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The authors wish to thank J.M. Torrejón for the organization of the Athena-multi-messenger Workshops, held on November 27 - 29, 2018, in Alicante, Spain and on 5 May 2020 - 6 May 2020 in videoconference., In this paper we explore the scientific synergies between Athena and some of the key multi-messenger facilities that should be operative concurrently with Athena. These facilities include LIGO A+, Advanced Virgo+ and future detectors for ground-based observation of gravitational waves (GW), LISA for space-based observations of GW, IceCube and KM3NeT for neutrino observations, and CTA for very high energy observations. These science themes encompass pressing issues in astrophysics, cosmology and fundamental physics such as: the central engine and jet physics in compact binary mergers, accretion processes and jet physics in Super-Massive Binary Black Holes (SMBBHs) and in compact stellar binaries, the equation of state of neutron stars, cosmic accelerators and the origin of Cosmic Rays (CRs), the origin of intermediate and high-Z elements in the Universe, the Cosmic distance scale and tests of General Relativity and the Standard Model. Observational strategies for implementing the identified science topics are also discussed. A significant part of the sources targeted by multi-messenger facilities is of transient nature. We have thus also discussed the synergy of Athena with wide-field high-energy facilities, taking THESEUS as a case study for transient discovery. This discussion covers all the Athena science goals that rely on follow-up observations of high-energy transients identified by external observatories, and includes also topics that are not based on multi-messenger observations, such as the search for missing baryons or the observation of early star populations and metal enrichment at the cosmic dawn with Gamma-Ray Bursts (GRBs)., Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature., This research has been supported by the European Union’s Horizon 2020 programme under the AHEAD (grant agreement number 654215) and AHEAD2020 project (grant agreement n. 871158). FJC and SMN acknowledges funding under project RTI2018-096686-B-C21 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”, and by the Unidad de Excelencia María de Maeztu, ref. MDM-2017-0765.

The Athena X-ray Integral Unit (X-IFU) is the high resolution X-ray spectrometer, studied since 2015 for flying in the mid-30s on the Athena space X-ray Observatory, a versatile observatory designed to address the Hot and Energetic Universe science theme, selected in November 2013 by the Survey Science Committee. Based on a large format array of Transition Edge Sensors (TES), it aims to provide spatially resolved X-ray spectroscopy, with a spectral resolution of 2.5 eV (up to 7 keV) over an hexagonal field of view of 5 arc minutes (equivalent diameter). The X-IFU entered its System Requirement Review (SRR) in June 2022, at about the same time when ESA called for an overall X-IFU redesign (including the X-IFU cryostat and the cooling chain), due to an unanticipated cost overrun of Athena. In this paper, after illustrating the breakthrough capabilities of the X-IFU, we describe the instrument as presented at its SRR, browsing through all the subsystems and associated requirements. We then show the instrument budgets, with a particular emphasis on the anticipated budgets of some of its key performance parameters. Finally we briefly discuss on the ongoing key technology demonstration activities, the calibration and the activities foreseen in the X-IFU Instrument Science Center, and touch on communication and outreach activities, the consortium organisation, and finally on the life cycle assessment of X-IFU aiming at minimising the environmental footprint, associated with the development of the instrument. Thanks to the studies conducted so far on X-IFU, it is expected that along the design-to-cost exercise requested by ESA, the X-IFU will maintain flagship capabilities in spatially resolved high resolution X-ray spectroscopy, enabling most of the original X-IFU related scientific objectives of the Athena mission to be retained. (abridged)., Comment: 48 pages, 29 figures, Accepted for publication in Experimental Astronomy with minor editing

We join gravitational-wave and electromagnetic data to implement a combined simultaneous fit of the GW170817 event. The LIGO-Virgo analysis includes the estimation of the inclination, the angle of the binary with respect to the gravitationa-wave detector network line of sight. From the observations of the afterglow, instead, we can recover the viewing angle. The inclination and the viewing angle are supplementary angles, and can be treated as a single parameter. The value of the inclination that we recover from the fit is in agreement with the LIGO-Virgo previous works, with an uncertainty that is 10-fold smaller, thanks to contribution of the electromagnetic data. Moreover, with the inclusion of the gravitational-wave data, the degeneracy between the viewing angle and the jet opening angle is broken. This procedure is useful not only for analyzing GW170817, but any gravitational-wave event with an electromagnetic counterpart.

Scientific synergies between Athena and some of the key multi-messenger facilities that should be operative concurrently with Athena are presented. These facilities include LIGO A+, Advanced Virgo+ and future detectors for ground-based observation of gravitational waves (GW), LISA for space-based observations of GW, IceCube and KM3NeT for neutrino observations, CTA for very high energy observations. Multimessenger synergy science themes discussed here include pressing issues in the field of Astrophysics, Cosmology and Fundamental physics such as: the central engine and jet physics in compact binary mergers, accretion processes and jet physics in SMBBHs and in compact stellar binaries, the equation of state in neutron stars, cosmic accelerators and the origin of cosmic rays, the origin of intermediate and high-Z elements in the Universe, the Cosmic distance scale and tests of General Relativity and Standard Model. Observational strategies for implementing the identified science topics are also discussed.

