| 11. |
- Abdo, A. A., et al.
(författare)
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The second Fermi large area telescope catalog of gamma-ray pulsars
- 2013
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 208:2, s. 17-
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Tidskriftsartikel (refereegranskat)abstract
- This catalog summarizes 117 high-confidence ≥0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on the Fermi satellite. Half are neutron stars discovered using LAT data through periodicity searches in gamma-ray and radio data around LAT unassociated source positions. The 117 pulsars are evenly divided into three groups: millisecond pulsars, young radio-loud pulsars, and young radio-quiet pulsars. We characterize the pulse profiles and energy spectra and derive luminosities when distance information exists. Spectral analysis of the off-peak phase intervals indicates probable pulsar wind nebula emission for four pulsars, and off-peak magnetospheric emission for several young and millisecond pulsars. We compare the gamma-ray properties with those in the radio, optical, and X-ray bands. We provide flux limits for pulsars with no observed gamma-ray emission, highlighting a small number of gamma-faint, radio-loud pulsars. The large, varied gamma-ray pulsar sample constrains emission models. Fermi's selection biases complement those of radio surveys, enhancing comparisons with predicted population distributions.
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| 12. |
- Abdo, A. A., et al.
(författare)
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DISCOVERY OF HIGH-ENERGY GAMMA-RAY EMISSION FROM THE BINARY SYSTEM PSR B1259-63/LS 2883 AROUND PERIASTRON WITH FERMI
- 2011
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Ingår i: Astrophysical Journal Letters. - 2041-8205. ; 736:1, s. L11-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the discovery of >= 100 MeV gamma-rays from the binary system PSR B1259-63/LS 2883 using the Large Area Telescope (LAT) on board Fermi. The system comprises a radio pulsar in orbit around a Be star. We report on LAT observations from near apastron to similar to 128 days after the time of periastron, t(p), on 2010 December 15. No gamma-ray emission was detected from this source when it was far from periastron. Faint gamma-ray emission appeared as the pulsar approached periastron. At similar to t(p) + 30 days, the >= 100 MeV gamma-ray flux increased over a period of a few days to a peak flux 20-30 times that seen during the pre-periastron period, but with a softer spectrum. For the following month, it was seen to be variable on daily timescales, but remained at similar to(1-4) x 10(-6) cm(-2) s(-1) before starting to fade at similar to t(p) + 57 days. The total gamma-ray luminosity observed during this period is comparable to the spin-down power of the pulsar. Simultaneous radio and X-ray observations of the source showed no corresponding dramatic changes in radio and X-ray flux between the pre-periastron and post-periastron flares. We discuss possible explanations for the observed gamma-ray-only flaring of the source.
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| 13. |
- Abdo, A. A., et al.
(författare)
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The first fermi large area telescope catalog of gamma-ray pulsars
- 2010
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 187:2, s. 460-494
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Tidskriftsartikel (refereegranskat)abstract
- The dramatic increase in the number of known gamma-ray pulsars since the launch of the Fermi Gamma-ray Space Telescope (formerly GLAST) offers the first opportunity to study a sizable population of these high-energy objects. This catalog summarizes 46 high-confidence pulsed detections using the first six months of data taken by the Large Area Telescope (LAT), Fermi's main instrument. Sixteen previously unknown pulsars were discovered by searching for pulsed signals at the positions of bright gamma-ray sources seen with the LAT, or at the positions of objects suspected to be neutron stars based on observations at other wavelengths. The dimmest observed flux among these gamma-ray-selected pulsars is 6.0 x 10(-8) ph cm(-2) s(-1) (for E > 100 MeV). Pulsed gamma-ray emission was discovered from 24 known pulsars by using ephemerides (timing solutions) derived from monitoring radio pulsars. Eight of these new gamma-ray pulsars are millisecond pulsars. The dimmest observed flux among the radio-selected pulsars is 1.4 x 10(-8) ph cm(-2) s(-1) (for E > 100 MeV). The remaining six gamma-ray pulsars were known since the Compton Gamma Ray Observatory mission, or before. The limiting flux for pulse detection is non-uniform over the sky owing to different background levels, especially near the Galactic plane. The pulsed energy spectra can be described by a power law with an exponential cutoff, with cutoff energies in the range similar to 1-5 GeV. The rotational energy-loss rate ((E) over dot) of these neutron stars spans five decades, from similar to 3 x 10(33) erg s(-1) to 5 x 10(38) erg s(-1), and the apparent efficiencies for conversion to gammaray emission range from similar to 0.1% to similar to unity, although distance uncertainties complicate efficiency estimates. The pulse shapes show substantial diversity, but roughly 75% of the gamma-ray pulse profiles have two peaks, separated by greater than or similar to 0.2 of rotational phase. For most of the pulsars, gamma-ray emission appears to come mainly from the outer magnetosphere, while polar-cap emission remains plausible for a remaining few. Spatial associations imply that many of these pulsars power pulsar wind nebulae. Finally, these discoveries suggest that gamma-ray-selected young pulsars are born at a rate comparable to that of their radio-selected cousins and that the birthrate of all young gamma-ray-detected pulsars is a substantial fraction of the expected Galactic supernova rate.
