| 1. |
- Atwood, W. B., et al.
(författare)
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THE LARGE AREA TELESCOPE ON THE FERMI GAMMA-RAY SPACE TELESCOPE MISSION
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 697:2, s. 1071-1102
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Tidskriftsartikel (refereegranskat)abstract
- The Large Area Telescope (Fermi/LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view (FoV), high-energy gamma-ray telescope, covering the energy range from below 20 MeV to more than 300 GeV. The LAT was built by an international collaboration with contributions from space agencies, high-energy particle physics institutes, and universities in France, Italy, Japan, Sweden, and the United States. This paper describes the LAT, its preflight expected performance, and summarizes the key science objectives that will be addressed. On-orbit performance will be presented in detail in a subsequent paper. The LAT is a pair-conversion telescope with a precision tracker and calorimeter, each consisting of a 4 x 4 array of 16 modules, a segmented anticoincidence detector that covers the tracker array, and a programmable trigger and data acquisition system. Each tracker module has a vertical stack of 18 (x, y) tracking planes, including two layers (x and y) of single-sided silicon strip detectors and high-Z converter material (tungsten) per tray. Every calorimeter module has 96 CsI(Tl) crystals, arranged in an eight-layer hodoscopic configuration with a total depth of 8.6 radiation lengths, giving both longitudinal and transverse information about the energy deposition pattern. The calorimeter's depth and segmentation enable the high-energy reach of the LAT and contribute significantly to background rejection. The aspect ratio of the tracker (height/width) is 0.4, allowing a large FoV (2.4 sr) and ensuring that most pair-conversion showers initiated in the tracker will pass into the calorimeter for energy measurement. Data obtained with the LAT are intended to (1) permit rapid notification of high-energy gamma-ray bursts and transients and facilitate monitoring of variable sources, (2) yield an extensive catalog of several thousand high-energy sources obtained from an all-sky survey, (3) measure spectra from 20 MeV to more than 50 GeV for several hundred sources, (4) localize point sources to 0.3-2 arcmin, (5) map and obtain spectra of extended sources such as SNRs, molecular clouds, and nearby galaxies, (6) measure the diffuse isotropic gamma-ray background up to TeV energies, and (7) explore the discovery space for dark matter.
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| 2. |
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Modulated High-Energy Gamma-Ray Emission from the Microquasar Cygnus X-3
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 326:5959, s. 1512-
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Tidskriftsartikel (refereegranskat)abstract
- Microquasars are accreting black holes or neutron stars in binary systems with associated relativistic jets. Despite their frequent outburst activity, they have never been unambiguously detected emitting high-energy gamma rays. The Fermi Large Area Telescope (LAT) has detected a variable high-energy source coinciding with the position of the x-ray binary and microquasar Cygnus X-3. Its identification with Cygnus X-3 is secured by the detection of its orbital period in gamma rays, as well as the correlation of the LAT flux with radio emission from the relativistic jets of Cygnus X-3. The gamma-ray emission probably originates from within the binary system, opening new areas in which to study the formation of relativistic jets.
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| 3. |
- Axelsson, Magnus, et al.
(författare)
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Vanishing hardness-flux correlation in Cygnus X-1 : Signs of the disc moving out
- 2008
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 490:1, s. 253-258
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We investigate observations of the X-ray binary Cygnus X-1 with unusually high hardness and low flux. In particular, we study the characteristic frequencies seen in the PDS and the hardness-flux correlation within and between these observations.Methods. We analyse observations of Cyg X-1 during periods when the source reaches its highest hardness levels (≳ 1 for the 9-20 keV over 2-4 keV RXTE/PCA count ratios, corresponding to Γ ≲ 1.6). Using the relativistic precession model to interpret the PDS, we estimate a value for the inner radius of the accretion disc. We also study the hardness-flux correlation.Results. In the selected observations, the characteristic frequencies seen in the power spectrum are shifted to the lowest end of their frequency range. Within a single observation, the hardness-flux correlation is very weak, contrary to the negative correlation normally observed in the hard state. We suggest that this could be interpreted as the inner disc boundary being at large radii (≳50Rg), thereby requiring more time to adjust to a changing accretion rate than allowed by a single RXTE observation, and compare our findings to estimates of the viscous time scale responsible for small scale variability in the system.
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| 4. |
- Tanaka, T., et al.
