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Scientific payloads onboard CubeSats usually have complex geometries and occasionally narrower allowed temperature ranges with respect to the rest of the spacecraft. In these cases, the capability to correctly predict the thermal behaviour of the payload once in orbit is mandatory. To achieve this ability, a thermal balance test is required to correctly identify the thermal model of the payload. The test consists in the application of different external thermal boundary conditions together with the addition of heat dissipation to simulate the thermal load produced by active electronics during operation. Those experimental data are fundamental to validate the numerical thermal model and make its predictions reliable. This paper presents the configuration and procedures of the thermal balance test performed on the Demonstration Model of the payload to be embarked on each satellite of the HERMES constellation. The test data is compared with the results of a finite volume thermal model of the payload, proving the application of this method to be reliable for space thermal analyses. The obtained test results show the temperature jumps caused by the heat dissipation applied to active components. A weak correlation between the payload interface and internal equipment has been observed, thus proving that the payload is almost decoupled from the Service Module temperature variations. Based on test outcomes, some modifications in the payload design have been implemented, with the aim to lower the operative temperature on critical, temperature-sensitive equipment.

Pulsar wind nebulae are formed when outflows of relativistic electrons and positrons hit the surrounding supernova remnant or interstellar medium at a shock front. The Vela pulsar wind nebula is powered by a young pulsar (B0833-45, aged 11,000 years) 1 and located inside an extended structure called Vela X, which is itself inside the supernova remnant 2 . Previous X-ray observations revealed two prominent arcs that are bisected by a jet and counter jet 3 , 4 . Radio maps have shown high linear polarization of 60% in the outer regions of the nebula 5 . Here we report an X-ray observation of the inner part of the nebula, where polarization can exceed 60% at the leading edge—approaching the theoretical limit of what can be produced by synchrotron emission. We infer that, in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field. Polarization can exceed 60% at the leading edge of the inner part of the Vela pulsar wind nebula; in contrast with the case of the supernova remnant, the electrons in the pulsar wind nebula are accelerated with little or no turbulence in a highly uniform magnetic field.

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4 million (Sagittarius A * (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei 1 . Reflection of X-rays from Sgr A * by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years 2 . The shape of the X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A * are consistent with the reflection scenario 3 – 5 . If this interpretation is correct, the reflected continuum emission should be polarized 6 . Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of −48° ± 11°. The polarization angle is consistent with Sgr A * being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A * was briefly comparable to that of a Seyfert galaxy. A study reports the measurement of the polarization degree and angle of X-rays from Sagittarius A * reflected off a nearby cloud, indicating an X-ray flare about 200 years ago.

Using observations of X-ray pulsar Hercules X-1 by the Imaging X-ray Polarimetry Explorer we report a highly significant (>17σ) detection of the polarization signal from an accreting neutron star. The observed degree of linear polarization of ~10% is far below theoretical expectations for this object, and stays low throughout the spin cycle of the pulsar. Both the degree and angle of polarization exhibit variability with the pulse phase, allowing us to measure the pulsar spin position angle 57(2) deg and the magnetic obliquity 12(4) deg, which is an essential step towards detailed modelling of the intrinsic emission of X-ray pulsars. Combining our results with the optical polarimetric data, we find that the spin axis of the neutron star and the angular momentum of the binary orbit are misaligned by at least ~20 deg, which is a strong argument in support of the models explaining the stability of the observed superorbital variability with the precession of the neutron star.

The Crab pulsar and its nebula are among the most studied astrophysical systems, and constitute one of the most promising environments where high-energy processes and particle acceleration can be investigated. They are the only objects for which significant X-ray polarization was detected in the past. Here we present the Imaging X-ray Polarimetry Explorer (IXPE) observation of the Crab pulsar and nebula. The total pulsar pulsed emission in the [2–8] keV energy range is unpolarized. Significant polarization up to 15% is detected in the core of the main peak. The nebula has a total space integrated polarized degree of 20% and polarization angle of 145°. The polarized maps show a large variation in the local polarization, and regions with a polarized degree up to 45–50%. The polarization pattern suggests a predominantly toroidal magnetic field. Our findings for the pulsar are inconsistent with most inner magnetospheric models, and suggest emission is more likely to come from the wind region. For the nebula, the polarization map suggests a patchy distribution of turbulence, uncorrelated with the intensity, in contrast with simple expectations from numerical models.X-ray polarization measurements of the Crab nebula and pulsar by the IXPE satellite reveal a global toroidal magnetic field with large variations in local polarization, suggesting a more complex turbulence distribution than anticipated.

