[en] Continuous-wave laser properties of Nd : GYSGG crystal based on the transition from ⁴F_3/2 to ⁴I_13/2 are investigated. With different output couplers, single or multiple wavelength lasers operating at 1321, 1336, 1404 and 1424 nm are obtained, which provide new options in these wavelength bands. In the 1.3 µm region, the maximum output power is 2.5 W, corresponding to the conversion efficiency of 18.5%. In the 1.4 µm region, the maximum output power at 1424 nm is 707 mW, corresponding to the conversion efficiency of 6.8%. The output power and conversion efficiency can be improved with better anti-reflection coatings for the laser crystal. (paper)

[en] We report on the observation of the accreting pulsar GRO J1008-57 performed by Insight-Hard X-ray Modulation Telescope (HXMT) at the peak of the source’s 2017 outburst. Pulsations are detected with a spin period of 93.283(1) s. The pulse profile shows double peaks at soft X-rays, and only one peak above 20 keV. The spectrum is well described by the phenomenological models of X-ray pulsars. A cyclotron resonant scattering feature (CRSF) is detected with very high statistical significance at a centroid energy of $E_{\mathrm{c}\mathrm{y}\mathrm{c}}={90.32}_{-<!--\; ???\; -->0.28}^{+0.32}$ keV, for the reference continuum and line models, HIGHECUT and GABS, respectively. Detection is very robust with respect to different continuum models. The line energy is significantly higher than what is suggested from previous observations, which provided very marginal evidence for the line. This establishes a new record for the centroid energy of a fundamental CRSF observed in accreting pulsars. We also discuss the accretion regime of the source during the Insight-HXMT observation.

[en] We present the analysis of the brightest flare that was recorded in the Insight-HMXT data set in a broad energy range (2–200 keV) from the microquasar GRS 1915+105 during an unusual low-luminosity state. This flare was detected by Insight-HXMT among a series of flares during 2019 June 2 UTC 16:37:06–20:11:36 with a 2–200 keV luminosity of 3.4–7.27 × 10³⁸ erg s⁻¹. Basing on the broadband spectral analysis, we find that the flare spectrum shows different behaviors during bright and faint epochs. The spectrum of the flare can be fitted with a model dominated by a power-law component. Additional components show up in the bright epoch with a hard tail and in the faint epoch with an absorption line of ∼6.78 keV. The reflection component of the latter is consistent with an inner disk radius ∼five times larger than that of the former. These results on the giant flare during the “unusual” low-luminosity state of GRS 1915+105 may suggest that the source experiences a possible fast transition from a jet-dominated state to a wind-dominated state. We speculate that the evolving accretion disk and the large-scale magnetic field may play important roles in this peculiar huge flare.

[en] We performed the broadband (1–100 keV) spectral analysis of the first Galactic Be ultraluminous X-ray pulsar (BeULX) Swift J0243.6+6124 observed by Insight-HXMT during the 2017−2018 outburst. The results show spectral transitions at two typical luminosities, roughly consistently with those reported previously via pure timing analysis. We find that the spectrum evolves and becomes softer and has higher cutoff energies until the luminosity reaches L ₁ (∼1.5  ×  10³⁸ erg s⁻¹). Afterwards the spectrum becomes harder with lower cutoff energies until the luminosity increases to L ₂ (∼4.4  × 10³⁸ erg s⁻¹), around which the second spectral transition occurs. Beyond L ₂, the spectrum softens again and has larger cutoff energies. Similar behaviors were observed previously in other high-mass X-ray binary systems (HMXBs), especially for the second transition at higher luminosities, which is believed to have a correlation with the magnetic field of the harbored neutron star. Accordingly, we speculate that Swift J0243.6+6124 owns a neutron star with magnetic field strength >10¹³ G. The spectral transition at around L ₁ of Swift J0243.6+6124 is first observed thoroughly for any HMXB outburst characterized by strong evolution of the thermal component: the temperature of the blackbody drops sharply accompanied by a sudden increase of the blackbody radius. These spectral transitions can in principle be understood in a general scenario of balancing the emission patterns between the pencil and the fan beams at the magnetic pole, for which the extreme brightness of Swift J0243.6+6124 may provide an almost unique lab to probe the details.

[en] We report the timing and spectral analyses of the type-II X-ray bursts from the rapid burster (MXB 1730–335) observed by the Hard X-ray Modulation Telescope (Insight-HXMT) and Swift/X-Ray Telescope (XRT). By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging behind the soft X-rays by 3 s. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be nonthermal, thanks to the broadband coverage of Insight-HXMT. These findings provide new insights into the type-II bursts and require a temporally visible corona for possible interpretation.

