The Laser Interferometer Lunar Antenna (LILA) is a next-generation gravitational-wave (GW) facility on the Moon. By harnessing the Moon's unique environment, LILA fills a critical observational gap in the mid-band GW spectrum (0.1 - 10 Hz) between terrestrial detectors (LIGO, Virgo, KAGRA) and the future space mission LISA. Observations enabled by LILA will fundamentally transform multi-messenger astrophysics and GW probes of fundamental physics. LILA will measure the lunar deep interior better than any existing planetary seismic instruments. The LILA mission is designed for phased development aligned with capabilities of the U.S.'s Commercial Lunar Payload Services and Artemis programs. LILA is a unique collaboration between universities, space industries, U.S. government laboratories, and international partners.

ABSTRACT We present simultaneous Chandra X-ray Observatory and Hubble Space Telescope observations of three certain (X5, X7, W37) and two likely (X4, W17) quiescent neutron star low-mass X-ray binaries (qLMXBs) in the globular cluster 47 Tuc. We study these systems in the X-ray, optical, and near-ultraviolet (NUV) using the simultaneous data and additional non-contemporaneous HST data. We have discovered a blue and variable NUV counterpart to W17. We have not securely identified the eclipsing qLMXB W37 in the optical or NUV. Deeper high-resolution imaging is needed to further investigate the faint NUV excess near the centre of the W37 error circle. We suggest that a previously identified optical astrometric match to X7 is likely the true counterpart. The H α emission and the location of the counterpart in the colour–magnitude diagram, indicate that the secondary is probably a non-degenerate, H-rich star. This is consistent with previous results from fitting X7’s X-ray spectrum. In X4, the simultaneous X-ray and optical behaviour supports the earlier suggestion that the X-ray variability is driven by changes in accretion rate. The X-ray eclipses in X5 coincide with minima in the optical/NUV light curves. Comparison of the 47 Tuc qLMXBs with the cataclysmic variables (CVs) in the cluster confirms that overall the qLMXBs have larger X-ray to optical flux ratios. Based on their optical/NUV colours, we conclude that the accretion discs in the qLMXBs are less prominent than in CVs. This makes the ratio of X-ray flux to excess blue-optical flux a powerful discriminator between CVs and qLMXBs.

HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10" FWHM) and broad spectral coverage (0.2-80 keV) with an effective area far superior to current facilities' (including XMM-Newton and NuSTAR). These capabilities will enable revolutionary new insights into a variety of important astrophysical problems. We present scientific objectives and simulations of HEX-P observations of the Galactic Center (GC) and Bulge. We demonstrate the unique and powerful capabilities of the HEX-P observatory for studying both X-ray point sources and diffuse X-ray emission. HEX-P will be uniquely equipped to explore a variety of major topics in Galactic astrophysics, allowing us to (1) investigate broad-band properties of X-ray flares emitted from the supermassive black hole (BH) at Sgr A* and probe the associated particle acceleration and emission mechanisms; (2) identify hard X-ray sources detected by NuSTAR and determine X-ray point source populations in different regions and luminosity ranges; (3) determine the distribution of compact object binaries in the nuclear star cluster and the composition of the Galactic Ridge X-ray emission; (4) identify X-ray transients and measure fundamental parameters such as BH spin; (5) find hidden pulsars in the GC; (6) search for BH-OB binaries and hard X-ray flares from young stellar objects in young massive clusters; (7) measure white dwarf (WD) masses of magnetic CVs to deepen our understanding of CV evolution and the origin of WD magnetic fields; (8) explore primary particle accelerators in the GC in synergy with future TeV and neutrino observatories; (9) map out cosmic-ray distributions by observing non-thermal X-ray filaments; (10) explore past X-ray outbursts from Sgr A* through X-ray reflection components from giant molecular clouds.

