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The European Space Agency is studying two large-class missions bound to operate in the decade of the 30s, and aiming at investigating the most energetic and violent phenomena in the Universe. Athena is poised to study the physical conditions of baryons locked in large-scale structures from the epoch of their formation, as well as to yield an accurate census of accreting supermassive black holes down to the epoch of reionization; LISA will extend the hunt for Gravitational Wave (GW) events to the hitherto unexplored mHz regime. We discuss in this paper the science that their concurrent operation could yield, and present possible Athena observational strategies. We focus on Supermassive (M |$\lesssim 10^7\, \rm {M_\odot }$|⁠) Black Hole Mergers (SMBHMs), potentially accessible to Athena up to z ∼ 2. The simultaneous measurement of their electromagnetic (EM) and GW signals may enable unique experiments in the domains of astrophysics, fundamental physics, and cosmography, such as the magnetohydrodynamics of fluid flows in a rapidly variable space–time, the formation of coronae and jets in Active Galactic Nuclei, and the measurement of the speed of GW, among others. Key to achieve these breakthrough results will be the LISA capability of locating a SMBHM event with an error box comparable to, or better than the field-of-view of the Athena Wide Field Imager (⁠|$\simeq 0.4\,$|  deg2) and Athena capability to slew fast to detect the source during the inspiral phase and the post-merger phase. Together, the two observatories will open in principle the exciting possibility of truly concurrent EM and GW studies of the SMBHMs [ABSTRACT FROM AUTHOR]

We are developing the Cryogenic AntiCoincidence detector (CryoAC) of the ATHENA X-IFU spectrometer. It is a TES-based particle detector aimed to reduce the background of the instrument. Here, we present the result obtained with the last CryoAC single-pixel prototype. It is based on a 1 cm2 silicon absorber sensed by a single 2 mm × 1 mm Ir/Au TES, featuring an on-chip heater for calibration and diagnostic purposes. We have illuminated the sample with 55Fe (6 keV line) and 241Am (60 keV line) radioactive sources, thus studying the detector response and the heater calibration accuracy at low energy. Furthermore, we have operated the sample in combination with a past-generation CryoAC prototype. Here, by analyzing the coincident detections between the two detectors, we have been able to characterize the background spectrum of the laboratory environment and disentangle the primary (i.e. cosmic muons) and secondaries (mostly secondary photons and electrons) signatures in the spectral shape. [ABSTRACT FROM AUTHOR]

The X-IFU is the cryogenic spectrometer onboard the future ATHENA X-ray observatory. It is based on a large array of TES microcalorimeters, which work in combination with a Cryogenic AntiCoincidence detector (CryoAC). This is necessary to reduce the particle background level thus enabling part of the mission science goals. Here we present the first joint test of X-IFU TES array and CryoAC Demonstration Models, performed in a FDM setup. We show that it is possible to operate properly both detectors, and we provide a preliminary demonstration of the anti-coincidence capability of the system achieved by the simultaneous detection of cosmic muons. [ABSTRACT FROM AUTHOR]

X-ray emission from the gravitational wave transient GW170817 is well described as non-thermal afterglow radiation produced by a structured relativistic jet viewed off-axis. We show that the X-ray counterpart continues to be detected at 3.3 years after the merger. Such long-lasting signal is not a prediction of the earlier jet models characterized by a narrow jet core and a viewing angle ≈20 deg, and is spurring a renewed interest in the origin of the X-ray emission. We present a comprehensive analysis of the X-ray dataset aimed at clarifying existing discrepancies in the literature, and in particular the presence of an X-ray rebrightening at late times. Our analysis does not find evidence for an increase in the X-ray flux, but confirms a growing tension between the observations and the jet model. Further observations at radio and X-ray wavelengths would be critical to break the degeneracy between models. [ABSTRACT FROM AUTHOR]

GRB 200522A 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 data set 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 GRB 200522A, carried out with the Karl G. Jansky Very Large Array at 288 d after the burst. For kilonova ejecta with energy E ej  |$\approx 10^{53}\, \rm erg$|⁠ , as expected for a long-lived magnetar remnant, we can already rule out ejecta masses |$M_{\rm ej}\lesssim 0.03\, \mathrm{M}_\odot$| for the most likely range of circumburst densities n ≳ 10−3 cm−3. Observations on timescales of ≈ 3–10 yr after the merger will probe larger ejecta masses up to M ej ∼ 0.1 M⊙, providing a robust test to the magnetar scenario. [ABSTRACT FROM AUTHOR]

The 50 mK cryogenic focal plane anti-coincidence detector of the Athena X-ray observatory (CryoAC) is a silicon suspended absorber sensed by a network of about 400 Ir/Au Transition Edge Sensors (TES) and connected through silicon bridges to a surrounding silicon frame plated with gold (rim). The device is shaped by Deep Reactive Ion Etching (DRIE) from a single silicon wafer of 500 μ m. There are two different possible geometries: a single Monolithic absorber and a Segmented one with 4 distinct absorber structures. As part of the payload of space mission the detector must resist to several mechanical excitation. We have tested a set of structural prototypes of the CryoAC vibrating several hexagonal silicon samples by using the vibrational mask provided by SRON which is responsible for FPA design. The aim is to have a first information on the mechanical response of the silicon bridges that connect the absorber to the rim, to start a trade-off over the two geometries and to validate the elastic-mechanical response. [ABSTRACT FROM AUTHOR]