The current projects of future X-rays space telescopes foresee high resolving power transition edge sensor (TES) micro-calorimeters arrays as focal plane instrument. In order to full exploit their sensitivity in the space environment, they need an anticoincidence detector to reject the background due to cosmic charged particles. High-energy protons (> 150 MeV) and other charged particle may release in the X-ray calorimeter, the same energy of the X-rays sources under observation. We report the description of the last prototype based on TES calorimeter where a 1-cm2 silicon chip is used as both substrate and absorber. The readout is made of 96 iridium/gold TESs uniformly distributed on its surface and parallel wired. In this paper, we will describe the design, with the preliminary phonon dynamics simulation, the fabrication, of first demonstration model for the ATHENA space telescope project.

The ATHENA observatory is the second large class ESA mission to be launched on 2031 at L2 orbit. One of the two onboard instruments is X-IFU, a TES-based kilo-pixel array able to perform simultaneous high-grade energy spectroscopy (FWHM 2.5 eV@7 keV) and imaging over the 5' field of view. The X-IFU sensitivity is degraded by primary particle background of both solar and galactic cosmic ray (GCR) origins, and by secondary electrons produced by primaries, interacting with the materials surrounding the detector: These particles cannot be distinguished by the scientific photons, thus degrading the instrument performance. Results from studies regarding the GCR component performed by Geant4 simulations address the necessity to use background reduction techniques to enable the study of several key science topics. This is feasible by combining an active Cryogenic AntiCoincidence detector (CryoAC) and a passive electron shielding to reach the required residual particle background of 0.005 cts/cm2/s/keV inside the 2-10 keV scientific energy band. The CryoAC is a four-pixel detector made of Si-suspended absorbers sensed by a network of IrAu TESes and placed at a distance < 1 mm below the TES array. Here we will provide an overview of the CryoAC program, starting with some details on the background assessment having impacts on the CryoAC design; then, we continue with its design concept including electronics and the Demonstration Model results, to conclude with programmatic aspects.

We present the results of a multiwavelength campaign of FRB 20201124A, the third closest repeating fast radio burst recently localized in a nearby (z=0.0978) galaxy. Deep VLA observations led to the detection of quiescent radio emission, also marginally visible in X-rays with Chandra. Imaging at 22 GHz allowed us to resolve the source on a scale of $\gtrsim$ 1 arcsec and locate it at the position of the FRB, within an error of 0.2 arcsec. EVN and e-MERLIN observations sampled small angular scales, from 2 to 100 mas, providing tight upper limits on the presence of a compact source and evidence for diffuse radio emission. We argue that this emission is associated with enhanced star formation activity in the proximity of the FRB, corresponding to a star formation rate of $\approx 10\ {\rm M}_\odot {\rm yr}^{-1}$. The surface star formation rate at the location of FRB 20201124A is two orders of magnitude larger than typically observed in other precisely localized FRBs. Such a high SFR is indicative of this FRB source being a new-born magnetar produced from a SN explosion of a massive star progenitor. Upper limits to the X-ray counterparts of 49 radio bursts observed in our simultaneous FAST, SRT and Chandra campaign are consistent with a magnetar scenario., Comment: Accepted by A&A letters

X-ray observations are limited by the background, due to the intrinsic faintness or diffuse nature of the sources. The future Athena X-ray observatory has among its goals the characterization of these sources. We aim at characterizing the particle-induced background of the Athena microcalorimeter, in both its low- (soft protons) and high-energy (galactic cosmic rays—GCR) induced components, to assess the instrument capability to characterize background-dominated sources such as the outskirts of clusters of galaxies. We compare two radiation environments, namely the L1 and L2 Lagrangian points, and derive indications against the latter. We estimate the particle-induced background level on the X-IFU microcalorimeter with Monte Carlo simulations, before and after all of the solutions adopted to reduce its level. Concerning the GCR-induced component, the background level is compliant with the mission requirement. Regarding the soft-proton component, the analysis does not predict dramatically different backgrounds in the L1 and L2 orbits. However, the lack of data concerning the L2 environment labels it as very weakly characterizable, and thus we advise against choosing it as the orbit for X-ray missions. We then use these background levels to simulate the observation of a typical galaxy cluster from its center out to 1.2 R 200 to probe the characterization capabilities of the instrument out to the outskirts. We find that without any background reduction, it is not possible to characterize the properties of the cluster in the outer regions. We also find no improvement in the observations when carried out during the solar maximum with respect to solar minimum conditions.