Stellar-mass black holes (BHs), with masses comparable to stars, are a major constituent of our Milky Way galaxy. This chapter describes the landscape of challenging, and long-sought efforts to identify these objects in the Galaxy. The first stellar-mass BHs were identified as persistent, but highly variable cosmic X-ray sources. Later, transient BH candidates were detected, and now far outnumber the persistent sources. Decades of effort have also yielded candidate BHs via gravitational microlensing and their orbital effect on binary companions. Populations of BH systems have begun to emerge from these detection strategies, offering insight into the astrophysical context in which BHs exist and driving questions about the formation, assembly, and ongoing evolution of these enigmatic objects.

ProtoEXIST2 (P2) was a prototype imaging x-ray detector plane developed for widefield time-domain astrophysics (TDA) in the 5 to 200 keV energy band. It was composed of an 8 x 8 array of 5 mm thick, 2 cmx 2 cm pixelated (32 x 32) CdZnTe (CZT) detectors with a 0.6-mm pitch that utilize the NuSTAR ASIC (NuASIC) for readout. During the initial detector development process leading up to postflight examination of the entire detector plane, distortions in expected pixel positions and shapes were observed in a significant fraction of the detectors. The High Resolution Energetic x-ray Imager (HREXI) calibration facility (HCF) was designed and commissioned to improve upon these early experiments and to rapidly map out and characterize pixel nonuniformities and defects within CZT detector planes at resolutions down to 50 mu m. Using this facility, the subpixel level detector response of P2 was measured at 100 mu m5 resolution and analyzed to extract and evaluate the area and profile of individual pixels, their morphology across the entire P2 detector plane for comparison with previous measurements and to provide additional characterization. In this article, we evaluate the imaging performance of a coded-aperture telescope using the observed pixel morphology for P2 detectors. This investigation will serve as an initial guide for detector selection in the development of HREXI detector planes, for the future implementation of the 4pi X-Ray Imaging Observatory o4piXIOTHORN6 mission, which aims to provide simultaneous and continuous imaging of the full sky (4p sr) in the 3 to 200 keV energy band with. 2 arcmin angular resolution and similar or equal to 10 arcsec source localization, as well as other, future coded-aperture instruments. (c) 2023 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JATIS.9.1.018005]

Detector commanding, processing and readout of spaceborne instrumentation is often accomplished with Application Specific Integrated Circuits (ASICs). The ASIC designed for the Nuclear Spectroscopic Telescope Array (NuSTAR) mission (NuASIC) enables future tiled CdZnTe (CZT) detector array readout for x-ray detectors such as the High Resolution Energetic X-ray Imager (HREXI). Modified NuASIC gain settings have been implemented for HREXI's broader targeted imaging energy range (3-300 keV) compared to NuSTAR (2-79 keV), which may require updated NuASIC internal parameters for optimal energy resolution. To reach HREXI's targeted low energy threshold, we have also enabled the NuASIC's "Charge Pump Mode" (CPM), which introduces an additional tuning parameter. In this paper, we describe the mechanics of the NuASIC's adjustable parameters and use our recently developed ASIC Test Stand (ATS) to probe a "bare" NuASIC using its internal test pulser. We record the effects of parameter tuning on the device's electronics noise and low energy threshold and report the optimal set of parameters for HREXI's updated gain setting. We detail a semi-automated procedure to derive the optimal parameters for each of HREXI's large area, closely tiled NuASIC/CZT detectors to expedite instrument integration.

