Context. The high sensitivity of the XMM-Newton instrumentation offers the opportunity to study faint and extended sources in the Milky Way and nearby galaxies such as the Large Magellanic Cloud ...(LMC) in detail. The ROSAT PSPC survey of the LMC has revealed more than 700 X-ray sources, among which there are 46 supernova remnants (SNRs) and candidates. Aims. We have observed the field around one of the most promising SNR candidates in the ROSAT PSPC catalogue, labelled HP99 456 with XMM-Newton, to determine its nature. Methods. We investigated the XMM-Newton data along with new radio-continuum, near infrared and optical data. In particular, spectral and morphological studies of the X-ray and radio data were performed. Results. The X-ray images obtained in different energy bands reveal two different structures. Below 1.0 keV the X-ray emission shows the shell-like morphology of an SNR with a diameter of ~73 pc, one of the largest known in the LMC. For its thermal spectrum we estimate an electron temperature of (0.49 ± 0.12) keV assuming non-equilibrium ionisation. The X-ray images above 1.0 keV reveal a less extended source within the SNR emission, located 1′ west of the centre of the SNR and coincident with bright point sources detected in radio-continuum. This hard component has an extent of 0.9′ (i.e. ~13 pc at a distance of ~50 kpc) and a non-thermal spectrum. The hard source coincides in position with the ROSAT source HP99 456 and shows an indication for substructure. Conclusions. We firmly identify a new SNR in the LMC with a shell-like morphology and a thermal spectrum. Assuming the SNR to be in the Sedov phase yields an age of ~23 kyr. We explore possible associations of the hard non-thermal emitting component with a pulsar wind nebula (PWN) or background active galactic nucleus (AGN).
Pulsars are rapidly spinning, highly magnetized neutron stars, created in the gravitational collapse of massive stars. We report the detection of pulsed giga–electron volt gamma rays from the young ...pulsar PSR J0540–6919 in the Large Magellanic Cloud, a satellite galaxy of the Milky Way. This is the first gamma-ray pulsar detected in another galaxy. It has the most luminous pulsed gamma-ray emission yet observed, exceeding the Crab pulsar's by a factor of 20. PSR J0540–6919 presents an extreme test case for understanding the structure and evolution of neutron star magnetospheres.
Context. Since the launch of the Fermi satellite, the number of known gamma-ray pulsars has increased tenfold. Most gamma-ray detected pulsars are young and energetic, and many are associated with ...TeV sources. PSR J1357−6429 is a high spin-down power pulsar (Ė = 3.1 × 1036 erg s-1), discovered during the Parkes multibeam survey of the Galactic plane, with significant timing noise typical of very young pulsars. In the very-high-energy domain (E > 100 GeV), H.E.S.S. has reported the detection of the extended source HESS J1356−645 (intrinsic Gaussian width of 12′) whose centroid lies 7′ from PSR J1357−6429. Aims. We search for gamma- and X-ray pulsations from this pulsar, characterize the neutron star emission and explore the environment of PSR J1357−6429. Methods. Using a rotational ephemeris obtained with 74 observations made with the Parkes telescope at 1.4 GHz, we phase-fold more than two years of gamma-ray data acquired by the Large Area Telescope on-board Fermi as well as those collected with XMM-Newton, and perform gamma-ray spectral modeling. Results. Significant gamma- and X-ray pulsations are detected from PSR J1357−6429. The light curve in both bands shows one broad peak. Gamma-ray spectral analysis of the pulsed emission suggests that it is well described by a simple power-law of index 1.5 ± 0.3stat ± 0.3syst with an exponential cut-off at 0.8 ± 0.3stat ± 0.3syst GeV and an integral photon flux above 100 MeV of (6.5 ± 1.6stat ± 2.3syst) × 10-8 cm-2 s-1. The X-ray spectra obtained from the new data provide results consistent with previous work. Upper limits on the gamma-ray emission from its potential pulsar wind nebula (PWN) are also reported. Conclusions. Assuming a distance of 2.4 kpc, the Fermi LAT energy flux yields a gamma-ray luminosity for PSR J1357−6429 of Lγ = (2.13 ± 0.25stat ± 0.83syst) × 1034 erg s-1, consistent with an \hbox{$L_\gamma \propto \sqrt{\dot E}$}Lγ∝Ė relationship. The Fermi non-detection of the pulsar wind nebula associated with HESS J1356−645 provides new constraints on the electron population responsible for the extended TeV emission.