(författare)
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Data acquisition system for the PoGOLite astronomical hard X-ray polarimeter
- 2007
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Ingår i: Nuclear Science Symposium Conference Record, 2007. - 9781424409228 ; , s. 445-449, s. 445-449
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The PoGOLite is a new balloon-borne instrument to measure the polarization of hard X-rays/soft gamma-rays in the 25-80 keV energy range for the first time. In order to detect the polarization, PoGOLite measures the azimuthal angle asymmetry of Compton scattering and the subsequent photo-absorption in an array of detectors. This array consists of 217 well-type phoswich detector cells (PDCs) surrounded by a side anti-coincidence shield (SAS) composed of 54 segments of BGO crystals. At balloon altitude, the intensity of backgrounds due to cosmic-ray charged particles, atmospheric gamma-rays and neutrons is extremely high, typically a few hundred Hz per unit. Hence the data acquisition (DAQ) system of PoGOLite is required to handle more than 270 signals simultaneously, and detect weak signals from astrophysical objects (100mCrab, 1.5 cs(-1) in 25-80 keV) under such a severe environment. We have developed a new DAQ system consisting of front-end electronics, waveform digitizer, Field Programmable Gate Array (FPGA) and a microprocessor. In this system, all output signals of PDC / SAS are fed into individual charge-sensitive amplifier and then digitized to 12 bit accuracy at 24 MSa/s by pipelined analog to digital converters. A DAQ board for the PDC records waveforms which will be examined in an off-line analysis to distinguish signals from the background events and measure the energy spectrum and polarization of targets. A board for the SAS records hit pattern to be used for background rejection. It also continuously records a pulse-height analysis (PHA) histogram to monitor incident background flux. These basic functions of the DAQ system were verified in a series of beam tests.
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| 5. |
- Axelsson, Magnus, et al.
(författare)
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Measuring energy dependent polarization in soft gamma-rays using compton scattering in PoGOLite
- 2007
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Ingår i: Astroparticle physics. - : Elsevier. - 0927-6505 .- 1873-2852. ; 28:3, s. 327-337
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Tidskriftsartikel (refereegranskat)abstract
- Linear polarization in X-and gamma-rays is an important diagnostic of many astrophysical sources, foremost giving information about their geometry, magnetic fields, and radiation mechanisms. However, very few X-ray polarization measurements have been made, and then only mono-energetic detections, whilst several objects are assumed to have energy dependent polarization signatures. In this paper, we investigate whether detection of energy dependent polarization from cosmic sources is possible using the Compton technique, in particular with the proposed PoGOLite balloon-experiment, in the 25-100 keV range. We use Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6 and 35 h were simulated, corresponding to a standard and a long duration flight, respectively. Both smooth and sharp energy variations of the polarization are investigated and compared to constant polarization signals using chi-square statistics. We can reject constant polarization, with energy, for the Cygnus X-1 spectrum (in the hard state), if the reflected component is assumed to be completely polarized, whereas the distinction cannot be made for weaker polarization. For the accreting pulsar, constant polarization can be rejected in the case of polarization in a narrow energy band with at least 50% polarization, and similarly for a negative step distribution from 30% to 0% polarization.
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| 6. |
- Axelsson, Magnus, et al.
(författare)
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The aperiodic broad-band X-ray variability of Cygnus X-3
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 394:3, s. 1544-1550
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Tidskriftsartikel (refereegranskat)abstract
- We study the soft X-ray variability of Cygnus X-3. By combining data from the All-Sky Monitor and Proportional Counter Array instruments on the RXTE satellite with EXOSAT/Medium Energy (ME) detector observations, we are able to analyse the power density spectrum (PDS) of the source from 10-9 to 0.1 Hz, thus covering time-scales from seconds to years. As the data on the longer time-scales are unevenly sampled, we combine traditional power spectral techniques with simulations to analyse the variability in this range. The PDS at higher frequencies (≳10-3 Hz) are for the first time compared for all states of this source. We find that it is for all states well described by a power law, with index ∌ -2 in the soft states and a tendency for a less steep power law in the hard state. At longer time-scales, we study the effect of the state transitions on the PDS, and find that the variability below ∌10-7 Hz is dominated by the transitions. Furthermore, we find no correlation between the length of a high/soft-state episode and the time since the previous high/soft state. On intermediate time-scales, we find evidence for a break in the PDS at time-scales of the order of the orbital period. This may be interpreted as evidence for the existence of a tidal resonance in the accretion disc around the compact object, and constraining the mass ratio to M2/M1 ≲ 0.3.
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| 7. |
- Mizuno, T., et al.
(författare)
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High sensitivity balloon-borne hard X-ray/soft Gamma-Ray Polarimeter PoGOLite
- 2007
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Ingår i: Nuclear Science Symposium Conference Record, 2007. NSS ’07. IEEE. - : IEEE. - 9781424409228 ; , s. 2538-2544
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Konferensbidrag (refereegranskat)abstract
- The Polarized Gamma-ray Observer - Lightweight version (PoGOLite) is a new balloon experiment capable of detecting 10% polarization from a 200 mCrab source in the 25-80 keV energy range in a single 6-hour flight for the first time. Polarization measurements of hard X-rays and soft gamma-rays are expected to provide a powerful probe into high-energy emission mechanisms as well as source geometries. PoGOLite uses Compton scattering and photo-absorption to measure polarization in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators. The adoption of a well-type phoswich counter concept and a thick polyethylene neutron shield provides a narrow field-of-view (1.25 msr), a large effective area ( gt; 250 cm2 at 40-50 keV), a high modulation factor (more than 25%) and the low background ( 100 mCrab) required to conduct high-sensitivity polarization measurements. Through tests in laboratories and accelerator facilities of a scaled-down prototype with the front-end electronics of flight design and an extensive study by Monte Carlo simulation, we have demonstrated high instrument performance. PoGOLite will be ready for a first engineering flight in 2009 and a science flight in 2010, during which polarization signals from the Crab Nebula/pulsar, Cygnus X-1 and other objects will be observed.
