The third observing run by LVC has brought the discovery of many compact binary coalescences. Following the detection of the first binary neutron star merger in this run (LIGO/Virgo S190425z), we performed a dedicated follow-up campaign with the Zwicky Transient Facility (ZTF) and Palomar Gattini-IR telescopes. The initial skymap of this single-detector gravitational wave (GW) trigger spanned most of the sky observable from Palomar Observatory. Covering 8000 deg² of the initial skymap over the next two nights, corresponding to 46% integrated probability, ZTF system achieved a depth of ≈21 m _AB in g- and r-bands. Palomar Gattini-IR covered 2200 square degrees in J-band to a depth of 15.5 mag, including 32% integrated probability based on the initial skymap. The revised skymap issued the following day reduced these numbers to 21% for the ZTF and 19% for Palomar Gattini-IR. We narrowed 338,646 ZTF transient “alerts” over the first two nights of observations to 15 candidate counterparts. Two candidates, ZTF19aarykkb and ZTF19aarzaod, were particularly compelling given that their location, distance, and age were consistent with the GW event, and their early optical light curves were photometrically consistent with that of kilonovae. These two candidates were spectroscopically classified as young core-collapse supernovae. The remaining candidates were ruled out as supernovae. Palomar Gattini-IR did not identify any viable candidates with multiple detections only after merger time. We demonstrate that even with single-detector GW events localized to thousands of square degrees, systematic kilonova discovery is feasible.

We report X-ray, optical, and near-infrared monitoring of the new X-ray transient MAXI J1820+070 discovered with MAXI on 2018 March 11. Its X-ray intensity reached ∼2 crab at 2–20 keV at the end of March, and then gradually decreased until the middle of June. In this period, the X-ray spectrum was described by Comptonization of the disk emission, with a photon index of ∼1.5 and an electron temperature of ∼50 keV, which is consistent with a black hole X-ray binary in the low/hard state. The electron temperature was slightly decreased, and the photon index increased, with increasing flux. The source showed significant X-ray flux variation on a timescale of seconds. This short-term variation was found to be associated with changes in the spectral shape, and the photon index became slightly harder at higher fluxes. This suggests that the variation was produced by a change in the properties of the hot electron cloud responsible for the strong Comptonization. Modeling a multi-wavelength spectral energy distribution around the X-ray flux peak at the end of March, covering the near-infrared to X-ray bands, we found that the optical and near-infrared fluxes were likely contributed substantially by the jet emission. Before this outburst, the source was never detected in the X-ray band with MAXI (with a 3 σ upper limit of ∼0.2 mcrab at 4–10 keV, obtained from seven years of data from 2009 to 2016), whereas weak optical and infrared activity was found at flux levels ∼3 orders of magnitude lower than the peak fluxes in the outburst.

The hard x-ray imaging spectroscopy system of “Hitomi” x-ray observatory is composed of two sets of hard x-ray imagers (HXI) coupled with hard x-ray telescopes (HXT). With a 12-m focal length, the system provides fine (1 ′ . 7 half-power diameter) imaging spectroscopy covering about 5 to 80 keV. The HXI sensor consists of a camera, which is composed of four layers of Si and one layer of CdTe semiconductor imagers, and an active shield composed of nine Bi4Ge3O12 scintillators to provide low background. The two HXIs started observation on March 8 and 14, 2016 and were operational until 26 March. Using a Crab observation, 5 to 80 keV energy coverage and good detection efficiency were confirmed. The detector background level of 1 to 3 × 10 − 4 counts s − 1 keV − 1 cm − 2 (in detector geometrical area) at 5 to 80 keV was achieved, by cutting the high-background time-intervals, adopting sophisticated energy-dependent imager layer selection, and baffling of the cosmic x-ray background and active-shielding. This level is among the lowest of detectors working in this energy band. By comparing the effective area and the background, it was shown that the HXI had a sensitivity that is same to that of NuSTAR for point sources and 3 to 4 times better for largely extended diffuse sources.

