J. Chenevez

The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-Ray Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a National Aeronautics and Space Administration (NASA) Small Explorer mission that carried the first focusing hard X-ray (6-79 keV) telescope into orbit. It was launched on a Pegasus rocket into a low-inclination Earth orbit on June 13, 2012, from Reagan Test Site, Kwajalein Atoll. NuSTAR will carry out a two-year primary science mission. The NuSTAR observatory is composed of the X-ray instrument and the spacecraft. The NuSTAR spacecraft is three-axis stabilized with a single articulating solar array based on Orbital Sciences Corporations LEOStar-2 design. The NuSTAR science instrument consists of two co-aligned grazing incidence optics focusing on to two shielded solid state CdZnTe pixel detectors. The instrument was launched in a compact, stowed configuration, and after launch, a 10-meter mast was deployed to achieve a focal length of 10.15 m. The NuSTAR instrument provides sub-arcminute imaging with excellent spectral resolution over a 12-arcminute field of view. The NuSTAR observatory will be operated out of the Mission Operations Center (MOC) at UC Berkeley. Most science targets will be viewed for a week or more. The science data will be transferred from the UC Berkeley MOC to a Science Operations Center (SOC) located at the California Institute of Technology (Caltech). In this paper, we will describe the mission architecture, the technical challenges during the development phase, and the post-launch activities.

MAXI J1659-152: the shortest orbital period black-hole transient in outburst

MAXI J1659−152 is a bright X-ray transient black-hole candidate binary system discovered in September 2010. We report here on MAXI, RXTE, Swift, and XMM-Newton observations during its 2010/2011 outburst. We find that during the first one and a half week of the outburst the X-ray light curves display drops in intensity at regular intervals, which we interpret as absorption dips. About three weeks into the outbursts, again drops in intensity are seen. These dips have, however, a spectral behaviour opposite to that of the absorption dips, and are related to fast spectral state changes (hence referred to as transition dips). The absorption dips recur with a period of 2.414  ±  0.005 h, which we interpret as the orbital period of the system. This implies that MAXI J1659−152 is the shortest period black-hole candidate binary known to date. The inclination of the accretion disk with respect to the line of sight is estimated to be 65-80°. We propose the companion to the black-hole candidate to be close to an M5 dwarf star, with a mass and radius of about 0.15-0.25 M⊙ and 0.2-0.25 R⊙, respectively. We derive that the companion had an initial mass of about 1.5 M⊙, which evolved to its current mass in about 5-6 billion years. The system is rather compact (orbital separation of ≳1.33 R⊙), and is located at a distance of 8.6  ±  3.7 kpc, with a height above the Galactic plane of 2.4  ±  1.0 kpc. The characteristics of short orbital period and high Galactic scale height are shared with two other transient black-hole candidate X-ray binaries, i.e., XTE J1118+480 and Swift J1735.5−0127. We suggest that all three are kicked out of the Galactic plane into the halo, rather than being formed in a globular cluster.

Correlated optical, X-ray, and γ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni

After 25 years of quiescence, the microquasar V404 Cyg entered a new period of activity in June 2015. This X-ray source is known to undergo extremely bright and variable outbursts seen at all wavelengths. It is therefore an object of prime interest to understand the accretion-ejection connections. These can, however, only be probed through simultaneous observations at several wavelengths. We made use of the INTEGRAL instruments to obtain long, almost uninterrupted observations from 2015 June 20, 15:50 UTC to June 25, 4:05 UTC, from the optical V band up to the soft γ-rays. V404 Cyg was extremely variable in all bands, with the detection of 18 flares with fluxes exceeding 6 Crab (20–40 keV) within three days. The flare recurrence can be as short as ~20 min from peak to peak. A model-independent analysis shows that the >6 Crab flares have a hard spectrum. A simple 10–400 keV spectral analysis of the off-flare and flare periods shows that the variation in intensity is likely to be only due to variations of a cut-off power-law component. The optical flares seem to be at least of two different types: one occurring in simultaneity with the X-ray flares, the other showing a delay greater than 10 min. The former could be associated with X-ray reprocessing by either an accretion disk or the companion star. We suggest that the latter are associated with plasma ejections that have also been seen in radio.

