration time of 300 s assuming a simple power-law spectrum with photon index of −1.2 and −2.0, respectively. Our result does not support the association of this FRB with a high-energy counterpart, in agreement with growing observational evidence that most FRBs are not associated with catastrophic events such as gamma-ray bursts.

T Sakamoto↗
NASA NTRS2021

Modeling the Prompt Optical Emission of GRB 180325A: The Evolution of a Spike from the Optical to Gamma Rays

The transition from prompt to afterglow emission is one of the most exciting and least understood phases in gamma-ray bursts (GRBs). Correlations among optical, X-ray, and gamma-ray emission in GRBs have been explored, to attempt to answer whether the earliest optical emission comes from internal and/or external shocks. We present optical photometric observations of GRB 180325A collected with the TAROT and RATIR ground-based telescopes. These observations show two strong optical flashes with separate peaks at ∼50 and ∼120 s, followed by a temporally extended optical emission. We also present X-rays and gamma-ray observations of GRB 180325A, detected by the Burst Alert Telescope and X-ray Telescope, on the Neil Gehrels Swift observatory, which both observed a narrow flash at ∼80 s. We show that the prompt gamma-ray and X-ray early emission shares similar temporal and spectral features consistent with internal dissipation within the relativistic outflow (e.g., by internal shocks or magnetic reconnection), while the early optical flashes are likely generated by the reverse shock that decelerates the ejecta as it sweeps up the external medium.

R. L. Becerra↗
NASA NTRS2021

Seventeen Tidal Disruption Events from the First Half of ZTF Survey Observations: Entering a New Era of Population Studies

While tidal disruption events(TDEs) have long been heralded as laboratories for the study of quiescent blackholes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress toward this promise. Here we present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light-curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, lower optical luminosities, and higher disruption rates compared to the rest of the sample. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities, and the lowest rates. A high detection rate of Bowen lines in TDEs with small photometric radii could be explained by the high density that is required for this fluorescence mechanism. The stellar debris can provide a source for this dense material. Diffusion of photons through this debris may explain why the rise and fade timescale of the TDEs in our sample are not correlated. We also report, for the first time, the detection of soft X-ray flares from a TDE on∼day timescales. Based on the fact that the X-ray flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow.

Sjoert van Velzen↗
NASA NTRS2021

Reflection Modeling of the Black Hole Binary 4U 1630–47: The Disk Density and Returning Radiation

We present the analysis of X-ray observations of the black hole binary 4U 1630−47 using relativistic reflection spectroscopy. We use archival data from the Rossi X-ray Timing Explorer, Neil Gehrels Swift Observatory, and Nuclear Spectroscopic Telescope Array observatories, taken during different outbursts of the source between 1998 and 2015. Our modeling includes two relatively new advances in modern reflection codes: high-density disks, and returning thermal disk radiation. Accretion disks around stellar-mass black holes are expected to have densities well above the standard value assumed in traditional reflection models (i.e., n(e) ∼ 10^(15) per cu. cm). New high-density reflection models have important implications in the determination of disk truncation (i.e., the disk inner radius). This is because one must retain self-consistency in the irradiating flux and corresponding disk ionization state, which is a function of disk density and system geometry. We find that the disk density is n(e) ≥ 10^(20) per cu. cm across all spectral states. This density, combined with our constraints on the ionization state of the material, implies an irradiating flux impinging on the disk that is consistent with the expected theoretical estimates. Returning thermal disk radiation—the fraction of disk photons that bend back to the disk producing additional reflection components—is expected predominantly in the soft state. We show that returning radiation models indeed provide a better fit to the soft-state data, reinforcing previous results that show that in the soft state, the irradiating continuum may be blackbody emission from the disk itself.

