| 1. |
- Abdo, A. A., et al.
(författare)
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Insights into the high-energy γ-ray emission of Markarian 501 from extensive multifrequency observations in the Fermi era
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 727:2, s. 129-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the gamma-ray activity of the blazar Mrk 501 during the first 480 days of Fermi operation. We find that the average Large Area Telescope (LAT) gamma-ray spectrum of Mrk 501 can be well described by a single power-law function with a photon index of 1.78 +/- 0.03. While we observe relatively mild flux variations with the Fermi-LAT (within less than a factor of two), we detect remarkable spectral variability where the hardest observed spectral index within the LAT energy range is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpected spectral changes do not correlate with the measured flux variations above 0.3 GeV. In this paper, we also present the first results from the 4.5 month long multifrequency campaign (2009 March 15-August 1) on Mrk 501, which included the Very Long Baseline Array (VLBA), Swift, RXTE, MAGIC, and VERITAS, the F-GAMMA, GASP-WEBT, and other collaborations and instruments which provided excellent temporal and energy coverage of the source throughout the entire campaign. The extensive radio to TeV data set from this campaign provides us with the most detailed spectral energy distribution yet collected for this source during its relatively low activity. The average spectral energy distribution of Mrk 501 is well described by the standard one-zone synchrotron self-Compton (SSC) model. In the framework of this model, we find that the dominant emission region is characterized by a size less than or similar to 0.1 pc (comparable within a factor of few to the size of the partially resolved VLBA core at 15-43 GHz), and that the total jet power (similar or equal to 10(44) erg s(-1)) constitutes only a small fraction (similar to 10(-3)) of the Eddington luminosity. The energy distribution of the freshly accelerated radiating electrons required to fit the time-averaged data has a broken power-law form in the energy range 0.3 GeV-10 TeV, with spectral indices 2.2 and 2.7 below and above the break energy of 20 GeV. We argue that such a form is consistent with a scenario in which the bulk of the energy dissipation within the dominant emission zone of Mrk 501 is due to relativistic, proton-mediated shocks. We find that the ultrarelativistic electrons and mildly relativistic protons within the blazar zone, if comparable in number, are in approximate energy equipartition, with their energy dominating the jet magnetic field energy by about two orders of magnitude.
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| 2. |
- Abdiyev, Kaldibek, et al.
(författare)
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Review of Slow Sand Filtration for Raw Water Treatment with Potential Application in Less-Developed Countries
- 2023
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Ingår i: Water (Switzerland). - 2073-4441. ; 15:11
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Forskningsöversikt (refereegranskat)abstract
- Providing safe drinking water to people in developing countries is an urgent worldwide water problem and a main issue in the UN Sustainable Development Goals. One of the most efficient and cheapest methods to attain these goals is to promote the use of slow sand filters. This review shows that slow sand filters can efficiently provide safe drinking water to people living in rural communities not served by a central water supply. Probably, the most important aspect of SSF for developing and less-developed countries is its function as a biological filter. WASH problems mainly relate to the spread of viruses, bacteria, and parasites. The surface and shallow groundwater in developing countries around urban areas and settlements are often polluted by domestic wastewater containing these microbes and nutrients. Thus, SSF’s function is to treat raw water in the form of diluted wastewater where high temperature and access to nutrients probably mean a high growth rate of microbes and algae but probably also high predation and high efficiency of the SSF. However, factors that may adversely affect the removal of microbiological constituents are mainly low temperature, high and intermittent flow rates, reduced sand depth, filter immaturity, and various filter amendments. Further research is thus needed in these areas, specifically for developing countries.
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| 3. |
- Abdo, A. A., et al.
