The Compton Spectrometer and Imager (COSI) is a Compton telescope designed to survey the 0.2–5 MeV sky, consisting of a compact array of cross-strip germanium detectors. It is planned to be launched in 2027 into an equatorial low-Earth (530 km) orbit with a prime mission duration of 2 yr. The observation of MeV gamma rays is dominated by background. Thus, background simulation and identification are crucial for predicting the sensitivities of instruments. In this work we perform Monte Carlo simulations of the background for the first 3 months in orbit, and we extrapolate the results to 2 yr in orbit in order to determine the buildup of the activation due to long-lived isotopes. The simulations account for the known background components and include time-dependent rate variations due to the geomagnetic cutoff and South Atlantic Anomaly (SAA) passages. In addition, they include detailed modeling of the delayed activation due to short- and long-lived isotopes. We determine the rates of events induced by the background that are reconstructed as Compton events in the simulated COSI data. We find that the extragalactic background photons dominate at low energies (<660 keV), while delayed activation from cosmic-ray primaries (proton/alpha) and albedo photons dominate at higher energies. As part of this work, a comparison at low latitude (∣ b ∣ ≤ 1°) between recent measurement of the SAA and the AP9/AE9 model has been made, showing an overestimation of the flux by a factor ∼9 by the model. The systematic uncertainties associated with these components are quantified.
This paper addresses the characterization of the subsonic flow in the 1.2 MW Inductively Coupled Plasma (ICP) wind tunnel at the von Karman Institute for Fluid Dynamics (VKI), targeting chamber pressures of 50 and 100 mbar, and input electric powers between 150 and 300 kW. Ultraviolet to near-infrared optical emission spectroscopy measurements of the free-jet flow are carried out with an updated experimental set-up, calibration procedure, and data processing, providing high-quality absolute spatially-resolved emission spectra. Emission measurements agree with thermochemical equilibrium predictions within a range of conditions, allowing to extract experimental maps of cold-wall heat flux and dynamic pressure against the inferred free-jet enthalpy. A detailed comparison with the characterization methodology traditionally employed is presented, highlighting the need for an improved modeling strategy. Using the measured free-jet temperature and dynamic pressure only, a forward procedure for the computation of the stagnation line flow is proposed. The latter agrees with intrusive heat flux measurements through a range of test conditions, and for values of the recombination coefficient of the reference copper probe commonly found in the literature. Results demonstrate that a consistent framework between numerical simulations and experimental data can be achieved, defining an improved method for the characterization of subsonic ICP jets.
VISTA (Volatile In-Situ Thermogravimeter Analyzer) is a QCM (Quartz Crystal Microbalance) based device designed to characterize the dust environment of the Didymos asteroid system by detecting the presence of dust particles with sizes smaller than 5-10 μ m and sub- μ m, as well as volatiles and light organics, within the framework of the European Space Agency (ESA) Hera Mission. Thanks to its customized subsystems design, VISTA is capable of monitoring deposition and desorption/sublimation processes in the space environment, as well as molecular contamination (in support to other instruments) originating from outgassing sources during different mission phases, and of performing Thermo-Gravimetric Analysis (TGA) on the collected materials. The VISTA Model development for Hera started in 2020 and continued through 2023, and included an Engineering Qualification Model 0 (EM0), an Engineering Qualification Model (EQM), a Flight Model (FM) and a Flight Spare (FS). The EM0 was electrically and mechanically representative of VISTA, while the EQM, FM and FS share the same mechanical structure and electrical connections. The prototype, as well as the Engineering, Flight and Spare Models, were delivered to the prime contractor, Tyvak International. VISTA Models are composed by three different subsystems: two quartz crystals mounted in a sandwich-like configuration; the Proximity Electronics; and the Thermal Control System, which includes two integrated and customized heaters and a Thermo-Electric Cooler to cool the sensor and facilitate dust and volatiles deposition. The VISTA EQM, FM and FS successfully passed the Qualification and Acceptance Test Campaigns. The main results obtained from simulations of particles and volatiles deposition in a vacuum chamber, heating cycles, and Thermo-Gravimetric Analyses are reported in this work.
RCW 86 is a supernova remnant whose origin has recently been linked to an off-center explosion within a cavity created by its progenitor star. In the southwestern region, the forward shock is thought to have reached the cavity wall, encountering diverse environmental conditions. We report on the spatially resolved X-ray polarimetric observation of RCW 86 with the Imaging X-ray Polarimetry Explorer (IXPE). In the 2-4.5 keV energy band we find no significant detection of polarization. Employing a dedicated background subtraction procedure and Bayesian spectropolarimetric fitting, we derive 99% upper limits on the polarization degree of the synchrotron component: 15% in higher-statistics regions and 30%-40% in lower-statistics regions. These upper limits on the polarization degree in several regions exclude the possibility of a strongly coherent magnetic field down to the subparsec scale, and that of a moderately coherent one on the scale of the synchrotron features as resolved by IXPE. The results indicate that the shocks in the southwestern rim of RCW 86 propagate more slowly than the unshocked ejecta at their locations, yet exceed the measured proper motion speeds. This behavior is consistent with reflected shocks occurring in tenuous regions of the shocked ejecta, distinct from regions that are radio-bright.
We analyzed optical/near-IR Large Binocular Telescope spectra of 16 sources alerted by Gaia between 2021 and 2024 due to significant photometric variability. Half of the spectra were taken during quiescence and the rest during a burst or at intermediate brightness. Our analysis of their 10 yr light curves and photometric/spectroscopic features provides evidence that all 16 sources are accreting young stellar objects (YSOs). One object, Gaia23bab, is a known EXor source. Other light curves either have peaks over a stable baseline or significant variability throughout the entire observation period, suggesting multiple contributing processes. All spectra exhibit emission lines from accretion columns, and over half of them show atomic forbidden lines as signatures of outflowing gas. We determined stellar parameters, accretion luminosity (Lacc), and mass accretion rate ( Macc ) at different brightness phases. Only two sources showed variability primarily due to extinction. During quiescence, our sources exhibit Lacc and Macc values typical of T Tauri and Herbig Ae/Be (HAeBe) sources, supporting the hypothesis that any YSO may undergo episodic accretion. In bursts, the Lacc and Macc of sources with photometric variations exceeding 2 mag follow a shallower relation with stellar luminosity and mass, typical of known EXor sources. This group includes one Class I, one flat-spectrum, and two Class II sources. Notably, the other Class I source, Gaia24beh, shows an Lacc value about 10 times higher than typical EXor bursts of the same mass. In the other cases, Lacc and Macc align with variability seen in T Tauri and HAeBe sources.