[en] Glitches correspond to sudden jumps of rotation frequency (ν) and its derivative ($\stackrel{\dot{}<!--\; ??\; -->}{\mathrm{\nu <!--\; ??\; -->}}$) of pulsars, the origin of which remains not well understood yet, partly because the jump processes of most glitches are not well time-resolved. There are three large glitches of the Crab pulsar, detected in 1989, 1996, and 2017, which were found to have delayed spin-up processes before the normal recovery processes. Here we report two additional glitches of this pulsar that occurred in 2004 and 2011 for which we discovered delayed spin-up processes, and present refined parameters of the largest glitch, which occurred in 2017. The initial rising time of the glitch is determined as <0.48 hr. The two glitches that occurred in 2004 and 2011 had delayed spin-up time scales (τ ₁) of 1.7 ± 0.8 days and 1.6 ± 0.4 days, respectively. We also carried out a statistical study of these five glitches with observed spin-up processes. We find that the Δν versus $|\mathrm{\Delta <!--\; ??\; -->}\stackrel{\dot{}<!--\; ??\; -->}{\mathrm{\nu <!--\; ??\; -->}}|$ relation of these five glitches is similar to those with no detected delayed spin-up process, indicating that they are similar to the others in nature except that they have larger amplitudes. For these five glitches, the amplitudes of the delayed spin-up process ($|\mathrm{\Delta <!--\; ??\; -->}{\mathrm{\nu <!--\; ??\; -->}}_{\mathrm{d}1}|$) and recovery process (Δν _d2), their time scales (τ ₁, τ ₂), and permanent changes in spin frequency (Δν _p) and total frequency step (Δν _g) have positive correlations. From these correlations, we suggest that the delayed spin-up processes are common for all glitches, but are too short and thus difficult to be detected for most glitches.

[en] We present a detailed spectral-timing analysis of the Kilohertz quasiperiodic oscillations (kHz QPOs) in Sco X-1 using the data of the Rossi X-ray Timing Explorer (RXTE) and the Hard X-ray Modulation Telescope (Insight-HXMT). The energy band with detectable kHz QPOs is studied for the first time: on the horizontal branch, it is ∼6.89–24.01 and ∼8.68–21.78 keV for the upper and lower kHz QPOs, respectively, detected by the RXTE, and ∼9–27.5 keV for the upper kHz QPOs by the Insight-HXMT; on the lower normal branch, the energy band is narrower. The fractional root mean square (rms) of the kHz QPOs increases with energy at a lower energy, reaches a plateau at about 16 and 20 keV for the lower and upper peaks, and then levels off though with a large uncertainty. The simulation of the deadtime effect of RXTE/PCA shows that the deadtime does not affect much the search of the kHz QPOs but makes the rms amplitude underestimated. No significant QPO is detected below ∼6 keV as shown by the RXTE data, implying that the kHz QPOs do not originate from the blackbody emission of the accretion disk and neutron star surface. In addition, with the combined analysis of the energy spectra and the absolute rms spectra of kHz QPOs, we suggest that the kHz QPOs in Sco X-1 originate from the Comptonization of the inner part of the transition layer, where the rotation sets the frequency and the inward bulk motion makes the spectrum harder.

[en] We report the energy-resolved broadband timing analysis of the black hole X-ray transient MAXI J1631-479 during its 2019 outburst from February 11 to April 9, using data from the Insight−Hard X-ray Modulation Telescope (Insight-HXMT), which caught the source from its hard-intermediate state to the soft state. Thanks to the large effective area of Insight-HXMT at high energies, we are able to present the energy dependence of fast variability up to ∼100 keV. Type-C quasi-periodic oscillations (QPOs) with a frequency varying between 4.9 and 6.5 Hz are observed in the 1–100 keV energy band. While the QPO fractional rms increases with photon energy from 1 keV to ∼10 keV and remains more or less constant from ∼10 keV to ∼100 keV, the rms of the flat-top noise first increases from 1 keV to ∼8 keV and then drops to less than 0.1% above ∼30 keV. We suggest that the disappearance of the broadband variability above 30 keV could be caused by the nonthermal acceleration in the Comptonizing plasma. At the same time, the QPOs could be produced by the precession of either a small-scale jet or a hot inner flow model.