(författare)
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Detection of High-Energy Gamma-Ray Emission from the Globular Cluster 47 Tucanae with Fermi
- 2009
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 325:5942, s. 845-848
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Tidskriftsartikel (refereegranskat)abstract
- We report the detection of gamma-ray emissions above 200 megaelectron volts at a significance level of 17 sigma from the globular cluster 47 Tucanae, using data obtained with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. Globular clusters are expected to emit gamma rays because of the large populations of millisecond pulsars that they contain. The spectral shape of 47 Tucanae is consistent with gamma-ray emission from a population of millisecond pulsars. The observed gamma-ray luminosity implies an upper limit of 60 millisecond pulsars present in 47 Tucanae.
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| 4. |
- Abdo, A. A., et al.
(författare)
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FERMI-LAT OBSERVATIONS OF THE GEMINGA PULSAR
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 720:1, s. 272-283
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Tidskriftsartikel (refereegranskat)abstract
- We report on the Fermi-LAT observations of the Geminga pulsar, the second brightest non-variable GeV source in the gamma-ray sky and the first example of a radio-quiet gamma-ray pulsar. The observations cover one year, from the launch of the Fermi satellite through 2009 June 15. A data sample of over 60,000 photons enabled us to build a timing solution based solely on gamma-rays. Timing analysis shows two prominent peaks, separated by Delta phi = 0.497 +/- 0.004 in phase, which narrow with increasing energy. Pulsed gamma-rays are observed beyond 18 GeV, precluding emission below 2.7 stellar radii because of magnetic absorption. The phase-averaged spectrum was fitted with a power law with exponential cutoff of spectral index Gamma = (1.30 +/- 0.01 +/- 0.04), cutoff energy E-0 = (2.46 +/- 0.04 +/- 0.17) GeV, and an integral photon flux above 0.1 GeV of (4.14 +/- 0.02 +/- 0.32) x 10(-6) cm(-2) s(-1). The first uncertainties are statistical and the second ones are systematic. The phase-resolved spectroscopy shows a clear evolution of the spectral parameters, with the spectral index reaching a minimum value just before the leading peak and the cutoff energy having maxima around the peaks. The phase-resolved spectroscopy reveals that pulsar emission is present at all rotational phases. The spectral shape, broad pulse profile, and maximum photon energy favor the outer magnetospheric emission scenarios.
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| 5. |
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| 6. |
- Abdo, A. A., et al.
(författare)
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A population of gamma-ray emitting globular clusters seen with the Fermi Large Area Telescope
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 524, s. A75-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Globular clusters with their large populations of millisecond pulsars (MSPs) are believed to be potential emitters of high-energy gamma-ray emission. The observation of this emission provides a powerful tool to assess the millisecond pulsar population of a cluster, is essential for understanding the importance of binary systems for the evolution of globular clusters, and provides complementary insights into magnetospheric emission processes. Aims. Our goal is to constrain the millisecond pulsar populations in globular clusters from analysis of gamma-ray observations. Methods. We use 546 days of continuous sky-survey observations obtained with the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope to study the gamma-ray emission towards 13 globular clusters. Results. Steady point-like high-energy gamma-ray emission has been significantly detected towards 8 globular clusters. Five of them (47 Tucanae, Omega Cen, NGC 6388, Terzan 5, and M 28) show hard spectral power indices (0.7 < Gamma < 1.4) and clear evidence for an exponential cut-off in the range 1.0-2.6 GeV, which is the characteristic signature of magnetospheric emission from MSPs. Three of them (M 62, NGC 6440 and NGC 6652) also show hard spectral indices (1.0 < Gamma < 1.7), however the presence of an exponential cut-off can not be unambiguously established. Three of them (Omega Cen, NGC 6388, NGC 6652) have no known radio or X-ray MSPs yet still exhibit MSP spectral properties. From the observed gamma-ray luminosities, we estimate the total number of MSPs that is expected to be present in these globular clusters. We show that our estimates of the MSP population correlate with the stellar encounter rate and we estimate 2600-4700 MSPs in Galactic globular clusters, commensurate with previous estimates. Conclusions. The observation of high-energy gamma-ray emission from globular clusters thus provides a reliable independent method to assess their millisecond pulsar populations.
