We present results based on 2 yr of intense Swift monitoring of three supergiant fast X-ray transients (SFXTs), IGR J16479−4514, XTE J1739−302 and IGR J17544−2619, which we started in 2007 October. ...Our out-of-outburst intensity-based X-ray (0.3-10 keV) spectroscopy yields absorbed power laws characterized by hard photon indices (Γ∼ 1 -2). The broad-band (0.3-150 keV) spectra of these sources, obtained while they were undergoing new outbursts observed during the second year of monitoring, can be fitted well with models typically used to describe the X-ray emission from accreting neutron stars in high-mass X-ray binaries. We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. By considering our monitoring as a casual sampling of the X-ray light curves, we can infer that the time these sources spend in bright outbursts is between 3 and 5 per cent of the total. The most probable X-ray flux for these sources is ∼(1 -2) × 10−11 erg cm−2 s−1 (2-10 keV, unabsorbed), corresponding to luminosities of the order of a few 1033 to a few 1034 erg s−1 (two orders of magnitude lower than the bright outbursts). In particular, the duty-cycle of inactivity is ∼19, 39 and 55 per cent (∼5 per cent uncertainty) for IGR J16479−4514, XTE J1739−302 and IGR J17544−2619, respectively. We present a complete list of BAT onboard detections, which further confirm the continued activity of these sources. This demonstrates that true quiescence is a rare state and that these transients accrete matter throughout their life at different rates. Variability in the X-ray flux is observed at all time-scales and intensity ranges we can probe. Superimposed on the day-to-day variability is intraday flaring, which involves flux variations up to one order of magnitude that can occur down to time-scales as short as ∼1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses M
cl∼ (0.3 -2) × 1019 g. Thanks to the Swift observations, the general picture we obtain is that, despite individual differences, common X-ray characteristics of this class are now well defined, such as outburst lengths well in excess of hours, with a multiple peaked structure, and a high dynamic range (including bright outbursts), up to approximately four orders of magnitude.
We have observed four low-luminosity active galactic nuclei (AGNs) classified as type 1 Low-Ionization Nuclear Emission-Line Regions (LINERs) with the X-Ray Telescope (XRT) and the ...Ultraviolet–Optical Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering mechanism of this class of nearby sources. Among our targets, we detect X-ray variability in NGC 3998 for the first time. The light curves of this object reveal variations of up to 30 per cent amplitude in half a day, with no significant spectral variability on this time-scale. We also observe a decrease of ∼30 per cent over 9 d, with significant spectral softening. Moreover, the X-ray flux is ∼40 per cent lower than observed in previous years. Variability is detected in M81 as well, at levels comparable to those reported previously: a flux increase in the hard X-rays (1–10 keV) of 30 per cent in ∼3 h and variations by up to a factor of 2 within a few years. This X-ray behaviour is similar to that of higher luminosity, Seyfert-type objects. Using previous high-angular-resolution imaging data from the Hubble Space Telescope (HST), we evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear flux in our UVOT observations. All sources are detected in the UV band, at levels similar to those of the previous observations with HST. The XRT (0.2–10 keV) spectra are well described by single power laws and the UV-to-X-ray flux ratios are again consistent with those of Seyferts and radio-loud AGNs of higher luminosity. The similarity in X-ray variability and broad-band energy distributions suggests the presence of similar accretion and radiation processes in low- and high-luminosity AGNs.
This paper presents a quantitative description of field emission in superconducting rf cavities. The goal is to fully reconstruct the physics involved in the phenomenon, from electron current ...generation at the emitter to the radiation pattern measured by the x-ray detector. The field emission process in the cavity was reconstructed using CST Particle Studio, and then the generation of x-ray radiation and its propagation toward the detectors were modeled analytically. The entire model was then applied to the specific case of the European Spallation Source medium beta superconducting cavities, and the theoretical predictions were cross-checked with experimental data collected during cavity vertical tests.