Riley M. T. Connors↗
NASA NTRS2022

X-Ray Imaging Observations of the High-Mass γ-Ray Binary HESS J0632+057

The Chandra X-ray Observatory (CXO)imaged the high-massγ-ray binary (HMGB) HESS8J0632+057 with the Advanced CCD Imaging Spectrometer (ACIS). We analyzed the CXO data to-9gether with 967 ks of archival imaging Swift-XRT observations. On arcsecond scales we find a hint10of asymmetric extended emission. On arcminute scales, there is a region of extended emission (“the11blob”) which is located≈5.5′east of the binary. The blob is seen in both the CXO-ACIS and the12Swift-XRT images but it does not have a counterpart in the radio, NIR, IR, or optical images. The13ACIS spectrum of the blob fits either an absorbed power-law model with Γ'2.6, or a thermal plasma14model withkT'3 keV. Since the blob’s NH is significantly larger than that of the binary we conclude15that the blob and binary are not directly related. The somewhat larger very deep XRT image suggests16that the binary may be located within a shell (or cavity). The four ACIS spectra taken within the∼2017day interval near the light curve minimum suggests that the NH is varying on timescales of days, pos-18sibly due to the inhomogenous circumbinary environment. The XRT spectra extracted from the wider19orbital phase intervals support significant changes in NH near the light curve maximum/minimum,20which may be responsible for the substantial systematic residuals seen near 1 keV, and provide tenta-21tive evidence for a Fe line at 6.4 keV. We find no significant periodic signal in the ACIS data up to 0.156 Hz.

O Kargaltsev↗
NASA NTRS2021

A Tale of Two Mergers: Constraints on Kilonova Detection in Two Short GRBs at z∼ 0.5

We present a detailed multiwavelength analysis of two short gamma-ray bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at z = 0.483 and GRB 200522A at z = 0.554. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated with a late-type galaxy with an old stellar population (≈3 Gyr) and moderate ongoing star formation (≈1 M⊙ yr−1). Hubble and Gemini limits on optical/near-infrared emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavouring large masses of wind ejecta (≲0.03 M⊙). In contrast, observations of GRB 200522A uncovered a luminous (LF125W ≈ 1042 erg s−1 at 2.3 d) and red (r − H ≈ 1.3 mag) counterpart. The red colour can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 M⊙ ≲ M ≲ 0.1 M⊙) or moderate extinction, E(B − V) ≈ 0.1−0.2 mag, along the line of sight. The location of this sGRB in the inner regions of a young (≈0.1 Gyr) star-forming (≈2−6 M⊙ yr−1) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red colour.

B O'Connor↗
NASA NTRS2021

HILIGT, Upper Limit Servers II - Implementing the Data Servers

The High-Energy Light curve Generator(HILIGT) is a new web-based tool which allows the user to generate long-term light curves of X-ray sources. It provides historical data and calculates upper limits from image data in real-time. HILIGT utilizes data from twelve satellites, both modern missions such as XMM-Newton and Swift, and earlier facilities such as ROSAT, EXOSAT, Einstein or Ariel V. Together, this enables the user to query 50 years of X-ray data and, for instance, study outburst behavior of transient sources. In this paper we focus on the individual back-end servers for each satellite, detailing the software layout, database design, catalog calls, and image footprints. We compile all relevant calibration information of these missions and provide an in-depth summary of the details of X-ray astronomical instrumentation and data.

Ole Konig↗
NASA NTRS2021

NICER Observations of the Black Hole Candidate MAXI J0637-430 during the 2019-2020 Outburst

We present detailed timing and spectral studies of the black hole candidate MAXI J0637–430 during its 2019–2020 outburst using observations with the Neutron Star Interior Composition Explorer (NICER)and the Neil Gehrels Swift Observatory. We find that the source evolves through the soft-intermediate, high-soft, hard-intermediate, and low-hard states during the outburst. No evidence of quasi-periodic oscillations is found in the power-density spectra of the source. Weak variability with fraction alarms amplitude<5 per cent is found in the softer spectral states. In the hard-intermediate and hard states, high variability with the fractional rms amplitude of>20 per cent is observed. The 0.7–10 keV spectra with NICER are studied with a combined disc-black-body and nth comp model along with the interstellar absorption. The temperature of the disc is estimated to be 0.6 keV in the rising phase and decreased slowly to 0.1 keV in the declining phase. The disc component was not detectable or absent during the low-hard state. From the state-transition luminosity and the inner edge of the accretion flow, we estimate the mass of the black hole to be in the range of 5–12 M, assuming the source distance of d<10 kpc.