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| 14. |
- Tinetti, Giovanna, et al.
(författare)
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The EChO science case
- 2015
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 40:2-3, s. 329-391
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Tidskriftsartikel (refereegranskat)abstract
- The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet population. We see gas giants in few-day orbits, whole multi-planet systems within the orbit of Mercury, and new populations of planets with masses between that of the Earth and Neptune-all unknown in the Solar System. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work and what causes the exceptional diversity observed as compared to the Solar System? The EChO (Exoplanet Characterisation Observatory) space mission was conceived to take up the challenge to explain this diversity in terms of formation, evolution, internal structure and planet and atmospheric composition. This requires in-depth spectroscopic knowledge of the atmospheres of a large and well-defined planet sample for which precise physical, chemical and dynamical information can be obtained. In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample within its 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allows us to measure atmospheric signals from the planet at levels of at least 10(-4) relative to the star. This can only be achieved in conjunction with a carefully designed stable payload and satellite platform. It is also necessary to provide broad instantaneous wavelength coverage to detect as many molecular species as possible, to probe the thermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellar photosphere. This requires wavelength coverage of at least 0.55 to 11 mu m with a goal of covering from 0.4 to 16 mu m. Only modest spectral resolving power is needed, with R similar to 300 for wavelengths less than 5 mu m and R similar to 30 for wavelengths greater than this. The transit spectroscopy technique means that no spatial resolution is required. A telescope collecting area of about 1 m(2) is sufficiently large to achieve the necessary spectro-photometric precision: for the Phase A study a 1.13 m(2) telescope, diffraction limited at 3 mu m has been adopted. Placing the satellite at L2 provides a cold and stable thermal environment as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. EChO has been conceived to achieve a single goal: exoplanet spectroscopy. The spectral coverage and signal-to-noise to be achieved by EChO, thanks to its high stability and dedicated design, would be a game changer by allowing atmospheric composition to be measured with unparalleled exactness: at least a factor 10 more precise and a factor 10 to 1000 more accurate than current observations. This would enable the detection of molecular abundances three orders of magnitude lower than currently possible and a fourfold increase from the handful of molecules detected to date. Combining these data with estimates of planetary bulk compositions from accurate measurements of their radii and masses would allow degeneracies associated with planetary interior modelling to be broken, giving unique insight into the interior structure and elemental abundances of these alien worlds. EChO would allow scientists to study exoplanets both as a population and as individuals. The mission can target super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300-3000 K) of F to M-type host stars. The EChO core science would be delivered by a three-tier survey. The EChO Chemical Census: This is a broad survey of a few-hundred exoplanets, which allows us to explore the spectroscopic and chemical diversity of the exoplanet population as a whole. The EChO Origin: This is a deep survey of a subsample of tens of exoplanets for which significantly higher signal to noise and spectral resolution spectra can be obtained to explain the origin of the exoplanet diversity (such as formation mechanisms, chemical processes, atmospheric escape). The EChO Rosetta Stones: This is an ultra-high accuracy survey targeting a subsample of select exoplanets. These will be the bright "benchmark" cases for which a large number of measurements would be taken to explore temporal variations, and to obtain two and three dimensional spatial information on the atmospheric conditions through eclipse-mapping techniques. If EChO were launched today, the exoplanets currently observed are sufficient to provide a large and diverse sample. The Chemical Census survey would consist of > 160 exoplanets with a range of planetary sizes, temperatures, orbital parameters and stellar host properties. Additionally, over the next 10 years, several new ground- and space-based transit photometric surveys and missions will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets.
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| 15. |
- Freire, P. C. C., et al.