The origin of radio emission in the majority of active galactic nuclei is still poorly understood. Various competing mechanisms are likely involved in the production of radio emission and precise diagnostic tools are needed to disentangle them, of which variability is among the most powerful. For the first time, we show evidence for significant radio variability at 5 GHz at milli-arcsecond scales on days to weeks time-scales in the highly accreting and extremely radio-quiet (RQ) Narrow Line Seyfert 1 (NLSy1) Mrk 110. The simultaneous Swift/XRT light curve indicates stronger soft than hard X-ray variability. The short-term radio variability suggests that the GHz emitting region has a size smaller than ∼180 Schwarzschild radii. The high brightness temperature and the radio and X-ray variability rule out a star-formation and a disc wind origin. Synchrotron emission from a low-power jet and/or an outflowing corona is then favoured. © 2021 The Author(s)., FP acknowledges support from a grant PRIN-INAF SKA-CTA 2016 and from Agenzia Spaziale Italiana (ASI) under contract n. 2019-35-HH.0. MPT acknowledges financial support from the State Agency for Research of the Spanish MCIU through the ‘Center of Excellence Severo Ochoa’ award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and through grants PGC2018-098915-B-C21 and PID2020-117404GB-C21 (MCI/AEI/FEDER, UE). LHG acknowledges funds by ANID - Millennium Science Initiative Program - ICN12_009 awarded to the Millennium Institute of Astrophysics (MAS).

Neutron star mergers produce a substantial amount of fast-moving ejecta, expanding outwardly for years after the merger. The interaction of these ejecta with the surrounding medium may produce a weak isotropic radio remnant, detectable in relatively nearby events. We use late-time radio observations of short duration gamma-ray bursts (sGRBs) to constrain this model. Two samples of events were studied: four sGRBs that are possibly in the local (, 13 pages, 8 figures, 3 tables. Accepted for pubblication in MNRAS

GRB200522A is a short duration gamma-ray burst (GRB) at redshift $z$=0.554 characterized by a bright infrared counterpart. A possible, although not unambiguous, interpretation of the observed emission is the onset of a luminous kilonova powered by a rapidly rotating and highly-magnetized neutron star, known as magnetar. A bright radio flare, arising from the interaction of the kilonova ejecta with the surrounding medium, is a prediction of this model. Whereas the available dataset remains open to multiple interpretations (e.g. afterglow, r-process kilonova, magnetar-powered kilonova), long-term radio monitoring of this burst may be key to discriminate between models. We present our late-time upper limit on the radio emission of GRB200522A, carried out with the Karl G. Jansky Very Large Array at 288 days after the burst. For kilonova ejecta with energy $E_{\rm ej} \approx 10^{53} \rm erg$, as expected for a long-lived magnetar remnant, we can already rule out ejecta masses $M_{\rm ej} \lesssim0.03 \mathrm{M}_\odot$ for the most likely range of circumburst densities $n\gtrsim 10^{-3}$ cm$^{-3}$. Observations on timescales of $\approx$3-10 yr after the merger will probe larger ejecta masses up to $M_{\rm ej} \sim 0.1 \mathrm{M}_\odot$, providing a robust test to the magnetar scenario., Accepted for publication on MNRAS letters

The Athena observatory is the second large class ESA mission to be launched on early 2030’s. One of the two on board instruments is the X-IFU, which is a TES based kilo-pixels array able to perform simultaneous high grade energy spectroscopy (2.5eV@7keV) and imaging over the 5' FoV. The X-IFU sensitivity is degraded by primary particles background (bkg) of both solar and Galactic Cosmic Rays origin, and secondary electrons produced by primaries interacting with the materials surrounding the detector. The TES-array main sensor therefore needs a Cryogenic AntiCoincidence detector (CryoAC) to veto as much as possible such particles. The required residual bkg is 0.005 cts/cm2 /s/keV in 2-10 keV energy bandwidth. The CryoAC is at present baselined as 4 pixels detector made of Silicon suspended absorbers sensed by a network of IrAu TESes, and placed at a distance < 1 mm below the TES-array. On November 2019, Athena has successfully passed the Mission Formulation Review (MFR), thus entering in Phase B. Next close goal is the MAR (Mission Adoption Review) planned in second half of 2022 where all the critical technologies must demonstrate a Technology Readiness Level (TRL) equal to 5. Here we will provide an overview of the CryoAC program advancement involving: 1) the present particle background assessment; 2) the assembly design concept and the related trade-off studies between the present baseline (4 pixels) against a monolithic solution (1 pixel); 2) the technology status (i.e., some results from the integrated chipset test; warm electronics). We will conclude with programmatic aspects.

CNES (French Space Agency) is in charge of the development of the X-IFU instrument for Athena. The main sensor array detection chain sub-system of the X-IFU instrument is one of the major sub-subsystem of the instrument, as the main contributor to the performance. This sub-system involves major partners of the X-IFU instrument, e.g GFSC, SRON, VTT, APC, and IRAP. The purpose of this paper is to present the baseline of the definition of the X-IFU detection chain in the frame at end of phase A/beginning of phase B. The readout is based on Time Domain Multiplexing (TDM). There are strong design issues which couple the different sub-components of the detection chain (the main sensor array, the cold electronics stages, and the warm electronics). The detection chain environment (thermal, mechanical and EMI/EMC environment) also requires a transverse analysis. This paper focuses on those aspects while providing design description of the sub-components of the detection chain.

