While the successful launch and operation in space of the Gas Pixel Detectors onboard the PolarLight cubesat and the Imaging X-ray Polarimetry Explorer demonstrate the viability and the technical ...soundness of this class of detectors for astronomical X-ray polarimetry, it is clear that the current state of the art is not ready to meet the challenges of the next generation of experiments, such as the enhanced X-ray Timing and Polarimetry mission, designed to allow for a significantly larger data throughput.
In this paper we describe the design and test of a new custom, self-triggering readout ASIC, dubbed XPOL-III, specifically conceived to address and overcome these limitations. While building upon the overall architecture of the previous generations, the new chip improves over its predecessors in several, different key areas: the sensitivity of the trigger electronics, the flexibility in the definition of the readout window, as well as the maximum speed for the serial event readout. These design improvements, when combined, allow for almost an order of magnitude smaller dead time per event with no measurable degradation of the polarimetric, spectral, imaging or timing capability of the detector, providing a good match for the next generation of X-ray missions.
We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT ...observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR) and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission.
First Fermi-LAT Solar Flare Catalog Ajello, M.; Baldini, L.; Bastieri, D. ...
The Astrophysical journal. Supplement series,
02/2021, Letnik:
252, Številka:
2
Journal Article
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We present the first Fermi-Large Area Telescope (LAT) solar flare catalog covering the 24th solar cycle. This catalog contains 45 Fermi-LAT solar flares (FLSFs) with emission in the γ-ray energy band ...(30 MeV-10 GeV) detected with a significance of ≥5 over the years 2010-2018. A subsample containing 37 of these flares exhibits delayed emission beyond the prompt-impulsive hard X-ray phase, with 21 flares showing delayed emission lasting more than two hours. No prompt-impulsive emission is detected in four of these flares. We also present in this catalog observations of GeV emission from three flares originating from active regions located behind the limb of the visible solar disk. We report the lightcurves, spectra, best proton index, and localization (when possible) for all FLSFs. The γ-ray spectra are consistent with the decay of pions produced by >300 MeV protons. This work contains the largest sample of high-energy γ-ray flares ever reported and provides a unique opportunity to perform population studies on the different phases of the flare and thus allowing a new window in solar physics to be opened.
ABSTRACT We report the Fermi Large Area Telescope detection of extended γ-ray emission from the lobes of the radio galaxy Fornax A using 6.1 years of Pass 8 data. After Centaurus A, this is now the ...second example of an extended γ-ray source attributed to a radio galaxy. Both an extended flat disk morphology and a morphology following the extended radio lobes were preferred over a point-source description, and the core contribution was constrained to be % of the total γ-ray flux. A preferred alignment of the γ-ray elongation with the radio lobes was demonstrated by rotating the radio lobes template. We found no significant evidence for variability on ∼0.5 year timescales. Taken together, these results strongly suggest a lobe origin for the γ-rays. With the extended nature of the γ-ray emission established, we model the source broadband emission considering currently available total lobe radio and millimeter flux measurements, as well as X-ray detections attributed to inverse Compton (IC) emission off the cosmic microwave background (CMB). Unlike the Centaurus A case, we find that a leptonic model involving IC scattering of CMB and extragalactic background light (EBL) photons underpredicts the γ-ray fluxes by factors of about ∼2-3, depending on the EBL model adopted. An additional γ-ray spectral component is thus required, and could be due to hadronic emission arising from proton-proton collisions of cosmic rays with thermal plasma within the radio lobes.
We report a combined molecular modeling and experimental effort to predict and measure the heat of sorption of target molecules in a polymer film. The primary focus of this work is to demonstrate and ...validate the applicability of molecular modeling techniques as a predictive tool to evaluate polymeric sensing films for micro-calorimetric chemical sensor applications. In the current investigation, molecular modeling studies were performed prior to experimental measurements to predict the heat of sorption (Δ
H
sorpt) of a target analyte in a polymer film. Experimental measurements for the heat of sorption were performed using a quartz crystal microbalance/heat conducting calorimeter setup. The system under investigation was isopropanol in ethyl cellulose polymer film. The average Δ
H
sorpt predicted by molecular modeling at 22
°C was found to be 39.7
kJ/mol (standard deviation
=
2.6
kJ/mol), which compares well with the average measured experimental value of 30.2
kJ/mol (standard deviation
=
7.2
kJ/mol).
The detection of high-redshift ( ) blazars enables the study of the evolution of the most luminous relativistic jets over cosmic time. More importantly, high-redshift blazars tend to host massive ...black holes and can be used to constrain the space density of heavy black holes in the early universe. Here, we report the first detection with the Fermi-Large Area Telescope of five γ-ray-emitting blazars beyond z = 3.1, more distant than any blazars previously detected in γ-rays. Among these five objects, NVSS J151002+570243 is now the most distant known γ-ray-emitting blazar at z = 4.31. These objects have steeply falling γ-ray spectral energy distributions (SEDs), and those that have been observed in X-rays have a very hard X-ray spectrum, both typical of powerful blazars. Their Compton dominance (ratio of the inverse Compton to synchrotron peak luminosities) is also very large ( ). All of these properties place these objects among the most extreme members of the blazar population. Their optical spectra and the modeling of their optical-UV SEDs confirm that these objects harbor massive black holes ( ). We find that, at , the space density of black holes hosted in radio-loud and radio-quiet active galactic nuclei are similar, implying that radio-loudness may play a key role in rapid black hole growth in the early universe.