We report timing and broadband spectral analysis of a Galactic X-ray source, CXOGBS J174517.0−321356 (J1745), with a 614 s periodicity. Chandra discovered the source in the direction of the Galactic Bulge. Gong proposed that J1745 was either an intermediate polar (IP) with a mass of ∼1 M ⊙ , or an ultracompact X-ray binary (UCXB). To confirm J1745's nature, we jointly fit XMM-Newton and NuSTAR spectra, ruling out a UCXB origin. We have developed a physically realistic model that considers a finite magnetosphere radius, X-ray absorption from the preshock region, and reflection from the white-dwarf (WD) surface to properly determine the IP properties, especially its WD mass. To assess systematic errors on the WD mass measurement, we consider a broad range of specific accretion rates ( m ̇ = 0.6 – 44 g cm −2 s −1 ) based on the uncertain source distance ( d = 3–8 kpc) and fractional accretion area ( f = 0.001–0.025). Our model properly implements the fitted accretion column height in the X-ray reflection model and accounts for the underestimated mass accretion rate due to the (unobserved) soft X-ray blackbody and cyclotron cooling emissions. We found that the lowest accretion rate of m ̇ = 0.6 g cm −2 s −1 , which corresponds to the nearest source distance and maximum f value, yields a WD mass of (0.92 ± 0.08) M ⊙ . On the other hand, as long as the accretion rate is m ̇ ≳ 3 g cm −2 s −1 , the WD mass is robustly measured to be (0.81 ± 0.06) M ⊙ , nearly independent of m ̇ . The derived WD mass range is consistent with the mean WD mass of nearby IPs. Assuming spin equilibrium between the WD and accretion disk, we constrained the WD magnetic field to B ≳ 7 MG, indicating that it could be a highly magnetized IP. Our analysis presents the most comprehensive methodology for constraining the WD mass and B field of an IP by consolidating the effects of cyclotron cooling, finite magnetospheric radius, and accretion column height.

In this paper, we review some of the extant literature on the study of interstellar objects (ISOs). With the forthcoming Vera C. Rubin Telescope and Legacy Survey of Space and Time (LSST), we find that 0.38-84 'Oumuamua-like interstellar objects are expected to be detected in the next 10 years, with 95% confidence. The feasibility of a rendezvous trajectory has been demonstrated in previous work. In this paper, we investigate the requirements for a rendezvous mission with the primary objective of producing a resolved image of an interstellar object. We outline the rendezvous distances necessary as a function of resolution elements and object size. We expand upon current population synthesis models to account for the size dependency on the detection rates for reachable interstellar objects. We assess the trade-off between object diameter and occurrence rate, and conclude that objects with the size range between a third of the size and the size of 'Oumuamua will be optimal targets for an imaging rendezvous. We also discuss expectations for surface properties and spectral features of interstellar objects, as well as the benefits of various spacecraft storage locations.

The discovery of the electromagnetic counterpart to the binary neutron star (NS) merger GW170817 has opened the era of gravitational-wave multimessenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multiwavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of NS mergers and other gravitational-wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving NSs (∼tens per year) out to distances of several hundred megaparsecs. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of NS and other compact-object mergers, and yet unknown classes of gravitational-wave events.

In 2018, we reported our discovery of a dozen quiescent X-ray binaries in the central parsec (pc) of the Galaxy (Hailey et al. 2018). In a recent follow-up paper (Mori et al. 2021), we published an extended analysis of these sources and other X-ray binaries (XRBs) in the central pc and beyond, showing that most if not all of the 12 non-thermal sources are likely black hole low-mass X-ray binary (BH-LMXB) candidates. In response, Maccarone et al. 2022 (TM22 hereafter) argued, primarily on the claim that neutron star low-mass X-ray binaries (NS-LMXBs) often do not have short outburst recurrence times (<~ 10 yr), that they cannot be excluded as a designation for the 12 quiescent X-ray binary sources. TM22 cites three main factors in their study: (1) X-ray outburst data of NS transients detected by RXTE and MAXI, (2) the Galactic population of NS-LMXBs, and (3) (persistently) quiescent NS-LMXBs in globular clusters. We address these arguments of TM22 and correct their misunderstandings of our work and the literature, even though most of these points have already been thoroughly addressed by Mori et al. 2021. We also correct TM22's assertion that our arguments are based solely on NS transients' recurrence times.

We report and correct a zero-point error in the HST ACS/WFC F658N (Hα) magnitudes for stars in the globular cluster NGC 6397 presented in the published article (hereafter C10).The Hα magnitudes reported in Table 1 of C10 are too faint by 3 mag and consequently the horizontal axes of Figures 3 and4 (right panels) and Figure 7 in C10 are shifted by this amount.This error does not affect any of the conclusions of C10, which are based on relative values of the Hα magnitudes.However, the correct calibration of the Hα magnitudes is necessary for other applications, such as for comparison of the Hα photometry of other clusters with that reported for NGC 6397 by C10 and for scaling the exposure times for other clusters from those of HST program GO-10257 (PI: Anderson) using the Hα magnitudes given by Table 1 of C10.We have therefore rectified the zero-point error and present here a revised Table 1 with corrected Hα magnitudes and revised Figures 3,4, and 7 that incorporate this correction by recalibrating the horizontal axes.We first noted the zero-point error in C10 in 2013.We recently came to appreciate its significance, for computing exposure times by scaling from NGC 6397 to other clusters, while writing an HST proposal.We used a correct Hα calibration procedure in our studies of NGC 6752 reported in Lugger et al. (2017) andCohn et al. (2021).We apologize for any confusion the error in C10 may have caused.