The X-ray Integral Field Unit (X-IFU) will be the cryogenic X-ray spectro-imager of the ESA Athena space observatory. It will implement a matrix of 3168 superconducting Transition Edge Sensors at 90 mK to provide a very high spectral resolution of 2.5 eV up to the energy of 7 keV, across a 5’ large field of view. The X-IFU successfully passed its Preliminary Requirements Review in mid-2019. After providing an overview of the driving science cases, the key requirements, I will present a status on the instrument development.

Gamma-ray bursts (GRBs) associated with gravitational wave events are, and will likely continue to be, viewed at a larger inclination than GRBs without gravitational wave detections. As demonstrated by the afterglow of GW170817, this requires an extension of the common GRB afterglow models which typically assume emission from an on-axis top hat jet. We present a characterization of the afterglows arising from structured jets, providing a framework covering both successful and choked jets. We compute new closure relations for structured jets and compare with the established relations for energy injection and refreshed shock models. The temporal slope before the jet break is found to be a simple function of the ratio between the viewing angle and effective opening angle of the jet. A numerical model to calculate synthetic light curves and spectra is publicly available as the open source Python package afterglowpy., Comment: 29 pages, 13 figures, submitted to ApJ

Recent observations with the Chandra X-ray telescope continue to detect X-ray emission from the transient GW170817. In a total exposure of 96.6 ks, performed between March 9 and March 16 2020 (935 d to 942 d after the merger), a total of 8 photons are measured at the source position, corresponding to a significance of about 5 sigma. Radio monitoring with the Australian Telescope Compact Array (ATCA) shows instead that the source has faded below our detection threshold (, 10 pages, 6 figures, 2 tables, submitted to MNRAS

Multi-messenger observations of GW170817 have not conclusively established whether the merger remnant is a black hole (BH) or a neutron star (NS). We show that a long-lived magnetized NS with a poloidal field $B\approx 10^{12}$G is fully consistent with the electromagnetic dataset, when spin down losses are dominated by gravitational wave (GW) emission. The required ellipticity $\epsilon\gtrsim 10^{-5}$ can result from a toroidal magnetic field component much stronger than the poloidal component, a configuration expected from a NS newly formed from a merger. Abrupt magnetic dissipation of the toroidal component can lead to the appearance of X-ray flares, analogous to the one observed in gamma-ray burst (GRB) afterglows. In the X-ray afterglow of GW170817 we identify a low-significance ($\gtrsim 3\sigma$) temporal feature at 155 d, consistent with a sudden reactivation of the central NS. Energy injection from the NS spin down into the relativistic shock is negligible, and the underlying continuum is fully accounted for by a structured jet seen off-axis. Whereas radio and optical observations probe the interaction of this jet with the surrounding medium, observations at X-ray wavelengths, performed with adequate sampling, open a privileged window on to the merger remnant., Comment: 10 pages, 7 figs, MNRAS Accepted 2018 November 5

The ATHENA X-IFU development program foresees to build and characterize an instrument Demonstration Model (DM), in order to probe the system critical technologies before the mission adoption. In this respect, we are now developing the DM of the X-IFU Cryogenic Anticoincidence Detector (CryoAC), which will be delivered to the Focal Plane Assembly (FPA) development team for the integration with the TES array. Before the delivery, we will characterize and test the CryoAC DM in our CryoLab at INAF/IAPS. In this paper we report the main results of the activities performed to improve our cryogenic test setup, making it suitable for the DM integration. These activities mainly consist in the development of a mechanichal setup and a cryogenic magnetic shielding system, whose effectiveness has been assessed by FEM simulations and a measurement at warm. The preliminary performance test has been performed by means of the last CryoAC single pixel prototype, the AC-S8 pre-DM sample., Accepted for publication in the Journal of Low Temperature Physics for LTD-17 special issue

We present our broadband study of GW170817 from radio to hard X-rays, including NuSTAR and Chandra observations up to 165 days after the merger, and a multi-messenger analysis including LIGO constraints. The data are compared with predictions from a wide range of models, providing the first detailed comparison between non-trivial cocoon and jet models. Homogeneous and power-law shaped jets, as well as simple cocoon models are ruled out by the data, while both a Gaussian shaped jet and a cocoon with energy injection can describe the current dataset for a reasonable range of physical parameters, consistent with the typical values derived from short GRB afterglows. We propose that these models can be unambiguously discriminated by future observations measuring the post-peak behaviour, with slope -1.0 for the cocoon and -2.5 for the jet model., Comment: 6 pages, 4 figures, accepted by MNRAS Letters. Minor changes to match the accepted version

This research was supported by the Spanish Ministry of Economy and Competitiveness, ESP2014-53672-C3-1-P and AYA2015-64346-C2-1-P.