Numerical models of magma chambers in the Oman ophiolite

Long standing studies conducted in the Oman ophiolite have led to models of fast oceanic spreading centers which in some aspects contradict the results of geophysical studies carried out along the East Pacific Rise (EPR); this is true in particular for the shape, size and internal magmatic flow of magma chambers, thus justifying numerical modeling. Our modeling, founded on the equations of fluid mechanics, is based on a two-dimensional finite difference method that takes into account high viscosity contrasts at the walls of the magma chamber, boundary conditions deduced from the geophysical data of the EPR, and geological observations from the Oman ophiolite. The model predicts a magma chamber shape consistent with field studies. Whether the magma feeding is from the top by the melt lens or from the bottom by basaltic sills, it has only a weak influence on the shape of the chamber. The viscosity of the magma plays a crucial role for the setting of convection in the chamber. According to geological observations which argue against the existence of convective flow patterns, the viscosity of the gabbroic mush within the chamber must exceed 1014–1015 Pa s, depending on the oceanic plate velocity. The flow pattern in the magma chamber and the associated foliation pattern rely critically on whether the underlying upwelling mantle is passive (foliations oriented inward) or active (foliations oriented outward). The active structural analysis in the Oman model is supported by ophiolite data.

Pollutant transport schemes integrated in a numerical weather prediction model: model description and verification results

Regional air pollution models are usually off-line coupled with numerical weather prediction models. The present study, however, focuses on on-line coupled modelling, for which the air pollution model is an integral part of the meteorological model. To this purpose, simulations of the first European Tracer Experiment (ETEX), which supplies the best suited verification data for dispersion modelling of a passive tracer, have been performed using versions of the Danish Meteorological Institute High Resolution Limited-Area Model (DMI-HIRLAM). Simulation results have been compared with observed values as well as with results from other models. The operational version of DMI-HIRLAM using well-known basic advection schemes has been tested, showing the need for improvements for air pollution purposes. The fourth-order Bott algorithm for advection has thus been implemented in the semi-Lagrangian version of DMI-HIRLAM. Due to its practically perfect mass conservation properties and its low computational cost, this method turns out to be efficient for simulations of transport of pollutants in the atmosphere. This represents a first effort towards a fully integrated air pollution model, e.g. for forecasting purposes. Copyright

Intermediate long X-ray bursts from the ultra-compact binary candidate SLX 1737-282

Aims. The low persistent flux X-ray burster source SLX 1737-282 is classified as an ultra-compact binary candidate. We compare the data on SLX 1737-282 with the other similar objects and attempt to derive constraints on the physical processes responsible for the formation of intermediate long bursts. Methods. Up to now only four bursts, all with duration between ≃ 15 − 30 minutes, have been recorded for SLX 1737282. The properties of three of these intermediate long X-ray bursts observed by INTEGRAL are investigated and compared to other burster sources. The broadband spectrum of the persistent emission in the 3–100 keV energy band is studied with the INTEGRAL data. Results. The persistent emission is measured to be 0.5% Eddington luminosity. From the photospheric radius expansion observed during at least one burst we derive the source distance at 7.3 kpc assuming a pure helium atmosphere. The observed intermediate long burst properties from SLX 1737-282 are consistent with helium ignition at the column depth of 5-8 ×10 9 g cm 2 and a burst energy release of ∼ 10 41 erg. The apparent recurrence time of ≃ 86 days between the intermediate long bursts from SLX 1737-282 suggests a regime of unstable burning of a thick, pure helium layer slowly accreted from a helium donor star.

THE SMOOTH CYCLOTRON LINE IN HER X-1 AS SEEN WITH NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY

Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for a detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35 day superorbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broadband spectra can be fitted with a power law with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape in all observations and at all pulse phases. We compare the residuals of a line with a Gaussian optical-depth profile to a Lorentzian optical-depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum only changes marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot corona of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date.