Arghajit Jana↗
NASA NTRS2021

GRB 191016A: A Long Gamma-Ray Burst Detected by TESS

The Transiting Exoplanet Survey Satellite (TESS) exoplanet-hunting mission detected the rising and decaying optical afterglow of GRB 191016A, a long gamma-ray burst (GRB) detected by Swift-BAT but without prompt XRT or UVOT follow-up due to proximity to the Moon. The afterglow has a late peak at least 1000 s after the BAT trigger, with a brightest-detected TESS data point at 2589.7 s post-trigger. The burst was not detected by Fermi-LAT, but was detected by Fermi-GBM without triggering, possibly due to the gradual nature of the rising light curve. Using ground-based photometry, we estimate a photometric redshift of z(sub phot) = 3.29 ± 0.40. Combined with the high-energy emission and optical peak time derived from TESS, estimates of the bulk Lorentz factor Γ(sub BL) range from 90 to 133. The burst is relatively bright, with a peak optical magnitude in ground-based follow-up of R = 15.1 mag. Using published distributions of GRB afterglows and considering the TESS sensitivity and sampling, we estimate that TESS is likely to detect ∼1 GRB afterglow per year above its magnitude limit.

Gamma-ray bursts↗
NASA NTRS2020

A GaN-based Four-Switch Buck-Boost Converter using Ripple Correlation Control for Maximum Power Point Tracking in Dynamic Deep Space Environments

As the demand for high-performance power conversion in spacecraft continues to grow and spacecraft mass and volume budgets become increasingly tight, it is essential to design DC-DC converters with higher efficiency and power density. Although photovoltaic (PV) efficiency has increased over time, solar irradiance and temperatures can fluctuate dramatically in deep space. This causes significant variations in the maximum power point (MPP) of the PV array, which can decrease the overall system efficiency unless accounted for. Thus, it is imperative to track the MPP of the PV panels to maintain optimal efficiency. This paper presents the experimental development of a four-switch, GaN-based buck-boost converter with an implementation of the Ripple Correlation Control (RCC) MPPT algorithm for dynamic deep space environments. Due to the use of GaN HEMTs, the experimental system achieves better efficiency and power density compared to the previous state of the art implementations. A simulation of the prototype buck-boost converter was implemented in SaberRD (Synopsis), and a digital design of the RCC-based MPPT controller utilizing the StateAMS tool is presented. The simulation results show that this controller swiftly and precisely converged to the MPP of the source PV panels in a dynamic solar irradiance condition.

Mahmud, Sadab↗
NASA NTRSDate not supplied

SMOS Salinity Retrieved from New Seawater Dielectric Constant Models at L-band

The accuracy of the Sea Surface Salinity (SSS) retrieved from L-Band radiometer measurements is strongly dependent on the accuracy of the modelling of the dielectric constant. Two new parametrizations have recently been developed based on one hand on the Soil Moisture and Ocean Salinity (SMOS) satellite multi-angular brightness temperature measurements [1] (BV) and on the other hand on new laboratory measurements [3] (GW2020). These two approaches are fully independent. The brightness temperatures, Tb, simulated with the BV and GW2020 parametrizations are compared with each other and with the ones derived from dielectric constant models previously in use in the SMOS, Soil Moisture Active Passive (SMAP) and Aquarius SSS retrievals. Tb simulated with the BV and GW2020 parametrizations agree particularly well for most SSS and SST commonly observed over the open ocean and are found to be in closer agreement than with earlier parametrizations. Nevertheless, uncertainty remains at low SST where a ∼ 0.1 K relative difference between the two models is observed. A complete reprocessing of SMOS SSS (2010–2020) has been performed using the BV parametrization instead of the Klein and Swift (1977) model previously used in SMOS processing. When compared with Argo derived near surface salinity maps, clear improvements are observed in warm and cold regions. Remaining uncertainties in cold waters will be discussed relatively to the uncertainties in SMOS Tb linked to sea ice contamination and given the constraints given by the GW2020 laboratory measurements.