Application Specific Integrated Circuits (ASICs) are commonly used to efficiently process the signals from sensors and detectors in space. Wire bonding is a space qualified technique of making interconnections between ASICs and their substrate packaging board for power, control and readout of the ASICs. Wire bonding is nearly ubiquitous in modern space programs, but their exposed wires can be prone to damage during assembly and subject to electric interference during operations. Additional space around the ASICs needed for wire bonding also impedes efficient packaging of large arrays of detectors. Here we introduce the Through Silicon Vias (TSV) technology that replaces wire bonds and eliminates their shortcomings. We have successfully demonstrated the feasibility of implementing TSVs to existing ASIC wafers (a.k.a. a via-last process) developed for processing the X-ray signals from the X-ray imaging CdZnTe detectors on the Nuclear Spectroscopic Telescope Array (NuSTAR) Small Explorer mission that was launched in 2012. While TSVs are common in the semiconductor industry, this is the first (to our knowledge) successful application for Astrophysics imaging instrumentation. We expect that the TSV technology will simplify the detector assembly, and thus will enable significant cost and schedule savings in assembly of large area CdZnTe detectors.

The REgolith X-ray Imaging Spectrometer (REXIS) instrument on board NASA’s OSIRIS-REx mission to the asteroid Bennu is a Class-D student collaboration experiment designed to detect fluoresced X-rays from the asteroid’s surface to measure elemental abundances. In July and November 2019 REXIS collected ∼615 hours of integrated exposure time of Bennu’s sun-illuminated surface from terminator orbits. As reported in Hoak et al. (Results from the REgolith X-ray Imaging Spectrometer (REXIS) at Bennu, 2021) the REXIS data do not contain a clear signal of X-ray fluorescence from the asteroid, in part due to the low incident solar X-ray flux during periods of observation. To support the evaluation of the upper limits on the detectable X-ray signal that may provide insights for the properties of Bennu’s regolith, we present an overview of the REXIS instrument, its operation, and details of its in-flight calibration on astrophysical X-ray sources. This calibration includes the serendipitous detection of the transient X-ray binary MAXI J0637-430 during Bennu observations, demonstrating the operational success of REXIS at the asteroid. We convey some lessons learned for future X-ray spectroscopy imaging investigations of asteroid surfaces.

The REgolith X-ray Imaging Spectrometer (REXIS) is a soft x-ray spectrometer and the student collaboration instrument aboard NASA's OSIRIS-REx asteroid sample return mission. REXIS utilizes MIT Lincoln Laboratory CCID-41 x-ray detectors coated with a directly deposited optical blocking filter (OBF) with a thickness of 320 nm. The aluminum coating, developed at MIT Lincoln Laboratory, is designed to block visible light from the detector, to maintain high sensitivity to soft x-rays in the presence of reflected sunlight from the surface of the target asteroid Bennu. The scientific objective for the REXIS instrument is to measure the stimulated x-ray flux fluoresced from Bennu to discern elemental abundances present on the asteroid's surface. The coating technique applied for blocking visible light had not previously been used on the CCD-41s in an extended space flight mission. The performance of the OBF on the flight detectors was not characterized before and after environmental stress testing. Therefore, to mature the OBF to technology readiness level (TRL) 6, the flight spare detectors were tested while the instrument was on the way to the asteroid. The flight spare hardware underwent vibration and thermal environmental stress testing to test the durability and effectiveness of the OBF. This testing informed our expectations of the in-flight OBF once it reached the asteroid and helped mature the TRL level of this directly deposited OBF. We discuss the setup and results of those tests and address the performance of the flight OBF at the asteroid. We conclude that depositing an aluminum OBF onto the surface of a charge-coupled device is able to withstand stresses of launch and an extended life-mission in interplanetary space. (C) 2021 Society of Photo-Optical Instrumentation Engineers (SPIE)

