arXiv subjects

W. Neil Johnson

Publications and source records attributed to W. Neil Johnson.

13 recordsLinked to original sources

arXiv2024

SIRI-2 Detection of the Gamma-ray Burst 221009A

SIRI-2 is a collection of Strontium Iodide gamma-ray detectors sensitive at approximately 400 keV to 10 MeV, launched on the Department of Defense's STPSat-6 to geosynchronous orbit. SIRI-2 detected the gamma-ray burst (GRB) 221009A and, unlike most GRB detectors, was not saturated and did not require any pulse pile-up corrections. The energetics of this burst as measured by SIRI-2 are consistent with those found by other instruments, and the Band function fits to the spectra are consistent with that from the unsaturated Insight and GECAM instruments, and similar to corrected spectra from the Fermi Gamma-ray Burst Monitor, but softer than those found by Konus-Wind when that instrument was saturated. The total fluence measured with SIRI-2 was measured to be 0.140 +/- 0.002 erg cm-2, lower than other instruments, likely due to the increasing background of SIRI-2 forcing the calculation to use a smaller time interval. An extrapolation of the distributions of fluences from GRBs to the fluence of 221009A measured with SIRI-2 indicates bursts brighter than this one should occur about once every 4,000 years.

astro-ph.HE↗

arXiv2024

Development of the cadmium zinc TElluride Radiation Imager (TERI)

The cadmium zinc TElluride Radiation Imager, or TERI, is an instrument to space qualify large-volume $4 \times 4 \times 1.5 \ \mathrm{cm}^3$ pixelated CdZnTe (CZT) detector technology. The CZT's anode is composed of a $22 \times 22$ array of pixels while the cathode is planar. TERI will contain four of those crystals with each pixel having an energy range of $40 \ \mathrm{keV}$ up to $3 \ \mathrm{MeV}$ with a resolution of $1.3 \%$ full-width-at-half maximum at $662 \ \mathrm{keV}$ all while operating in room temperature. As the detectors are 3D position sensitive, TERI can Compton image events. TERI is fitted with a coded-aperture mask which permits imaging low energy photons in the photoelectric regime. TERI's primary mission is to space-qualify large-volume CZT and measure its degradation due to radiation damage in a space environment. Its secondary mission includes detecting and localizing astrophysical gamma-ray transients. TERI is manifested on DoD's STP-H10 mission for launch to the International Space Station in early 2025.

astro-ph.IM↗

arXiv2023

Development of Dual-Gain SiPM Boards for Extending the Energy Dynamic Range

Astronomical observations with gamma rays in the range of several hundred keV to hundreds of MeV currently represent the least explored energy range. To address this so-called MeV gap, we designed and built a prototype CsI:Tl calorimeter instrument using a commercial off-the-shelf (COTS) SiPMs and front-ends which may serve as a subsystem for a larger gamma-ray mission concept. During development, we observed significant non-linearity in the energy response. Additionally, using the COTS readout, the calorimeter could not cover the four orders of magnitude in energy range required for the telescope. We, therefore, developed dual-gain silicon photomultiplier (SiPM) boards that make use of two SiPM species that are read out separately to increase the dynamic energy range of the readout. In this work, we investigate the SiPM's response with regards to active area ($3\times3 \ \mathrm{mm}^2$ and $1 \times 1 \ \mathrm{mm}^2$) and various microcell sizes ($10$, $20$, and $35 \ μ\mathrm{m}$). We read out $3\times3\times6 \ \mathrm{cm}^3$ CsI:Tl chunks using dual-gain SiPMs that utilize $35 \ μ\mathrm{m}$ microcells for both SiPM species and demonstrate the concept when tested with high-energy gamma-ray and proton beams. We also studied the response of $17 \times 17 \times 100 \ \mathrm{mm}^3$ CsI bars to high-energy protons. With the COTS readout, we demonstrate a sensitivity to $60 \ \mathrm{MeV}$ protons with the two SiPM species overlapping at a range of around $2.5-30 \ \mathrm{MeV}$. This development aims to demonstrate the concept for future scintillator-based high-energy calorimeters with applications in gamma-ray astrophysics.