J. Boutin↗
NASA NTRSDate not supplied

Mixed Integer Linear Programming in Planning

This project, Activity Planning with Resources for the Exploration of Space (APRES), uses a mixed-integer linear program (MILP) to solve planning problems. This work enables APRES to interpret a model file and output a solution with improved human readability. A plan model is optimized using a MILP solver and the best solution is taken. Once a plan is generated, it is parsed allowing it to retain only desired information and modified for swift human readability.

Christina Erwin↗
NASA NTRS2022

Status of the Dielectric Constant of Sea Water at L-Band for Remote Sensing of Salinity

The model expressing the dielectric constant of sea water at microwave frequencies as a function of salinity and temperature is an important element in remote sensing of sea surface salinity. It is also important independently as a description of the physical properties of salt water. A major milestone was the development in the late 1970s by Klein and Swift of a model based on laboratory measurements at L- and S-band and a functional form supported by theory for polar molecules and previous work on freshwater. Much of the subsequent work has focused on measurements at higher frequency and determining model parameters tuned to apply for applications, such as remote sensing of sea surface temperature (SST). Interest in the dielectric constant at 1.4 GHz (L-band) increased again with the development of soil moisture ocean salinity (SMOS) and Aquarius to measure salinity from space, but there have been few new measurements at L-band and often confusion regarding the applicability of new models at 1.4 GHz. The objective of this article is to compare available models in the context of how well they represent the dielectric constant of sea water at 1.4 GHz. Among the criteria applied will be the recent measurements at the George Washington University of the dielectric constant at 1.4 GHz

Microwave Remote Sensing↗
NASA NTRS2023

First Detection of X-Ray Polarization from the Accreting Neutron Star 4U 1820−303

This paper reports the first detection of polarization in the X-rays for atoll-source 4U 1820−303, obtained with the Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed in the radio with the Australia Telescope Compact Array. The IXPE observations of 4U 1820−303 were coordinated with Swift X-ray Telescope, Neutron Star Interior Composition Explorer, and Nuclear Spectroscopic Telescope Array aiming to obtain an accurate X-ray spectral model covering a broad energy interval. The source shows a significant polarization above 4 keV, with a polarization degree of 2.0% ± 0.5% and a polarization angle of −55° ± 7° in the 4–7 keV energy range, and a polarization degree of 10% ± 2% and a polarization angle of −67° ± 7° in the 7–8 keV energy bin. This polarization also shows a clear energy trend with polarization degree increasing with energy and a hint for a position-angle change of ≃90° at 96% CL around 4 keV. The spectro-polarimetric fit indicates that the accretion disk is polarized orthogonally to the hard spectral component, which is presumably produced in the boundary/spreading layer. We do not detect linear polarization from the radio counterpart, with a 3σ upper limit of 50% at 7.25 GHz.

4U1820-303↗
NASA NTRSDate not supplied

Mrk 421: 2 Weeks of Multi-Wavelength Monitoring with IXPE

At z=0.03, Mrk 421 is one of the closest Blazars and hence an exceptional target for X-ray polarization. In December of 2023, IXPE monitored Mrk 421 for approximately two weeks, in order to investigate the possible origins of X-ray polarization variability previously seen in this target, and whether it might originate in a shock propagating through the field lines of an energy-stratified jet. We present the results of the IXPE campaign, which is also complemented by NuSTAR, XMM-Newton, Swift, and ground-based monitoring.