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| 7. |
- Abdo, A. A., et al.
(författare)
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FERMI/LARGE AREA TELESCOPE DISCOVERY OF GAMMA-RAY EMISSION FROM THE FLAT-SPECTRUM RADIO QUASAR PKS 1454-354
- 2009
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 697:1, s. 934-941
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Tidskriftsartikel (refereegranskat)abstract
- We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope of high-energy gamma-ray (GeV) emission from the flat-spectrum radio quasar PKS 1454-354 (z = 1.424). On 2008 September 4, the source rose to a peak flux of (3.5 +/- 0.7) x 10(-6) ph cm(-2) s(-1) (E > 100 MeV) on a timescale of hours and then slowly dropped over the following 2 days. No significant spectral changes occurred during the flare. Fermi/LAT observations also showed that PKS 1454-354 is the most probable counterpart of the unidentified EGRET source 3EG J1500-3509. Multiwavelength measurements performed during the following days (7 September with Swift; 6-7 September with the ground-based optical telescope Automated Telescope for Optical Monitoring; 13 September with the Australia Telescope Compact Array) resulted in radio, optical, UV, and X-ray fluxes greater than archival data, confirming the activity of PKS 1454-354.
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| 8. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE VIEW OF THE CORE OF THE RADIO GALAXY CENTAURUS A
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 719:2, s. 1433-1444
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Tidskriftsartikel (refereegranskat)abstract
- We present gamma-ray observations with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope of the nearby radio galaxy Centaurus A (Cen A). The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of Cen A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum (Gamma = 2.67 +/- 0.10(stat) +/- 0.08(sys) where the photon flux is Phi alpha E-Gamma). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004 to 2008. The fit requires a low Doppler factor, in contrast to BL Lac objects which generally require larger values to fit their broadband SEDs. This indicates that the gamma-ray emission originates from a slower region than that from BL Lac objects, consistent with previous modeling results from Cen A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow. The fit parameters are also consistent with Cen A being able to accelerate ultra-high energy cosmic-rays, as hinted at by results from the Auger observatory.
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| 9. |
- Abdo, A. A., et al.
(författare)
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FERMI OBSERVATIONS OF HIGH-ENERGY GAMMA-RAY EMISSION FROM GRB 080825C
- 2009
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 707:1, s. 580-592
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Tidskriftsartikel (refereegranskat)abstract
- The Fermi Gamma-ray Space Telescope has opened a new high-energy window in the study of gamma-ray bursts (GRBs). Here we present a thorough analysis of GRB 080825C, which triggered the Fermi Gamma-ray Burst Monitor (GBM), and was the first firm detection of a GRB by the Fermi Large Area Telescope (LAT). We discuss the LAT event selections, background estimation, significance calculations, and localization for Fermi GRBs in general and GRB 080825C in particular. We show the results of temporal and time-resolved spectral analysis of the GBM and LAT data. We also present some theoretical interpretation of GRB 080825C observations as well as some common features observed in other LAT GRBs.
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| 10. |
- Abdo, A. A., et al.
(författare)
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Gamma-Ray Flares from the Crab Nebula
- 2011
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 331:6018, s. 739-742
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Tidskriftsartikel (refereegranskat)abstract
- A young and energetic pulsar powers the well-known Crab Nebula. Here, we describe two separate gamma-ray (photon energy greater than 100 mega-electron volts) flares from this source detected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. The first flare occurred in February 2009 and lasted approximately 16 days. The second flare was detected in September 2010 and lasted approximately 4 days. During these outbursts, the gamma-ray flux from the nebula increased by factors of four and six, respectively. The brevity of the flares implies that the gamma rays were emitted via synchrotron radiation from peta-electron-volt (10(15) electron volts) electrons in a region smaller than 1.4 x 10(-2) parsecs. These are the highest-energy particles that can be associated with a discrete astronomical source, and they pose challenges to particle acceleration theory.
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