astro-ph.IM↗

arXiv2020

Radiation damage assessment of SensL SiPMs

Silicon Photomultipliers (SiPMs) are quickly replacing traditional photomultiplier tubes (PMTs) as the readout of choice for gamma-ray scintillation detectors in space. While they offer substantial size, weight and power saving, they have shown to be susceptible to radiation damage. SensL SiPMs with different cell sizes were irradiated with 64 MeV protons and 8 MeV electrons. In general, results show larger cell sizes are more susceptible to radiation damage with the largest 50 um SiPMs showing the greatest increase in current as a function of dose. Current increases were observed for doses as low at ~2 rad(Si) for protons and ~20 rad(Si) for electrons. The U.S. Naval Research Laboratory's (NRL) Strontium Iodide Radiation Instrument (SIRI-1) experienced a 528 uA increase in the bias current of the on-board 2x2 SensL J-series 60035 SiPM over its one-year mission in sun-synchronous orbit. The work here focuses on the increase in bulk current observed with increasing radiation damage and was performed to better quantify this effect as a function of dose for future mission. These include the future NRL mission SIRI-2, the follow on to SIRI-1, Glowbug and the GAGG Radiation Instrument (GARI).

physics.ins-det↗

arXiv2019

Strontium Iodide Radiation Instrument (SIRI) -- Early On-Orbit Results

The Strontium Iodide Radiation Instrument (SIRI) is a single detector, gamma-ray spectrometer designed to space-qualify the new scintillation detector material europium-doped strontium iodide (SrI2:Eu) and new silicon photomultiplier (SiPM) technology. SIRI covers the energy range from 0.04-8 MeV and was launched into 600 km sun-synchronous orbit on Dec 3, 2018 onboard STPSat5 with a one-year mission to investigate the detector's response to on-orbit background radiation. The detector has an active volume of 11.6 cm3 and a photo fraction efficiency of 50% at 662 keV for gamma-rays parallel to the long axis of the crystal. Its spectroscopic resolution of 4.3% was measured by the FWHM of the characteristic Cs-137 gamma-ray line at 662 keV. Measured background rates external to the trapped particle regions are 40-50 counts per second for energies greater than 40 keV and are largely the result of short- and long-term activation products generated by transits of the SAA and the continual cosmic-ray bombardment. Rate maps determined from energy cuts of the collected spectral data show the expected contributions from the various trapped particle regions. Early spectra acquired by the instrument show the presence of at least 10 characteristic gamma-ray lines and a beta continuum generated by activation products within the detector and surrounding materials. As of April 2019, the instrument has acquired over 1000 hours of data and is expected to continue operations until the space vehicle is decommissioned in Dec. 2019. Results indicate SrI2:Eu provides a feasible alternative to traditional sodium iodide and cesium iodide scintillators, especially for missions where a factor-of-two improvement in energy resolution would represent a significant difference in scientific return. To the best of our knowledge, SIRI is the first on-orbit use of SrI2:Eu scintillator with SiPM readouts.

astro-ph.IM↗

arXiv2002

X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4

We analyse five observations of the X-ray binary GX 339-4 by the soft gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE observations. The source was bright during four of them, with the luminosity of L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s. Our broad-band spectral fits show the mean electron energy of electrons in the Comptonizing plasma decreasing with increasing luminosity within the hard (bright) state. For the observation with the best statistics at soft gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably carried by non-thermal electrons. Then, considering the efficiency of Comptonized hybrid synchrotron emission allows us to obtain an upper limit on the strength of the magnetic field in the X-ray source. Furthermore, this synchrotron emission is capable of producing the optical spectrum observed in an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum is consistent with being dominated by bremsstrahlung. The unusually strong Fe K alpha line observed by the PCA during that state is found not to be intrinsic to the source but to originate mostly in the Galactic diffuse emission.

astro-ph↗

arXiv2000

Observations of Seyferts by OSSE and parameters of their X-ray/gamma-ray sources

We present a summary of spectra of Seyfert galaxies observed by the OSSE detector aboard Compton Gamma Ray Observatory. We obtain average spectra of Seyferts of type 1 and 2, and find they are well fitted by thermal Comptonization. We present detailed parameter ranges for the plasma temperature and the Compton parameter in the case of spherical and slab geometries. We find both the average and individual OSSE spectra of Seyfert 2s are significantly harder than those of Seyfert 1s, which difference can be due to anisotropy of Compton reflection and/or Thomson-thick absorption.