The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds a critical value known as the Eddington limit. The discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity considerably exceeds this limit. A high apparent luminosity might be produced due to the geometric beaming of radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we measured the X-ray polarization in the Galactic X-ray binary Cygnus X-3 (Cyg X-3). We found high, >20%, nearly energy-independent linear polarization orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow from the X-ray binary Cyg X-3 and constrain its half-opening angle to ≲15°. Thus, the source can be used as a laboratory for studying the supercritical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources. The accretion geometry of X-ray binary Cygnus X-3 is determined here from IXPE observations. X-ray polarization reveals a narrow funnel with reflecting walls, which focuses emission, making Cyg X-3 appear as an ultraluminous X-ray source.

The centre of the Milky Way Galaxy hosts a black hole with a solar mass of about 4million (Sagittarius A· (Sgr A)) that is very quiescent at present with a luminosity many orders of magnitude below those of active galactic nuclei1. Reflection of X-rays from Sgr A· by dense gas in the Galactic Centre region offers a means to study its past flaring activity on timescales of hundreds and thousands of years2. The shape ofthe X-ray continuum and the strong fluorescent iron line observed from giant molecular clouds in the vicinity of Sgr A· are consistent with the reflection scenario3-5. If this interpretation is correct, the reflected continuum emission should be polarized6. Here we report observations of polarized X-ray emission in the direction of the molecular clouds in the Galactic Centre using the Imaging X-ray Polarimetry Explorer. We measure a polarization degree of 31% ± 11%, and a polarization angle of -48° ± 11°. The polarization angle is consistent with Sgr A· being the primary source of the emission, and the polarization degree implies that some 200 years ago, the X-ray luminosity of Sgr A· was briefly comparable to that of a Seyfert galaxy.

HERMES-Pathfinder is a constellation of six 3U nano-satellites hosting simple but innovative X-ray detectors for determining the positions of, and monitoring cosmic high-energy transients such as gamma-ray bursts and the electromagnetic counterparts of gravitational Wave Events. The HERMES Technological Pathfinder project is funded by the Italian Space Agency, while the HERMES Scientific Pathfinder project is funded by the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No. 821896. HERMES-Pathfinder is an in-orbit demonstration, that should be tested in orbit starting in 2023. We present the main scientific goals of HERMES-Pathfinder, as well as a description of the HERMES-Pathfinder payload and performance.

HERMES Pathfinder (High Energy Rapid Modular Ensemble of Satellites Pathfinder) is a space mission based on a constellation of nano-satellites in a low Earth Orbit, hosting new miniaturized detectors to probe the X-ray temporal emission of bright high-energy transients such as Gamma-Ray Bursts and the electromagnetic counterparts of Gravitational Waves. This ambitious goal will be achieved exploiting at most Commercial offthe-shelf components. For HERMES-SP, a custom Power Supply Unit board has been designed to supply the needed voltages to the payload and, at the same time, protecting it from Latch-Up events.

Discovery of pulsations from a number of ULXs proved that accretion onto neutron stars can produce luminosities exceeding the Eddington limit by several orders of magnitude. The conditions necessary to achieve such high luminosities as well as the exact geometry of the accretion flow in the neutron star vicinity are, however, a matter of debate. The pulse phase-resolved polarization measurements that became possible with the launch of the Imaging X-ray Polarimetry Explorer (IXPE) can be used to determine the pulsar geometry and its orientation relative to the orbital plane. They provide an avenue to test different theoretical models of ULX pulsars. In this paper we present the results of three IXPE observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2023 outburst. We find strong variations in the polarization characteristics with the pulsar phase. The average polarization degree increases from about 5% to 15% as the flux dropped by a factor of three in the course of the outburst. The polarization angle (PA) as a function of the pulsar phase shows two peaks in the first two observations, but changes to a characteristic sawtooth pattern in the remaining data set. This is not consistent with a simple rotating vector model. Assuming the existence of an additional constant polarized component, we were able to fit the three observations with a common rotating vector model and obtain constraints on the pulsar geometry. In particular, we find the pulsar angular momentum inclination with respect to the line of sight of 15-40 deg, the magnetic obliquity of 60-80 deg, and the pulsar spin position angle of -50 deg, which significantly differs from the constant component PA of about 10 deg. Combining these X-ray measurements with the optical PA, we find evidence for at least a 30 deg misalignment between the pulsar angular momentum and the binary orbital axis.