The INTEGRAL Galactic bulge monitoring program: the first 1.5 years

Aims.The Galactic bulge region is a rich host of variable high-energy point sources. Since 2005, February 17 we are monitoring the source activity in the Galactic bulge region regularly and frequently, i.e., about every three days, with the instruments onboard INTEGRAL. Thanks to the large field of view, the imaging capabilities and the sensitivity at hard X-rays, we are able to present for the first time a detailed homogeneous (hard) X-ray view of a sample of 76 sources in the Galactic bulge region. Methods: We describe the successful monitoring program and show the first results from the start of the monitoring up to 2006, April 21, i.e., for a period of about one and a half year, during three visibility seasons. We focus on the short (hour), medium (month) and long-term (year) variability in the hard X-ray bands, i.e., 20-60 keV and 60-150 keV. When available, we discuss the simultaneous observations in the soft X-ray, 3-10 keV and 10-25 keV, bands. Results: Almost all the sources in the Galactic bulge region we detect in the 20-60 keV and 60-150 keV bands are variable. During the last two and a half weeks of the third visibility season most of the known persistent (hard) X-ray sources in the Galactic Center region were not detected. Of our sample of sources, per visibility season we detect 32/33 sources in the 20-60 keV band and 8/9 sources in the 60-150 keV band above a signal to noise of 7. On average, we find per visibility season one active bright (âa†100 mCrab, 20-60 keV) black-hole candidate X-ray transient and three active weaker (âa‰25 mCrab, 20-60 keV) neutron star X-ray transients. Most of the time a clear anti-correlation can be seen between the soft and hard X-ray emission in some of the X-ray bursters. Hard X-ray flares or outbursts in X-ray bursters, which have a duration of the order of weeks are accompanied by soft X-ray drops. On the other hand, hard X-ray drops can be accompanied by soft X-ray flares/outbursts. During the course of our program we found a number of new sources, IGR J17354-3255, IGR 17453-2853, IGR J17454-2703, IGR J17456-2901b, IGR J17536-2339, and IGR J17541-2252. We report here on some of the high-energy properties of these sources. Conclusions: The high-energy light curves of all the sources in the field of view, and the high-energy images of the region, are made available through the WWW, as soon as possible after the observations have been performed, at http://isdc.unige.ch/Science/BULGE/. Appendices are only available in electronic form at http://www.aanda.org

Crustal feeding in the Oman ophiolite: From the top and from the bottom? A thermal and mass balance model

The stability of the basalt melt lens identified below fast spreading ridges has been tested by a thermal and mass balance model in order to investigate how the generation of the crust by basaltic melt can be affected by spreading rate. The mpdel is constrained using field information from the Oman ophiolite, the main piece of evidence dealing with the nature and structure of the root zone of sheeted dikes which acts as a thermal boundary layer between the melt lens below, and the hydrothermally cooled sheeted dike unit above. For a mass balance calculation we first assume that the gabbro unit is entirely created by the melt issued from the lens. We examine how the heat generated by crystallization of this mass can be evacuated through the boundary layer on top of the melt lens. A steady-state situation is found for a given spreading rate and ratio of the Nusselt number to the field-deduced thickness of the boundary layer. If, as suggested by geophysical data, the melt lens exists only for spreading rates larger than 6 cm/yr, the boundary layer needs to be essentially conductive. With increasing spreading rates and increasing heat supply the system thermally adjusts itself until it must work in a new mode. Geological evidence suggests that this could be achieved by developing an additional feeding of the gabbro unit by a large number of sills being emplaced below the melt lens into the lower gabbros.

Two-phase X-ray burst from GX 3+1 observed by INTEGRAL

INTEGRAL detected on August 31, 2004, an unusual thermonuclear X-ray burst from the low-mass X-ray binary GX 3+1. Its duration was 30 min, which is between the normal burst durations for this source (