astro-ph↗

arXiv1998

Broad-band X-ray/gamma-ray spectra and binary parameters of GX 339-4 and their astrophysical implications

We present X-ray/gamma-ray spectra of the binary GX 339-4 observed in the hard state simultaneously by Ginga and CGRO OSSE during an outburst in 1991 September. The Ginga spectra are well represented by a power law with a photon spectral index of 1.75 and a moderately-strong Compton reflection component with a fluorescent Fe K alpha line. The OSSE data require a sharp high-energy cutoff in the power-law spectrum. The broad-band spectra are very well modelled by repeated Compton scattering in a thermal plasma with tau=1 and kT=50 keV. We also find the distance to the system to be > 3 kpc, ruling out earlier determinations of 1.3 kpc. Using this limit, the observed reddening and the orbital period, we find the allowed range of the mass of the primary is consistent with it being a black hole. The data are inconsistent with models of either homogenous or patchy coronae above the surface of an accretion disc. Rather, they are consistent with the presence of a hot inner hot disc accreting at a rate close to the maximum set by advection and surrounded by a cold outer disc. The seed photons for Comptonization are supplied by the outer cold disc and/or cold clouds within the hot disc. Pair production is negligible if electrons are thermal. The hot disc model, which scaled parameters are independent of the black-hole mass, is supported by the similarity of the spectrum of GX 339-4 to those of other black-hole binaries and Seyfert 1s. On the other hand, their spectra in the soft gamma-ray regime are significantly harder than those of weakly-magnetized neutron stars. Based on this difference, we propose that the presence of broad-band spectra corresponding to thermal Comptonization with kT of 50 keV or more represents a black-hole signature.

astro-ph↗

arXiv1998

X-ray and soft gamma-ray spectra of Broad-Line Radio Galaxies

We study X-ray and soft gamma-ray spectral properties of BLRGs using data from Ginga, ASCA, OSSE and EXOSAT. The X-ray spectra are well fitted by an intrinsic power-law continuum with an energy index of alpha ~ 0.7, moderately absorbed by a cold medium. In addition, the Ginga spectra show Fe K-alpha lines with an average equivalent width of ~ 100 eV, and, in some cases, Compton reflection humps. However, the latter components are significantly weaker than both those seen in radio-quiet Seyferts and those expected if the Fe K-alpha lines were due to reflection. Some ASCA and EXOSAT spectra show soft X-ray excesses below ~ 3 keV. When that component is taken into account, the Fe K-alpha lines in the ASCA data become unresolved with equivalent widths < 200 eV. Multiple observations of 3C 390.3 and 3C 382 show the Fe K-alpha line approximately constant in flux but accompanied by strong continuum variations. This indicates the bulk of the line is formed by matter at a distance much larger than an accretion-disk scale, consistent with the ASCA line width measurements. The column density of the matter required to account for the observed line fluxes is N_H > 10^23 cm^-2. Such a medium is in the line-of-sight in 3C 445 but it has to be out of it in other objects, in which the observed N_H are substantially lower. Thus, a cold medium with that N_H and covering a large solid angle is out of the line-of-sight in most objects. The spectra of BLRGs break and become softer above ~ 100 keV, as shown by a simultaneous ASCA/OSSE observation of 3C 120 and by the OSSE spectra being on average much softer than the X-ray spectra. Finally, we find the X-ray and gamma-ray spectral properties of Cen A intrinsically very similar to BLRGs studied here.

astro-ph↗

arXiv1997

X-ray and gamma-ray spectra of Cyg X-1 in the soft state

We present X-ray/gamma-ray observations of Cyg X-1 in the soft state during 1996 May-June. We analyze ASCA, RXTE and OSSE data. The spectrum consists of soft X-ray blackbody emission of an optically thick accretion disk in the vicinity of a black hole and a power law with an energy index alpha around 1.2-1.5 extending to at least several hundred keV. In the spectra, we find the presence of strong Compton reflection, which probably comes from the disk.