(författare)
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Fermi Detection of a Luminous gamma-Ray Pulsar in a Globular Cluster
- 2011
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 334:6059, s. 1107-1110
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Tidskriftsartikel (refereegranskat)abstract
- We report on the Fermi Large Area Telescope's detection of gamma-ray (>100 mega-electron volts) pulsations from pulsar J1823-3021A in the globular cluster NGC 6624 with high significance (similar to 7 sigma). Its gamma-ray luminosity, L(gamma) = (8.4 +/- 1.6) x 10(34) ergs per second, is the highest observed for any millisecond pulsar (MSP) to date, and it accounts for most of the cluster emission. The nondetection of the cluster in the off-pulse phase implies that it contains <32 gamma-ray MSPs, not similar to 100 as previously estimated. The gamma-ray luminosity indicates that the unusually large rate of change of its period is caused by its intrinsic spin-down. This implies that J1823-3021A has the largest magnetic field and is the youngest MSP ever detected and that such anomalous objects might be forming at rates comparable to those of the more normal MSPs.
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| 16. |
- Weigelt, G., et al.
(författare)
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VLTI-MATISSE chromatic aperture-synthesis imaging of eta Carinae's stellar wind across the Br alpha line Periastron passage observations in February 2020
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652
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Tidskriftsartikel (refereegranskat)abstract
- Context. Eta Carinae is a highly eccentric, massive binary system (semimajor axis similar to 15.5 au) with powerful stellar winds and a phase-dependent wind-wind collision (WWC) zone. The primary star, eta Car A, is a luminous blue variable (LBV); the secondary, eta Car B, is a Wolf-Rayet or O star with a faster but less dense wind. Aperture-synthesis imaging allows us to study the mass loss from the enigmatic LBV eta Car. Understanding LBVs is a crucial step toward improving our knowledge about massive stars and their evolution. Aims. Our aim is to study the intensity distribution and kinematics of eta Car's WWC zone. Methods. Using the VLTI-MATISSE mid-infrared interferometry instrument, we perform Br alpha imaging of eta Car's distorted wind. Results. We present the first VLTI-MATISSE aperture-synthesis images of eta Car A's stellar windin several spectral channels distributed across the Br alpha 4.052 mu m line (spectral resolving power R similar to 960). Our observations were performed close to periastron passage in February 2020 (orbital phase similar to 14.0022). The reconstructed iso-velocity images show the dependence of the primary stellar wind on wavelength or line-of-sight (LOS) velocity with a spatial resolution of 6 mas (similar to 14 au). The radius of the faintest outer wind regions is similar to 26 mas (similar to 60 au). At several negative LOS velocities, the primary stellar wind is less extended to the northwest than in other directions. This asymmetry is most likely caused by the WWC. Therefore, we see both the velocity field of the undisturbed primary wind and the WWC cavity. In continuum spectral channels, the primary star wind is more compact than in line channels. A fit of the observed continuum visibilities with the visibilities of a stellar wind CMFGEN model (CMFGEN is an atmosphere code developed to model the spectra of a variety of objects) provides a full width at half maximum fit diameter of the primary stellar wind of 2.84 +/- 0.06 mas (6.54 +/- 0.14 au). We comparethe derived intensity distributions with the CMFGEN stellar wind model and hydrodynamic WWC models.
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| 17. |
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| 18. |
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| 19. |
- Arndt, D. S., et al.
(författare)
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STATE OF THE CLIMATE IN 2017
- 2018
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Ingår i: Bulletin of The American Meteorological Society - (BAMS). - : American Meteorological Society. - 0003-0007 .- 1520-0477. ; 99:8, s. S1-S310
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Forskningsöversikt (refereegranskat)
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| 20. |
- Arndt, D. S., et al.