The Cryogenic AntiCoincidence Detector (CryoAC) of ATHENA X-IFU is designed to reduce the particle background of the instrument and to enable the mission science goals. It is a 4 pixel silicon microcalorimeter sensed by an Ir/Au TES network. We have developed the CryoAC Demonstration Model, a prototype aimed to probe the critical technologies of the detector, i.e. the suspended absorber with an active area of 1 cm2; the low energy threshold of 20 keV; and the operation connected to a 50 mK thermal bath with a power dissipation less than 40 nW. Here we report the test performed on the first CryoAC DM sample (namely the AC-S10 prototype), showing that it is fully compliant with its requirements., Accepted for publication in the Journal of Low Temperature Physics for LTD-18 special issue

We present the results of our year-long afterglow monitoring of GW170817, the first binary neutron star (NS) merger detected by advanced LIGO and advanced Virgo. New observations with the Australian Telescope Compact Array (ATCA) and the Chandra X-ray Telescope were used to constrain its late-time behavior. The broadband emission, from radio to X-rays, is well-described by a simple power-law spectrum with index ~0.585 at all epochs. After an initial shallow rise ~t^0.9, the afterglow displayed a smooth turn-over, reaching a peak X-ray luminosity of ~5e39 erg/s at 160 d, and has now entered a phase of rapid decline ~t^(-2). The latest temporal trend challenges most models of choked jet/cocoon systems, and is instead consistent with the emergence of a relativistic structured jet seen at an angle of ~22 deg from its axis. Within such model, the properties of the explosion (such as its blastwave energy E_K~2E50 erg, jet width theta_c~4 deg, and ambient density n~3E-3 cm^(-3)) fit well within the range of properties of cosmological short GRBs., Comment: 11 pages, 8 figures, 2 tables, MNRAS, in press. Final version, minor changes only relative to original submission dated 21 August 2018

Outflows from active galactic nuclei (AGN) are often invoked to explain the co-evolution of AGN and their host galaxies, and the scaling relations between the central black hole mass and the bulge velocity dispersion. Nuclear winds are often seen in the X-ray spectra through Fe K shell transitions and some of them are called ultra fast outflows (UFOs) due to their high velocities, up to some fractions of the speed of light. If they were able to transfer some percentage of the AGN luminosity to the host galaxy, this might be enough to trigger an efficient feedback mechanism. We aim to establish new constraints on the covering fraction and on the kinematic properties of the UFO in the powerful (L(bol) ~ 10^(47) erg/s) quasar PDS 456, an established Rosetta stone for studying AGN feedback from disk winds. This will allow us to estimate the mass outflow rate and the energy transfer rate of the wind, which are key quantities to understand the potential impact on the host galaxy. We analyze two sets of simultaneous XMM-Newton and NuSTAR observations taken in September 2013 and reported in Nardini et al. (2015) as having similar broadband spectral properties. We fit the Fe K features with a P-Cygni profile between 5 and 14 keV, using a novel Monte Carlo model for the WINd Emission (WINE). We find an outflow velocity ranging from 0.17 to 0.28 c, with a mean value of 0.23 c. We obtain an opening angle of the wind of 71(+13,-8) deg and a covering fraction of 0.7(+0.2,-0.3), suggesting a wide-angle outflow. We check the reliability of the WINE model by performing extensive simulations of joint XMM-Newton and NuSTAR observations. Furthermore, we test the accuracy of the WINE model in recovering the geometrical properties of UFOs by simulating observations with the forthcoming X-ray observatory ATHENA., Comment: 7 pages, 4 figures, accepted for publication in A&A

The recent discovery of a faint gamma-ray burst (GRB) coincident with the gravitational wave (GW) event GW 170817 revealed the existence of a population of low-luminosity short duration gamma-ray transients produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B was an analogue of GRB170817A located at a cosmological distance. GRB 150101B was a faint short duration GRB characterized by a bright optical counterpart and a long-lived X-ray afterglow. These properties are unusual for standard short GRBs and are instead consistent with an explosion viewed off-axis: the optical light is produced by a luminous kilonova component, while the observed X-rays trace the GRB afterglow viewed at an angle of ~13 degrees. Our findings suggest that these properties could be common among future electromagnetic counterparts of GW sources., 28 pages, 8 figures, 2 tables. Accepted for publication

