On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.
X-ray polarimetry in astronomy has not been exploited well, despite its importance. The recent innovation of instruments is changing this situation. We focus on a complementary MOS (CMOS) pixel detector with small pixel size and employ it as an x-ray photoelectron tracking polarimeter. The CMOS detector we employ is developed by GPixel Inc., and has a pixel size of 2.5 µm × 2.5 µm. Although it is designed for visible light, we succeed in detecting x-ray photons with an energy resolution of 176 eV (FWHM) at 5.9 keV at room temperature and the atmospheric condition. We measure the x-ray detection efficiency and polarimetry sensitivity by irradiating polarized monochromatic x-rays at BL20B2 in SPring-8, the synchrotron radiation facility in Japan. We obtain modulation factors of 7.63% ± 0.07% and 15.5% ± 0.4% at 12.4 keV and 24.8 keV, respectively. It demonstrates that this sensor can be used as an x-ray imaging spectrometer and polarimeter with the highest spatial resolution ever tested.
RX J1856.5−3754 is the brightest and nearest (∼ 120 pc) source among thermally emitting isolated neutron stars. Its spectra observed with XMM-Newton and Chandra satellites are wellfitted with the two-temperature (kT ∞ ∼ 32 and 63 eV) blackbody model. Fitting ten sets of the data from Suzaku XIS0, XIS1, XIS3 and XMM-Newton EPIC-pn with the two-temperature blackbody model, we discover an excess emission, 16-26% in 0.8-1.2 keV. We examine possible causes of this keV-X-ray excess; uncertainty in the background, pile up of the low energy photons and confusion of other sources. None of them succeeds in explaining the keV-X-ray excess observed with different instruments. We thus consider this keV-X-ray excess is most likely originated in RX J1856.5−3754. However, it is difficult to constrain the spectral shape of the keV-X-ray excess. The third blackbody component with kT ∞ = 137 −0.8 can reproduce the keV-X-ray excess. We also search for the periodicity of 0.8-1.2 keV data, since 7.055 s pulsation is discovered from 0.15-1.2 keV band in the XMM Newton EPIC-pn data (∼1.5%). We only obtain the upper limit of pulsed fraction < 3% in the keV-X-ray excess. We shortly discuss the possible origin of the keV-X-ray excess, such as synchrotron radiation and Comptonization of blackbody 1 photons.
Strongly magnetized isolated neutron stars (NSs) are categorized into two families, according mainly to their magnetic field strength. The one with a higher magnetic field of 10 14 -10 15 G is called magnetar, characterized with repeated short bursts, and the other is X-ray isolated neutron star (XINS) with 10 13 G. Both magnetars and XINSs show thermal emission in X-rays, but it has been considered that the thermal spectrum of magnetars is reproduced with a twotemperature blackbody (2BB), while that of XINSs shows only a single-temperature blackbody (1BB) and the temperature is lower than that of magnetars. On the basis of the magnetic field and temperature, it is often speculated that XINSs may be old and cooled magnetars.Here we report that all the seven known XINSs show a high-energy component in addition to the 1BB model. Analyzing all the XMM-Newton data of the XINSs with the highest statistics ever achieved, we find that their X-ray spectra are all reproduced with a 2BB model, similar to magnetars. Their emission radii and temperature ratios are also similar to those of magnetars except for two XINSs, which show significantly smaller radii than the others. The remarkable similarity in the X-ray spectra between XINSs and magnetars suggests that their origins of the 1 emission are also the same. The lower temperature in XINSs can be explained if XINSs are older than magnetars. Therefore, these results are the observational indication that supports the standard hypothesis on the classification of highly-magnetized NSs.
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