We announce the discovery of 1-100 GeV gamma-ray emission from the archetypal TeV pulsar wind nebula (PWN) HESS J1825--137 using 20 months of survey data from the Fermi-Large Area Telescope (LAT). ...The gamma-ray emission detected by the LAT is significantly spatially extended, with a best-fit rms extension of Delta *s = 056 ? 007 for an assumed Gaussian model. The 1-100 GeV LAT spectrum of this source is well described by a power law with a spectral index of 1.38 ? 0.12 ? 0.16 and an integral flux above 1 GeV of (6.50 ? 0.21 ? 3.90) X 10--9 cm--2 s--1. The first errors represent the statistical errors on the fit parameters, while the second ones are the systematic uncertainties. Detailed morphological and spectral analyses bring new constraints on the energetics and magnetic field of the PWN system. The spatial extent and hard spectrum of the GeV emission are consistent with the picture of an inverse Compton origin of the GeV-TeV emission in a cooling-limited nebula powered by the pulsar PSR J1826--1334.
We present the outcome of a mission concept study that designed a small atmospheric entry probe and examined the feasibility and benefit of a future multi-probe mission to Uranus. We call our design ...the Small Next-generation Atmospheric Probe (SNAP). The primary scientific objective of a multi-probe mission is to reveal spatial variability of atmospheric conditions. This article first highlights that not all measurements must be repeated by multiple probes; some quantities, notably the noble gas abundances and elemental isotopic ratios, are not expected to be variable, and thus need to be performed only by a single large Primary Probe. Our study demonstrates that, by focusing its measurements on spatially variable quantities including atmospheric vapor concentrations, thermal stratification and wind speed, a viable atmospheric probe design is realized with an entry system with 50-cm heatshield diameter and 30-kg atmospheric entry mass.
As a case study, we present a detailed analysis of adding SNAP to a notional Uranus Orbiter with Probe mission, which launches in 2031 and arrives at Uranus in 2043, designed by the NASA-funded Science Definition Team study in 2017. We demonstrate that, with minimal changes to the notional carrier mission, a large Primary Probe and SNAP can be delivered to the winter and summer hemispheres to examine seasonal atmospheric variabilities, and transmit data to the Orbiter, which in turn relays the data to Earth. The additional maneuvers needed to deliver SNAP totals a Delta-V of 84 m/s, and consumes 43 kg of propellant. The addition of SNAP is expected to cost $79.5 million in FY2018 dollars; thus, our study demonstrates that a multi-probe mission can be implemented with a 4% cost increase relative to the $2.0 billion cost estimate of the notional mission designed by NASA’s Ice Giant Flagship Science Definition Team study reported in 2017.
The SNAP design incorporates several technologies that are currently under development at various Technology Readiness Levels (TRL) between TRL = 4 and TRL = 6. In particular, our study recommends targeted technology development in Thermal Protection System materials, advanced batteries, and miniaturized instruments to enable and enhance future small atmospheric probes like SNAP.
We report on the Fermi Large Area Telescope's detection of γ-ray (> 100 mega-electron volts) pulsations from pulsar J1823-3021A in the globular cluster NGC 6624 with high significance (-~7σ). Its ...γ-ray luminosity, Lγ = (8.4 ± 1.6) ÷ 10³₄ 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 γ-ray MSPs, not -100 as previously estimated. The γ-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.
In this paper, we present the Fermi All-sky Variability Analysis (FAVA), a tool to systematically study the variability of the gamma-ray sky measured by the Large Area Telescope on board the Fermi ...Gamma-ray Space Telescope.For each direction on the sky, FAVA compares the number of gamma-rays observed in a given time window to the number of gamma-rays expected for the average emission detected from that direction. This method is used in weekly time intervals to derive a list of 215 flaring gamma-ray sources. We proceed to discuss the 27 sources found at Galactic latitudes smaller than 10 and show that, despite their low latitudes, most of them are likely of extragalactic origin.
We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi ...GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy-loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process.
We present the fourth Fermi Large Area Telescope catalog (4FGL) of γ-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range ...from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse γ-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4 significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.
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