IXPE↗
NASA NTRSDate not supplied

Spectral Measurement Errors due to CCD Serial Pixel-to-Pixel Readout Interference in the Ocean Color Instrument of the NASA PACE Mission

The Ocean Color Instrument on NASA’s PACE mission is a hyperspectral imager with a spatial resolution of 1km x 1km and spectral resolution of 5nm in 2.5nm steps over 320-890nm. The detection system is based on two Charge-Coupled Devices (CCDs) operating in Time Delay Integration (TDI) mode to achieve high signal-to-noise ratio. The front-end optical imager is a rotating mirror-based system that images the ground-scene onto a slit with a field of view of 16km x 1km. The slit-image is re-imaged and wavelength dispersed on the CCDs. As the ground-scene moves through the slit, it moves along the CCD columns as charge is moved along with it. The accumulated charge at the end of each column is collected in a serial pixel. Each of the 16 CCD outputs read out 32 columns of the same ground scene spaced 0.625nm apart. The 32-pixel serial register is swiftly read out before the next TDI cycle. The telescope is spinning at 5.77Hz to achieve the required spatial resolution. This results in a serial pixel readout speed of 8.5MHz. Each serial read-cycle goes through a reset and video period that are each sampled to create a low-noise correlated double sample value. This only allows 59ns for the reset and video to be asserted and settled before sampling. Due to the short time period, the response of the CCD exhibits serial pixel-to-pixel readout interference as the reset and video signals do not have time to fully settle before sampling. Each serial pixel value therefore has a dependence on the value of the preceding pixel value. This leads to a spectral measurement error of up to 0.3%. We explain the operation of the detection system, the behavior of the interference and the resulting wavelength error with results from ground testing and on-orbit characterization.

Ulrik B Gliese↗
NASA NTRSDate not supplied

Uneven Inflation Load Share Trends in Clusters

The use of parachute clusters for payload recovery is still seeing widespread use ever since the early days of WWII. By involving the (near) simultaneous deployment of several smaller and identical canopies connected to the payload, cluster systems offer flexibility in tailoring to needed descent rates and load management, as well as providing robustness against individual canopy deployment or opening failure. Their downside, of course, resides in the possibility of differing inflation rates by each cluster member as caused by deployment variability, canopy-to-canopy interference, etc. Such variability leads to the lead-lag phenomenon, which causes uneven loading among the parachutes, often times leaving a single canopy to take up a significant portion of the system’s inflation loads. Herein we investigate how serious such an effect can be in terms of the number N of cluster members, underinflation drag of the lagging canopies, inflation swiftness of the leader in comparison to the laggards’, disreefing cutter activation staggering and pre-disreefing drag area. Two new metrics are used to highlight load share unevenness, namely, the peak and average leader canopy riser load in comparison to the leader’s drag during no-lead-lag; and leader peak load, as compared to total peak load. Results are calculated from data collected in NASA’s Orion/CPAS test program, as well as from simple algebraic expressions informing leader drag as sustained in different cluster systems (i.e., of different N) and varying leader inflation time relative to the laggards. Generally, using large-N cluster systems confers better load sharing among canopies. However, and in deployments where significant lead-lag occur, large-N systems may feature greater leader overload excursions, oftentimes in excess of 50% the no-lead-lag levels. These excursions are particularly made worse when cutter activation among the members are far from simultaneous and the laggards’ pre-disreefing drag area is small in comparison to the leader’s.

parachutes↗
NASA NTRSDate not supplied

UAS Payload Design for In-Situ Smoke Plume Sensing

To aid in the detection of harmful aerosols and trace gases emitted in wildfires and prescribed burns, we are designing a smoke-specific payload which can be flown on the Black Swift S2™ Uncrewed Aerial System (UAS) platform. The payload will be custom and mounted within the nosecone, equipped with a trace gas sensor suite and particle spectrometer for characterization of wildfire and prescribed burn emissions. This data will aid in the improvement of HYSPLIT3 modeling to make more informed decisions regarding prescribed burns, particularly for mountain valleys in the Pacific Northwest. The trace gas suite will be composed of 8 low-cost trace gas sensors (CH4, CO2, CO, HCN, NO2, NO, O3, and Volatile Organic Compounds (VOCs)). Portable Optical Particle Spectrometer (POPS) measures particle size range between 0.14 to 3 µm, which is crucial for assessing public health impact, air quality, and visibility.

Henry Miller↗