The Soft X-ray Imager (SXI) is part of the scientific payload of the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. SMILE is a joint science mission between the European Space ...Agency (ESA) and the Chinese Academy of Sciences (CAS) and is due for launch in 2025. SXI is a compact X-ray telescope with a wide field-of-view (FOV) capable of encompassing large portions of Earth’s magnetosphere from the vantage point of the SMILE orbit. SXI is sensitive to the soft X-rays produced by the Solar Wind Charge eXchange (SWCX) process produced when heavy ions of solar wind origin interact with neutral particles in Earth’s exosphere. SWCX provides a mechanism for boundary detection within the magnetosphere, such as the position of Earth’s magnetopause, because the solar wind heavy ions have a very low density in regions of closed magnetic field lines. The sensitivity of the SXI is such that it can potentially track movements of the magnetopause on timescales of a few minutes and the orbit of SMILE will enable such movements to be tracked for segments lasting many hours. SXI is led by the University of Leicester in the United Kingdom (UK) with collaborating organisations on hardware, software and science support within the UK, Europe, China and the United States.
The limits of performance of the European XFEL 3.9 GHz superconducting cavities were investigated. Most cavities exhibited high field Q slope, reaching the breakdown field at approximately22MV/m. We ...hypothesize that this limit is a feature of high frequency cavities and can be explained by a thermal model incorporating field dependent surface resistance. The results obtained from simulations were in good agreement with experimental data obtained at 2 K.
The advent of Swift has allowed, for the first time, the possibility to give supergiant fast X-ray transients (SFXTs), the new class of high-mass X-ray binaries discovered by the International ...Gamma-Ray Astrophysics Laboratory, non-serendipitous attention throughout most phases of their life. In this paper, we present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479−4514, XTE J1739−302, IGR J17544−2619 and AX J1841.0−0536. We obtain the first assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is ∼17, 28, 39 and 55 per cent (∼5 per cent uncertainty), for IGR J16479−4514, AX J1841.0−0536, XTE J1739–302 and IGR J17544−2619, respectively, so that true quiescence, which is below our detection ability even with the exposures we collected in 1 yr, is a rare state, when compared with estimates from less sensitive instruments. This demonstrates that these transients accrete matter throughout their lifetime at different rates. AX J1841.0−0536 is the only source which has not undergone a bright outburst during our monitoring campaign. Although individual sources behave somewhat differently, common X-ray characteristics of this class are emerging, such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of ∼4 orders of magnitude has been observed in IGR J17544−2619 and XTE J1739−302, of ∼3 in IGR J16479−4514 and of about 2 in AX J1841.0−0536 (this lowest range is due to the lack of bright flares). We also present a complete list of Burst Alert Telescope (BAT) on-board detections, which complements our previous work, and further confirms the continuous activity of these sources. We performed out-of-outburst intensity-based spectroscopy. In particular, spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred metres, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We used the whole BAT data set, since the beginning of the mission, to search for periodicities due to orbital motion and found Porb= 3.32 d for IGR J16479−4514, confirming previous findings. We also present the Ultraviolet/Optical Telescope (UVOT) data of these sources; we show the UVOT light curves of AX J1841.0−0536 and the ones of XTE J1739−302 before, during and after the outbursts.
Until recently, X-ray flares during the afterglow of gamma-ray bursts (GRBs) were a rarely detected phenomenon; thus, their nature is unclear. During the afterglow of GRB 050502B, the largest X-ray ...flare ever recorded rose rapidly above the afterglow light curve detected by the Swift X-Ray Telescope. The peak flux of the flare was >500 times that of the underlying afterglow, and it occurred >12 minutes after the nominal prompt burst emission. The fluence of this X-ray flare, (1.0 c 0.05) x 10 super(-6) ergs cm super(-2) in the 0.2-10.0 keV energy band, exceeded the fluence of the nominal prompt burst. The spectra during the flare were significantly harder than those measured before and after the flare. Later in time, there were additional flux increases detected above the underlying afterglow, as well as a break in the afterglow light curve. All evidence presented below, including spectral and, particularly, timing information during and around the giant flare, suggests that this giant flare was the result of internal dissipation of energy due to late central engine activity, rather than an afterglow-related effect. We also find that the data are consistent with a second central engine activity episode, in which the ejecta is moving slower than that of the initial episode, causing the giant flare and then proceeding to overtake and refresh the afterglow shock, thus causing additional activity at even later times in the light curve.
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