We have measured and characterized the thermal crosstalk in two different arrays of transition-edge sensor (TES) X-ray microcalorimeters with frequency-domain multiplexing (FDM) readout. The TES arrays are fabricated at SRON and are a 8 × 8 and a 32 × 32 'kilo-pixel' uniform array. The amount of crosstalk is evaluated as the ratio between the averaged crosstalk signal and the X-ray pulse amplitudes. The crosstalk ratios (CRs) for our detectors are compliant with the requirements for future X-ray space missions, such as Athena X-IFU (CR < 10^{-3 for first-neighbor pixels): We measured a nearest neighbor thermal CR at a level of 10^{-4, with a highest CR of 4× 10^{-4 for the kilo-pixel array (worst case, center of array) and 1× 10^{-4 for the 8 × 8 array, with a margin of improvement achievable by optimizing the Cu metallization and the width of the Si supporting structures (muntins) in the backside of the TES array chip. Based on the measured CRs, we have estimated the impact on the spectral resolution by means of noise equivalent power considerations and a Monte Carlo simulation, finding an average degradation in quadrature of less than 40 meV, compliant with the <0.2 eV requirement for Athena X-IFU.

Large format arrays of transition edge sensor (TES) are crucial for the next generation of X-ray space observatories. Such arrays are required to achieve an energy resolution of Δ E < 3 eV full-width-half-maximum (FWHM) in the soft X-ray energy range. We are currently developing X-ray microcalorimeter arrays as a backup option for the X-IFU instrument on board of ATHENA space telescope, led by ESA and foreseen to be launched in 2031. In this contribution, we report on the development and the characterization of a uniform 32 × 32 pixel array with (length × width) 140 × 30 μ m2 TiAu TESs, which have a 2.3 μ m thick Au absorber for X-ray photons. The pixels have a typical normal resistance Rn = 121 m Ω and a critical temperature Tc ∼ 90 mK. We performed extensive measurements on 60 pixels out of the array in order to show the uniformity of the array. We obtained an energy resolutions between 2.4 and 2.6 eV (FWHM) at 5.9 keV, measured in a single-pixel mode at AC bias frequencies ranging from 1 to 5 MHz, with a frequency domain multiplexing (FDM) readout system, which is developed at SRON/VTT. We also present the detector energy resolution at X-ray with different photon energies generated by a modulated external X-ray source from 1.45 keV up to 8.9 keV. Multiplexing readout across several pixels has also been performed to evaluate the impact of the thermal crosstalk to the instrument's energy resolution budget requirement. This value results in a derived requirement, for the first neighbour, that is less than 1;× 10-3 when considering the ratio between the amplitude of the crosstalk signal to an X-ray pulse (for example at 5.9 keV).