Previous works have suggested a correlation between the X-ray luminosity and the rotational luminosity of radio pulsars. However, none of the obtained regression lines is statistically acceptable due to large scatter. We construct a statistical model that has an intrinsic – relation and reproduces the observed distribution about it by using a Monte Carlo simulator, which takes into account the effects obscuring the intrinsic relation, i.e., the anisotropy of radiation, additional heating, uncertainty in distance, and the detection limit of the instruments. From the ATNF pulsar catalog we collect 57 “ordinary radio pulsars” with significant detection and 42 with upper limits. The sample does not include high-magnetic-field pulsars (>10¹³ G), which are analyzed separately. We obtain a statistically acceptable relation with c ₁ = 1.03 ± 0.27 and L ₀ = 10^35.38. The distribution about the obtained – relation is reproduced well by the simulator. Pulsars with abnormally high fall into two types: one is the soft gamma-ray pulsars, and the other is pulsars that are thermally bright in comparison with the standard cooling curve. On the other hand, pulsars showing low are found to have dim pulsar wind nebulae (PWNs). We argue that there is an unknown mechanism that governs both the magnetospheric emission and the PWNs, and it might involve the production rate of electron–positron pairs. High-field pulsars form a population that is distinct from ordinary pulsars due to their excess luminosities.

Hitomi, formerly known as ASTRO-H, was the sixth Japanese x-ray astronomy satellite and was developed under extensive international collaboration between Japan and the United States with European and Canadian participation.¹ The soft x-ray spectrometer (SXS)^2,3 was a cryogenic high-resolution x-ray spectrometer onboard Hitomi, utilizing an x-ray microcalorimeter array.⁴ Operated at 50 mK, it was designed to achieve a resolving power of 1000 or larger at 6 keV. The detector array was cooled to 50 mK using a three-stage adiabatic demagnetization refrigerator (ADR).⁵ The detector assembly (DA)⁶ and the ADR composed a modular unit called a calorimeter spectrometer insert (CSI). The cooling chain from room temperature to the ADR heat sink consisted of two-stage Stirling-cycle (2ST) cryocoolers,⁷ a ${}^4\mathrm{He}$ Joule–Thomson (JT) cryocooler,⁷ and superfluid liquid helium (LHe). The lifetime requirement for nominal operation was 3 years (when launched with a minimum of 30 L of LHe), and at least for 9 months in the case of failure of a single cryocooler. It was also designed to operate even when the LHe was depleted.^5,8 The CSI, the aperture assembly with optical/infrared blocking filters,⁹ and control electronics for the ADR and the filter heaters were developed by NASA and the University of Wisconsin, and JAXA was responsible for the cooling system from room temperature to the ADR heat sink and cryocooler drive electronics, which were manufactured by Sumitomo Heavy Industries, Ltd. (SHI), with contribution by ESA.

Hitomi was the sixth Japanese x-ray astronomy satellite¹ 2. 3. 4. 5. ^{– 6} developed to investigate a variety of astrophysical problems, including the formation mechanisms and dynamics of galaxy clusters, formation and evolution of supermassive black holes, and to uncover the behavior of matter under extreme conditions, such as high density, strong magnetic field, and strong gravity in the vicinity of neutron stars and black holes. To fulfill the above objectives, the Hitomi satellite carried the following instruments: a very high-energy-resolution soft x-ray spectrometer (SXS) covering the 0.3- to 12-keV band consisting of thin-foil x-ray optics [Soft X-ray Telescope (SXT)]⁷ and a microcalorimeter array (SXS);⁸ a soft x-ray imaging spectrometer sensitive in the 0.4- to 12-keV band, consisting of an SXT focusing x-rays onto CCD sensors (Soft x-ray Imager);⁹ a hard x-ray imaging spectrometer, sensitive in the 3- to 80-keV band, consisting of multilayer-coated, focusing hard x-ray mirrors (Hard X-ray Telescope)¹⁰ and silicon (Si) and cadmium telluride (CdTe) cross-strip sensors [Hard X-ray Imager (HXI)];¹¹ 12. 13. 14. 15. 16. 17. 18. 19. ^{– 20} and a soft gamma-ray spectrometer covering the 60- to 600-keV band, utilizing multilayer semiconductor Compton cameras with an active shield [Soft Gamma-ray Detector (SGD)].¹¹ 12. 13. ^{– 14,21,22} 23. 24. ^{– 25}