(författare)
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State of the Climate in 2016
- 2017
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Ingår i: Bulletin of The American Meteorological Society - (BAMS). - 0003-0007 .- 1520-0477. ; 98:8, s. S1-S280
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Tidskriftsartikel (refereegranskat)abstract
- In 2016, the dominant greenhouse gases released into Earth's atmosphere-carbon dioxide, methane, and nitrous oxide-continued to increase and reach new record highs. The 3.5 +/- 0.1 ppm rise in global annual mean carbon dioxide from 2015 to 2016 was the largest annual increase observed in the 58-year measurement record. The annual global average carbon dioxide concentration at Earth's surface surpassed 400 ppm (402.9 +/- 0.1 ppm) for the first time in the modern atmospheric measurement record and in ice core records dating back as far as 800000 years. One of the strongest El Nino events since at least 1950 dissipated in spring, and a weak La Nina evolved later in the year. Owing at least in part to the combination of El Nino conditions early in the year and a long-term upward trend, Earth's surface observed record warmth for a third consecutive year, albeit by a much slimmer margin than by which that record was set in 2015. Above Earth's surface, the annual lower troposphere temperature was record high according to all datasets analyzed, while the lower stratospheric temperature was record low according to most of the in situ and satellite datasets. Several countries, including Mexico and India, reported record high annual temperatures while many others observed near-record highs. A week-long heat wave at the end of April over the northern and eastern Indian peninsula, with temperatures surpassing 44 degrees C, contributed to a water crisis for 330 million people and to 300 fatalities. In the Arctic the 2016 land surface temperature was 2.0 degrees C above the 1981-2010 average, breaking the previous record of 2007, 2011, and 2015 by 0.8 degrees C, representing a 3.5 degrees C increase since the record began in 1900. The increasing temperatures have led to decreasing Arctic sea ice extent and thickness. On 24 March, the sea ice extent at the end of the growth season saw its lowest maximum in the 37-year satellite record, tying with 2015 at 7.2% below the 1981-2010 average. The September 2016 Arctic sea ice minimum extent tied with 2007 for the second lowest value on record, 33% lower than the 1981-2010 average. Arctic sea ice cover remains relatively young and thin, making it vulnerable to continued extensive melt. The mass of the Greenland Ice Sheet, which has the capacity to contribute similar to 7 m to sea level rise, reached a record low value. The onset of its surface melt was the second earliest, after 2012, in the 37-year satellite record. Sea surface temperature was record high at the global scale, surpassing the previous record of 2015 by about 0.01 degrees C. The global sea surface temperature trend for the 21st century-to-date of +0.162 degrees C decade(-1) is much higher than the longer term 1950-2016 trend of +0.100 degrees C decade(-1). Global annual mean sea level also reached a new record high, marking the sixth consecutive year of increase. Global annual ocean heat content saw a slight drop compared to the record high in 2015. Alpine glacier retreat continued around the globe, and preliminary data indicate that 2016 is the 37th consecutive year of negative annual mass balance. Across the Northern Hemisphere, snow cover for each month from February to June was among its four least extensive in the 47-year satellite record. Continuing a pattern below the surface, record high temperatures at 20-m depth were measured at all permafrost observatories on the North Slope of Alaska and at the Canadian observatory on northernmost Ellesmere Island. In the Antarctic, record low monthly surface pressures were broken at many stations, with the southern annular mode setting record high index values in March and June. Monthly high surface pressure records for August and November were set at several stations. During this period, record low daily and monthly sea ice extents were observed, with the November mean sea ice extent more than 5 standard deviations below the 1981-2010 average. These record low sea ice values contrast sharply with the record high values observed during 2012-14. Over the region, springtime Antarctic stratospheric ozone depletion was less severe relative to the 1991-2006 average, but ozone levels were still low compared to pre-1990 levels. Closer to the equator, 93 named tropical storms were observed during 2016, above the 1981-2010 average of 82, but fewer than the 101 storms recorded in 2015. Three basins-the North Atlantic, and eastern and western North Pacific-experienced above-normal activity in 2016. The Australian basin recorded its least active season since the beginning of the satellite era in 1970. Overall, four tropical cyclones reached the Saffir-Simpson category 5 intensity level. The strong El Nino at the beginning of the year that transitioned to a weak La Nina contributed to enhanced precipitation variability around the world. Wet conditions were observed throughout the year across southern South America, causing repeated heavy flooding in Argentina, Paraguay, and Uruguay. Wetter-than-usual conditions were also observed for eastern Europe and central Asia, alleviating the drought conditions of 2014 and 2015 in southern Russia. In the United States, California had its first wetter-than-average year since 2012, after being plagued by drought for several years. Even so, the area covered by drought in 2016 at the global scale was among the largest in the post-1950 record. For each month, at least 12% of land surfaces experienced severe drought conditions or worse, the longest such stretch in the record. In northeastern Brazil, drought conditions were observed for the fifth consecutive year, making this the longest drought on record in the region. Dry conditions were also observed in western Bolivia and Peru; it was Bolivia's worst drought in the past 25 years. In May, with abnormally warm and dry conditions already prevailing over western Canada for about a year, the human-induced Fort McMurray wildfire burned nearly 590000 hectares and became the costliest disaster in Canadian history, with $3 billion (U.S. dollars) in insured losses.
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