astro-ph↗

arXiv1997

Simultaneous X-ray and gamma-ray observations of Cyg X-1 in the hard state by Ginga and OSSE

We present four X-ray/gamma-ray spectra of Cyg X-1 observed in the hard ('low') state simultaneously by Ginga and GRO/OSSE on 1991 July 6. The 3-30 keV Ginga spectra are well represented by power laws with an energy spectral index of alpha~0.6 and a Compton reflection component including a fluorescent Fe K-alpha corresponding to the solid angle of the reflector of ~0.3 times 2 Pi. The overall Ginga/OSSE spectra can be modelled by repeated Compton scattering in a mildly-relativistic, tau ~1, plasma. However, the high-energy cutoff is steeper than that due to single-temperature thermal Comptonisation. It can be described by a superposition of dominant optically-thin, thermal emission at kT~140 keV and a Wien-like component from an optically-thick plasma at kT~50 keV. The X-ray spectra do not show the presence of an anisotropy break required if thermal Compton scattering takes place in a corona above a cold disc. Also, the flat spectral index shows that the plasma is soft-photon starved, i.e., the luminosity in incident soft X-ray seed photons is very much less than that in the hard X-rays. Furthermore, the observed solid angle of the reflector is significantly less than 2 pi. These facts taken together strongly rule out a disc-corona geometry. Rather, the observed spectra are consistent with a geometry in which the cold accretion disc (which both supplies the seed soft X-rays and reflects hard X-rays) only exists at large radii, while the Comptonising hot plasma is located in an inner region with no cold disc. This hot plasma consists of either pure electron-positron pairs if the source size is ~5 Schwarzschild radii or it contains also protons if the size is larger.

astro-ph↗

arXiv1996

X-rays and gamma-rays from accretion flows onto black holes in Seyferts and X-ray binaries

We review observations and theoretical models of X-ray/gamma-ray spectra of radio-quiet Seyfert galaxies and of Galactic black-hole candidates (in the hard spectral state). The observed spectra share all their basic components: an underlying power law, a Compton-reflection component with an Fe K-alpha line, low-energy absorption by intervening cold matter, and a high-energy cutoff above ~ 200 keV. The X-ray energy spectral index, alpha, is typically in the range ~ 0.8-1 in Seyfert spectra from Ginga, EXOSAT and OSSE. The hard-state spectra of black-hole candidates Cyg X-1 and GX 339-4 from simultaneous Ginga/OSSE observations have alpha ~ 0.6-0.8. The Compton-reflection component corresponds to cold matter (e.g., inner or outer parts of an accretion disk) covering a solid angle of ~ (0.4-1) times 2 pi as seen from the X-ray source. The spectra are cut off in soft gamma-rays above ~ 200 keV. The broad-band spectra of both Seyferts and black-hole sources are well fitted by Compton upscattering of soft photons in thermal plasmas. Our fits yield the thermal plasma temperature of ~ 100 keV and the Thomson optical depth of tau 1. A fraction of the luminosity emitted nonthermally appears to be small and it can be constrained to < 15% in the Seyfert galaxy NGC 4151. The spectra are cut off before 511 keV, which is strongly suggestive of a thermostatic role of e+e- pair production in constraining the temperature and optical depth of the sources. The source geometry is compatible with a patchy corona above a cold disk in Seyferts, but not in Cyg X-1. In the latter, the relative weakness of reflection is compatible with reflection of emission of a hot inner disk from outer disk regions.

astro-ph↗

arXiv1995

The first simultaneous X-ray/gamma-ray observations of Cyg X-1 by Ginga and OSSE

We present the results of 4 simultaneous observations of Cygnus X-1 by Ginga and OSSE. The X-ray/gamma-ray spectra can be described by an intrinsic continuum and a component due to Compton reflection including an iron K-alpha line. The intrinsic spectrum at X-ray energies is a power-law with a photon spectral index of Gamma=1.6. The intrinsic gamma-ray spectrum can be phenomenologically described by either a power-law without cutoff up to 150 keV, and an exponential cutoff above that energy, or by an exponentially cutoff power law and a second hard component.

astro-ph↗