Both the interplanetary space and the Earth magnetosphere are populated by low energy ($\leq300$ keV) protons that are potentially able to scatter on the reflecting surface of Wolter-I optics of X-ray focusing telescopes and reach the focal plane. This phenomenon, depending on the X-ray instrumentation, can dramatically increase the background level, reducing the sensitivity or, in the most extreme cases, compromising the observation itself. The use of a magnetic diverter, deflecting protons away from the field of view, requires a detailed characterization of their angular and energy distribution when exiting the mirror. We present the first end-to-end Geant4 simulation of proton scattering by X-ray optics and the consequent interaction with the diverter field and the X-ray detector assembly, selecting the ATHENA Wide Field Imager as a case study for the evaluation of the residual soft proton induced background. We obtain that, in absence of a magnetic diverter, protons are indeed funneled towards the focal plane, with a focused Non X-ray Background well above the level required by ATHENA science objectives ($5\times10^{-4}$ counts cm$^{-2}$ s$^{-1}$ keV$^{-1}$), for all the plasma regimes encountered in both L1 and L2 orbits. These results set the proton diverter as a mandatory shielding system on board the ATHENA mission and all high throughput X-ray telescopes operating in the interplanetary space. For a magnetic field computed to deflect 99\% of the protons that would otherwise reach the WFI, Geant4 simulations show that this configuration, in the assumption of a uniform field, would efficiently shield the focal plane, yielding a residual background level of the order or below the requirement., Accepted for publication on The Astrophysical Journal on 28/08/2018

The ATHENA X-ray Observatory is the second large-class mission in the ESA Cosmic Vision 2015-2025 science programme. One of the two on-board instruments is the X-IFU, an imaging spectrometer based on a large array of TES microcalorimeters. To reduce the particle-induced background, the spectrometer works in combination with a Cryogenic Anticoincidence detector (CryoAC), placed less than 1 mm below the TES array. The last CryoAC single-pixel prototypes, namely AC-S7 and AC-S8, are based on large area (1 cm2) Silicon absorbers sensed by 65 parallel-connected iridium TES. This design has been adopted to improve the response generated by the athermal phonons, which will be used as fast anticoincidence flag. The latter sample is featured also with a network of Aluminum fingers directly connected to the TES, designed to further improve the athermals collection efficiency. In this paper we will report the main results obtained with AC-S8, showing that the additional fingers network is able to increase the energy collected from the athermal part of the pulses (from the 6% of AC-S7 up to the 26 % with AC-S8). Furthermore, the finger design is able to prevent the quasiparticle recombination in the aluminum, assuring a fast pulse rising front (L/R limited). In our road map, the AC-S8 prototype is the last step before the development of the CryoAC Demonstration Model (DM), which will be the detector able to demonstrate the critical technologies expected in the CryoAC development programme., Accepted for publication in the Journal of Low Temperature Physics for LTD-17 special issue

Since 2015, AHEAD (Integrated Activities in the High Energy Astrophysics Domain) has provided a reference framework to bring together Europe's science community. AHEAD is part of the EU Horizon 2020 programme for Research Infrastructures. Its main goal is to integrate the national efforts and keep the European community at the cutting edge of science and technology. AHEAD is offering funding opportunities, also open to participants outside Europe, for transnational visits, workshops and dissemination activities. The landmark for AHEAD is the future large observatory Athena. Significant effort is devoted to improve its design beyond the baseline and improve the related infrastructures. AHEAD is also active in the exploitation of current space missions, providing access to data archives and advanced tools and delivering new products and services. Finally, it supports technology innovation for the benefits of society and new design studies towards the definition of a future gamma-ray mission.

The X-ray Integral Field Unit (X-IFU) is an imaging microcalorimeter being developed for ESA's Athena X-ray observatory to providing high spectral resolution imaging between 0.2-12 keV, with moderate count-rate capability and a large field-of-view. The X-IFU focal plane assembly (FPA) will contain the instrument's large-format transition edge sensor (TES) microcalorimeter array and its superconducting readout electronics, plus a second TES detector, located behind the main sensor array, is used to detect high-energy cosmic rays and secondary particles passing through the sensor array and enable the rejection of false events that they generate in the sensor array's event list. A Kevlar thermal suspension is used to isolate the detectors at 55 mK from the 2 K environment of the X-IFU instrument cryostat's cold stage, while three layers of shielding are used to allow the detector's to achieve their low-noise performance in the expected on-ground and in-flight electromagnetic and microvibration environment. This paper will describe the preliminary design concept of the X-IFU focal plane assembly and its critical technology building blocks.

Copyright 2018 Society of Photo-Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. The X-ray Integral Field Unit (X-IFU) is one of the two instruments of the Athena astrophysics space mission approved by ESA in the Cosmic Vision 2015-2025 Science Programme. The X-IFU consists of a large array of transition edge sensor micro-calorimeters that will operate at 100 mK inside a sophisticated cryostat. A set of thin filters, highly transparent to X-rays, will be mounted on the opening windows of the cryostat thermal shields in order to attenuate the IR radiative load, to attenuate radio frequency electromagnetic interferences, and to protect the detector from contamination. Thermal filters are critical items in the proper operation of the X-IFU detector in space. They need to be strong enough to survive the launch stresses but very thin to be highly transparent to X-rays. They essentially define the detector quantum efficiency at low energies and are fundamental to make the photon shot noise a negligible contribution to the energy resolution budget. In this paper, we review the main results of modeling and characterization tests of the thermal filters performed during the phase A study to identify the suitable materials, optimize the design, and demonstrate that the chosen technology can reach the proper readiness before mission adoption.