We report on optical observations and modeling of HD96670, a single-line spectroscopic binary in the Carina OB2 association. We collected 10 epochs of optical spectroscopy, and optical photometry on 17 nonconsecutive nights on the source. We construct a radial velocity curve from the spectra, and update the orbital period of the binary to be P = 5.28388 +/- 0.00046 days. The spectra show oxygen and helium absorption, consistent with an O-type primary. We see no evidence for spectral lines from the secondary star in the binary. We model the optical light curve and radial velocity curve simultaneously using the Wilson-Devinney code and find a best-fit mass of M-1 = 22.7(-3.6)(+5.2) M-circle dot for the primary, and M-2 = 6.2(-0.7)(+0.9) M-circle dot for the secondary. An object of this mass is consistent with either a B-type star, or a black hole. Given that we see no absorption lines from the secondary, in combination with an observed hard power-law X-ray spectrum with Gamma = 2.6 detected past 10 keV that may have been produced by wind accretion onto the secondary, we conclude that the secondary is most likely a black hole. We see asymmetrical helium lines with a shape consistent with the presence of a third star. If the secondary is indeed a black hole, this system would add to the small sample of only four possible black hole high mass X-ray binaries in the galaxy.

We present an investigation of the quiescent and transient X-ray binaries (XRBs) of the Galactic Center (GC). We extended our Chandra analysis of the non-thermal X-ray sources, located in the central parsec, from Hailey et al. (), using an additional 4.6 Msec of ACIS-S data obtained in 2012–2018. The individual Chandra spectra of the 12 sources fit to an absorbed power-law model with a mean photon index Γ ≈ 2 and show no Fe emission lines. Long-term variability was detected from nine of them, confirming that a majority are quiescent XRBs. Frequent X-ray monitoring of the GC revealed that the 12 non-thermal X-ray sources, as well as four X-ray transients have shown at most a single outburst over the last two decades. They are distinct from the six known neutron star LMXBs in the GC, which have all undergone multiple outbursts with ≲ 5 year recurrence time on average. Based on the outburst history data of the broader population of X-ray transients, we conclude that the 16 sources represent a population of ∼240–630 tightly bound BH-LMXBs with ∼4−12 hr orbital periods, consistent with the stellar/binary dynamics modeling in the vicinity of Sgr A*. The distribution of the 16 BH-LMXB candidates is disk-like (at 87% CL) and aligned with the nuclear star cluster. Our results have implications for XRB formation and the rate of gravitational wave events in other galactic nuclei.

The High Resolution Energetic X-Ray Imager (HREXI) CZT detector development program at Harvard is aimed at developing tiled arrays of finely pixelated CZT detectors for use in wide-field coded aperture 3-200 keV X-ray telescopes. A pixel size of $\simeq$ 600 $\mu m$ has already been achieved in the ProtoEXIST2 (P2) detector plane with CZT read out by the NuSTAR ASIC. This paves the way for even smaller 300 $\mu m$ pixels in the next generation HREXI detectors. This article describes a new HREXI calibration facility (HCF) which enables a high resolution sub-pixel level (100 $\mu m$) 2D scan of a 256 $cm^2$ tiled array of 2 $\times$ 2 cm CZT detectors illuminated by a bright X-ray AmpTek Mini-X tube source at timescales of around a day. HCF is a significant improvement from the previous apparatus used for scanning these detectors which took $\simeq$ 3 weeks to complete a 1D scan of a similar detector plane. Moreover, HCF has the capability to scan a large tiled array of CZT detectors ($32cm \times 32cm$) at 100 $\mu m$ resolution in the 10 - 50 keV energy range which was not possible previously. This paper describes the design, construction, and implementation of HCF for the calibration of the P2 detector plane.