NGC 1068 is the most observed radio-quiet active galactic nucleus (AGN) in polarimetry, yet its high-energy polarization has never been probed before due to a lack of dedicated polarimeters. Using ...the first X-ray polarimeter sensitive enough to measure the polarization of AGNs, we want to probe the orientation and geometric arrangement of (sub)parsec-scale matter around the X-ray source. We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2-8 keV polarization of NGC 1068. We pointed IXPE at the target for a net exposure time of 1.15 Ms, in addition to using two Chandra snapshots of sim 10 ks each in order to account for the potential impact of several ultraluminous X-ray sources (ULXs) within IXPE's field of view. We measured a 2 -- 8 keV polarization degree of 12.4<!PCT!> pm 3.6<!PCT!> and an electric vector polarization angle of 101$^ pm 8$^ at a 68<!PCT!> confidence level. If we exclude the spectral region containing bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises to 21.3<!PCT!> pm 6.7<!PCT!> in the 3.5 -- 6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec-scale radio jet. Using a combined Chandra and IXPE analysis plus multiwavelength constraints, we estimated that the circumnuclear “torus” may sustain a half-opening angle of 50 -- 55$^ (from the vertical axis of the system). Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of the polarization angle orientation with respect to the radio jet and the torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN: the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic objects is now feasible thanks to X-ray polarimetry.
We use joint observations by the Swift X-ray Telescope (XRT) and the Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows to investigate the nature of the long-lived high-energy ...emission observed by Fermi LAT. Joint broadband spectral modeling of XRT and LAT data reveals that LAT nondetections of bright X-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the LAT and/or XRT energy ranges. Such a break is sufficient to suppress the high-energy emission so as to be below the LAT detection threshold. By contrast, LAT-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the XRT and LAT energy ranges. We speculate that the primary difference between GRBs with LAT afterglow detections and the nondetected population may be in the type of circumstellar environment in which these bursts occur, with late-time LAT detections preferentially selecting GRBs that occur in low wind-like circumburst density profiles. Furthermore, we find no evidence of high-energy emission in the LAT-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed X-ray emission. The lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-Compton (SSC) component in the LAT energy range. Alternatively, the peak of the SSC emission could be beyond the 0.1-100 GeV energy range considered for this analysis.
The Imaging X-Ray Polarimetry Explorer IXPE mission will perform polarization measures of 2-8 keV X-rays. Imaging, spectroscopy, and timing will complement this measurement for a comprehensive study ...of soft X-rays. The launch of the IXPE NASA small explorer mission to a low earth orbit is due late 2021. We designed a subsystem of the scientific payload, which has three identical telescopes based on the detector unit. The Gas Pixel Detector and its back-end electronics are the core of these units, performing data acquisition and processing, event sequencing, and on-line data compression. The back-end electronics processes the auto-triggered output of the detector of 300 photons per second with 30% of dead-time. A radiation-tolerant FPGA implements the electronics custom algorithms, including two digital serial interfaces with a central on-board computer. One interface is used for command and control of the unit, while the other for scientific data transmission. We also designed comprehensive test equipment to emulate the on-board computer and to operate the electronics. This equipment uses an FPGA on a VMEbus board as the electrical interface for the electronics, transferring data to a personal computer with dedicated software infrastructure. In this paper, we shall discuss the design process of the back-end electronics and the results of laboratory tests and measurements with X-ray sources.
ABSTRACT The Fermi Large Area Telescope (LAT) has an instantaneous field of view (FoV) covering ∼ 1 / 5 of the sky and it completes a survey of the entire sky in high-energy gamma-rays every 3 hr. It ...enables searches for transient phenomena over timescales from milliseconds to years. Among these phenomena could be electromagnetic counterparts to gravitational wave (GW) sources. In this paper, we present a detailed study of the LAT observations relevant to Laser Interferometer Gravitational-wave Observatory (LIGO) event GW150914, which is the first direct detection of gravitational waves and has been interpreted as being due to the coalescence of two stellar-mass black holes. The localization region for GW150914 was outside the LAT FoV at the time of the GW signal. However, as part of routine survey observations, the LAT observed the entire LIGO localization region within ∼70 minutes of the trigger and thus enabled a comprehensive search for a γ-ray counterpart to GW150914. The study of the LAT data presented here did not find any potential counterparts to GW150914, but it did provide limits on the presence of a transient counterpart above 100 MeV on timescales of hours to days over the entire GW150914 localization region.
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