Given the unprecedented effective area, the new ATHENA Silicon Pore Optics (SPO) focusing technology, the dynamic and variable L2 environment, where no X-ray mission has flown up to date, a dedicated Geant4 simulation campaign is needed to evaluate the impact of low energy protons scattering on the ATHENA mirror surface and the induced residual background level on its X-ray detectors. The Geant4 mass model is built as part of the ESA AREMBES project activities using the BoGEMMS framework. An SPO mirror module row is the atomic unit of the mass model, allowing the simulation of the full structure by means of 20 independent runs, one for each row. Thanks to the BoGEMMS configuration files, both single pores, mirror modules or the entire SPO row can be built with the same Geant4 geometry. Both Remizovich, in its elastic approximation, and Coulomb single scattering Geant4 models are used in the interaction of mono-energetic proton beams with a single SPO pore. The scattering efficiency for the first model is almost twice the efficiency obtained with the latter but for both cases we obtain similar polar and azimuthal angular distributions, with about 70-75% of scatterings generated by single or double reflections. The soft proton flux modelled for the plasma sheet region is used as input for the simulation of soft proton funnelling by the full SPO mass model. A much weaker soft proton vignetting than the one observed by XMM-Newton EPIC detectors is generated by ATHENA mirrors. The residual soft proton flux reaching the focal plane, defined as a 15 cm radius, is 10^4 times lower than the input L2 soft proton population entering the mirror, at the same energy, with rates comparable or higher than the ones observed in XMM EPIC-pn most intense soft proton flares., Comment: Proceeding of the SPIE Astronomical Telescopes + Instrumentation Conference, 10 - 15 June 2018

The X-ray Integral Field Unit (X-IFU) is the cryogenic imaging spectrometer on board the future X-ray observatory \textsl{Athena}. With a hexagonal array of 3840 AC-biased Transition Edge Sensors (TES), it will provide narrow-field observations (5$^{\prime}$ equivalent diameter) with unprecedented high spectral resolution (2.5 eV up to 7 keV) over the 0.2 - 12 keV bandpass. Throughout its observations, the X-IFU will face various sources of X-ray background. Specifically, the so-called Non-X-ray Background (NXB) caused by the interaction of high-energy cosmic rays with the instrument, may lead to a degradation of its sensitivity in the observation of faint extended sources (e.g. galaxy clusters outskirts). To limit this effect, a cryogenic anti-coincidence detector (CryoAC) will be placed below the detector plane to lower the NXB level down to the required level of $5 \times 10^{-3}$ cts/s/cm$^{2}$/keV over 2 - 10 keV. In this contribution, we investigate ways to accurately monitor the NXB and ensure the highest reproducibility in-flight. Using the limiting science case of the background-dominated observation of galaxy clusters outskirts, we demonstrate that a reproducibility of 2\% on the absolute knowledge of the background is required to perform driving science objectives, such as measuring abundances and turbulence in the outskirts. Monitoring of the NXB in-flight through closed observations, the detector's CryoAC or the companion instrument (Wide Field Imager) will be used to meet this requirement., 11 pages, 4 figures, SPIE proceeding Austin 2018

We report on a systematic investigation of the cold and mildly ionized gaseous baryonic metal components of our Galaxy, through the analysis of high resolution Chandra and XMM-Newton spectra of two samples of Galactic and extragalactic sources. The comparison between lines of sight towards sources located in the disk of our Galaxy and extragalactic sources, allows us for the first time to clearly distinguish between gaseous metal components in the disk and halo of our Galaxy. We find that a Warm Ionized Metal Medium (WIMM) permeates a large volume above and below the Galaxy's disk, perhaps up to the Circum-Galactic space (CGM). This halo-WIMM imprints virtually the totality of the OI and OII absorption seen in the spectra of our extragalactic targets, has a temperature of T(Halo-WIMM)=2900 +/- 900 K, a density (Halo-WIMM) = 0.023 +/- 0.009 cm-3 and a metallicity Z(Halo-WIMM) = (0.4 +/- 0.1) Z_Solar. Consistently with previous works, we also confirm that the disk of the Galaxy contains at least two distinct gaseous metal components, one cold and neutral (the CNMM: Cold Neutral Metal Medium) and one warm and mildly ionized, with the same temperature of the Halo-WIMM, but higher density ((Disk-WIMM) = 0.09 +/- 0.03 cm-3) and metallicity (Z(Disk-WIMM) = 0.8 +/- 0.1$ Z_Solar). By adopting a simple disk+sphere geometry for the Galaxy, we estimates masses of the CNMM and the total (disk + halo) WIMM of M(CNMM), Comment: 20 pages, 12 Figures, 9 Tables. Accepted my MNRAS