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| 8. |
- Pearce, Mark, et al.
(författare)
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PoGOLite : A balloon-borne soft gamma-ray polarimeter
- 2007
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Ingår i: Proceedings of the 30th International Cosmic Ray Conference, ICRC 2007. - : Universidad Nacional Autonoma de Mexico. ; , s. 479-482
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Konferensbidrag (refereegranskat)abstract
- Polarized gamma-rays are expected from a wide variety of sources including rotationpowered pulsars, accreting black holes and neutron stars, and jet-dominated active galaxies. Polarization measurements provide a powerful probe of the gamma-ray emission mechanism and the distribution of magnetic and radiation fields around the source. No measurements have been performed in the soft gamma-ray band where non-thermal processes are expected to produce high degrees of polarization. The PoGOLite experiment applies well-type phoswich detector technology to polarization measurements in the 25 - 80 keV energy range. The instrument uses Compton scattering and photoabsorption in an array of 217 phoswich detector cells made of plastic and BGO scintillators, and surrounded by active BGO shields. A prototype of the flight instrument has been tested with polarized gammarays and background generated with radioactive sources. The test results and computer simulations confirm that the instrument can detect 10% polarization of a 200 mCrab source in one 6 hour balloon observation. In flight, targets are constrained to within better than 5% of the field-of-view (~5 degrees squared) in order to maximize the effective detection area during observations. The pointing direction on the sky is determined by an attitude control system comprising star trackers, differential GPS receiver system, gyroscopes, accelerometers and magnetometers which provide correction signals to a reaction wheel and torque motor system. Additionally, the entire polarimeter assembly rotates around its viewing axis to minimize systematic bias during observations. Flights are foreseen to start in 2009- 2010 and will target northern sky sources including the Crab pulsar/nebula, Cygnus X-1, and Hercules X-1. These observations will provide valuable information about the pulsar emission mechanism, the geometry around the black hole, and photon transportation in the strongly magnetized neutron star surface, respectively. Future goals include a long duration balloon flight from the Esrange facility in Northern Sweden to Canada.
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| 9. |
- Takahashi, H., et al.
(författare)
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Beam test results of the polarized gamma-ray observer, PoGOLite
- 2008
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Ingår i: 2008 IEEE NUCLEAR SCIENCE SYMPOSIUM AND MEDICAL IMAGING CONFERENCE (2008 NSS/MIC), VOLS 1-9. - 9781424427147 ; , s. 732-736
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Konferensbidrag (refereegranskat)abstract
- The Polarized Gamma-ray Observer, PoGOLite, is a balloon experiment with the capability of detecting 10% polarization from a 200 mCrab celestial object in the energy range 25 #x2013;80 keV. During a beam test at KEK-PF in February 2008, 20 detector units were assembled, and a 50 keV X-ray beam with a polarization degree of #x223C;90% was irradiated at the center unit. Signals from all 20 units were fed into flightversion electronics consisting of six circuit boards (four waveform digitizer boards, one digital I/O board and one router board) and one microprocessor (SpaceCube), which communicate using a SpaceWire interface. One digitizer board, which can associate up to 8 PDCs, outputs a trigger signal. The digital I/O board handles the trigger and returns a data acquisition request if there is no veto signal (upper or pulse-shape discriminators) from any detector unit. This data acquisition system worked well, and the modulation factor was successfully measured to be #x223C;34%. These results confirmed the capabilities of both detector and data-acquisition system for a pathfinder flight planned in 2010.
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| 10. |
- Takahashi, H., et al.
(författare)
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The Polarized Gamma-Ray Observer, PoGOLite
- 2010
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Ingår i: Transactions of the Japanese Society for Artificial Intelligence, Aerospace Technology Japan. - 1346-0714 .- 1346-0714. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The Polarized Gamma-ray Observer, PoGOLite, is a balloon experiment with the capability of detecting 10% polarization from a 200 mCrab celestial object in the energy-range 25–80 keV. During a beam test at KEK-PF in 2008, 19 detector units and one anti-coincidence detector were assembled, and a 50 keV X-ray beam with a polarization degree of ∼90% was irradiated at the center unit. Signals from all 20 units were fed into flight-version electronics consisting of six circuit boards (four waveform digitizer boards, one digital I/O board and one router board) and one microprocessor (SpaceCube), which communicate using a SpaceWire interface. One digitizer board, which can associate up to 8 detectors, outputs a trigger signal. The digital I/O board handles the trigger and returns a data acquisition request if there is no veto signal (upper or pulse-shape discriminators) from any detector unit. This data acquisition system worked well, and the modulation factor was successfully measured to be ∼34%. These results confirmed the capabilities of the data-acquisition system for a “pathfinder” flight planned in 2010.
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