The ATHENA observatory is the second large-class ESA mission, in the context of the Cosmic Vision 2015–2025, scheduled to be launched on 2028 at L2 orbit. One of the two planned focal plane instruments is the X-ray Integral Field Unit (X-IFU), which will be able to perform simultaneous high-grade energy spectroscopy and imaging over the 5 arcmin FoV by means of a kilo-pixel array of transition-edge sensor (TES) microcalorimeters, coupled to a high-quality X-ray optics. The X-IFU sensitivity is degraded by the particle background, induced by primary protons of both solar and cosmic rays’ origin and secondary electrons. A Cryogenic AntiCoincidence (CryoAC) TES-based detector, located $$

It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest. A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (OVII) in the high signal-to-noise-ratio X-ray spectrum of a quasar at redshift >0.4. These absorbers show no variability over a 2-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (down to the sensitivity threshold of our data), agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium (WHIM). We conclude that the missing baryons have been found., Appeared in Nature (Volume 558, Issue 7710) on 21 June 2018. The posted PDF version is the pre-editorial-change version and includes the main paper, its Methods section and the Extended Data section. A link to the (view-only) PDF of the final published version of the paper, is available here: https://rdcu.be/1eak

International audience; The experience gained with the current generation of X-ray telescopes like Chandra and XMM-Newton has shown that low energy “soft” protons can pose a severe threat to the possibility to exploit scientific data, reducing the available exposure times by up to 50% and introducing a poorly reproducible background component. These soft protons are present in orbits outside the radiation belts and enter the mirrors, being concentrated towards the focal plane instruments, losing energy along their path and finally depositing their remaining energy in the detectors. Their contribution to the residual background will be even higher for ATHENA with respect to previous missions, given the much higher collecting area of the mirrors, even if the instruments will likely suffer no significant radiation damage from this particles flux. As a consequence this soft proton flux shall be damped with the use of a magnetic diverter to avoid excess background loading on the WFI or X-IFU instruments. We present here a first complete evaluation of this background component for the two focal plane instruments of the ATHENA mission in absence of a magnetic diverter, and derive the requirements for such device to reduce the soft protons induced background below the level required to enable the mission science. We estimate the soft proton flux expected in L2 for the interplanetary component and for the component generated locally by acceleration processes in the magnetotail. We produce a proton response matrix for each of the two instruments of ATHENA focal plane, exploiting two independent Monte Carlo simulations to estimate the optics concentration efficiency, and Geant4 simulations to evaluate the energy loss inside the radiation filters and deposited in the detector. With this modular approach we derive the expected fluxes and spectra for the soft protons component of the background. Finally, we calculate the specifics of a magnetic diverter able to reduce such flux below the required level for both X-IFU and WFI.

The background of the ATHENA X-IFU instrument is evaluated by Geant4 simulations. A new, highly detailed, mass model of the X-IFU and of its cryostat has been produced, a new model for the Galactic Cosmic Ray protons in L2 has been developed from satellite data, and a set of physics models tuned to ATHENA needs has been refined through extensive validations against experimental results. We are going to report the latest results in the estimate of the background of the X-IFU instrument, obtained after the update of all the elements of the Geant4 simulations and of the post processing software.

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5¿1 arcmin localization, an energy band extending from several MeV down to 0.3¿keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) follow-up with a 0.7¿m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift 10, signatures of Pop III stars, sources and physics of re-ionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late ¿20s/early ¿30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).© 2018 COSPAR, S.E. acknowledges the financial support from contracts ASI-INAF 1/009/10/0, NARO15 ASI-INAF 1/037/12/0 and ASI 2015-046-R.0. R.H. acknowledges GACR grant 13-33324S. S.V. research leading to these results has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 606176. D.S. was supported by the Czech grant 1601116S GA CR. Maria Giovanna Dainotti acknowledges funding from the European Union through the Marie Curie Action FP7-PEOPLE-2013-IOF, under grant agreement No. 626267 (>Cosmological Candles>).

A long-standing paradigm in astrophysics is that collisions- or mergers- of two neutron stars (NSs) form highly relativistic and collimated outflows (jets) powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the observational support for this model is only indirect. A hitherto outstanding prediction is that gravitational wave (GW) events from such mergers should be associated with GRBs, and that a majority of these GRBs should be off-axis, that is, they should point away from the Earth. Here we report the discovery of the X-ray counterpart associated with the GW event GW170817. While the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow from freshly synthesized r-process material in the merger ejecta, known as kilonova, observations at X-ray and, later, radio frequencies exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short GRBs and GWs from NS mergers, and gives independent confirmation of the collimated nature of the GRB emission., 38 pages, 10 figures, Nature, in press

International audience; In this paper we are going to review the latest estimates for the particle background expected on the X-IFU instrument onboard of the ATHENA mission. The particle background is induced by two different particle populations: the so called "soft protons" and the Cosmic rays. The first component is composed of low energy particles (100 MeV) that possess enough energy to cross the spacecraft and reach the detector from any direction, depositing a small fraction of their energy inside the instrument. Both these components are estimated using Monte Carlo simulations and the latest results are presented here.