Abstract
The X-ray pulsar 1A 0535+262 exhibited a giant outburst in 2020, offering us a unique opportunity for X-ray polarimetry of an accreting pulsar in the supercritical state. Measurement with ...PolarLight yielded a nondetection in 3–8 keV; the 99% upper limit of the polarization fraction (PF) is found to be 0.34 averaged over spin phases, or 0.51 based on the rotating vector model. No useful constraint can be placed with phase-resolved polarimetry. These upper limits are lower than a previous theoretical prediction of 0.6–0.8, but consistent with those found in other accreting pulsars, like Her X-1, Cen X-3, 4U 1626−67, and GRO J1008−57, which were in the subcritical state, or at least not confidently in the supercritical state, during the polarization measurements. Our results suggest that the relatively low PF seen in accreting pulsars cannot be attributed to the source not being in the supercritical state, but could be a general feature.
The gas pixel detector (GPD) is designed and developed for high-sensitivity astronomical X-ray polarimetry, which is a new window about to open in a few years. Due to the small mass, low power, and ...compact geometry of the GPD, we propose a CubeSat mission Polarimeter Light (PolarLight) to demonstrate and test the technology directly in space. There is no optics but a collimator to constrain the field of view to 2.3 degrees. Filled with pure dimethyl ether (DME) at 0.8 atm and sealed by a beryllium window of 100
μ
m thick, with a sensitive area of about 1.4 mm by 1.4 mm, PolarLight allows us to observe the brightest X-ray sources on the sky, with a count rate of, e.g., ∼0.2 counts s
− 1
from the Crab nebula. The PolarLight is 1U in size and mounted in a 6U CubeSat, which was launched into a low Earth Sun-synchronous orbit on October 29, 2018, and is currently under test. More launches with improved designs are planned in 2019. These tests will help increase the technology readiness for future missions such as the enhanced X-ray Timing and Polarimetry (eXTP), better understand the orbital background, and may help constrain the physics with observations of the brightest objects.
Gaseous polarimetry applies in soft X-ray polarization detection, typically like the Gas Pixel detector (GPD) and the micropattern time projection chamber (TPC). The polarization phase angles resolve ...from photoelectron track trajectories for both of them. The photoelectron track images derived from read-out pixels or strips are the key to determining the performance of the polarimetry. Gaseous X-ray polarimetry is regularly optimized and diagnosed by the Geant4 associated with Garfield++ and finite element analysis software (full simulation). Although the results are more realistic by the full simulation, the sample generation is low efficiency, especially for large gain values of the multiplier stage. The 2-D photoelectron track images regularly generate by a gaussian smear of initial ionization electrons at present. However, discrepancies exist in the modulation curves, resulting in fuzzy diagnoses and optimization of polarimetry. In this study, a fast simulation framework is constructed, the reconstruction of polarization phase angles and energy spectrum of the samples obtained from the fast simulation are consistent with those from experiments and the full simulation, and the deviation of modulation factor is within 2% for both situations of the GPD and TPC. In addition, the fast simulation greatly improves the sample generation efficiency. These results illustrate the fast simulation model is relative accuracy and high efficiency, which provides a useful tool for gaseous X-ray polarimetry.
ABSTRACT Dead time is a common instrumental effect of X-ray detectors, which would alter the behaviour of timing properties of astronomical signals, such as distorting the shape of power density ...spectra (PDS), affecting the root-mean-square of potential quasi-periodic oscillation signals, etc. We revisit the effects of the dead time of Medium Energy X-ray telescope (ME) onboard Insight-HXMT based on the simulation of electronic read-out mechanism that causes the dead time and the real data. We investigate dead time effects on the pulse profile as well as the quasi–periodic oscillation (QPO) signals. The dead time coefficient suggests a linear correlation with the observed count rate in each phase bin of the pulse profile according to the simulation of periodic signal as well as the real data observed on Swift J0243.6+6124. The Fourier-amplitude-difference (FAD) method could well recover the intrinsic shape of the observed PDS in the case that the PDS is from two identical detectors. We apply this technique on ME, by splitting the nine FPGA modules into two groups. The results indicate that the FAD technique suits the case when two groups of detectors are not largely different; and the recovered PDS of Sco X-1 observed by ME slightly enhances the significance of the previously known QPO signal, meanwhile the root-mean-square of QPO is significantly improved. We provide the FAD correction tool implemented in HXMTDAS for users in the future to better analyse QPO signals.
Abstract
We report the detection of X-ray polarization in the neutron-star low-mass X-ray binary Scorpius (Sco) X-1 with PolarLight. The result is energy-dependent, with a nondetection in 3–4 keV but ...a 4
σ
detection in 4–8 keV; it is also flux-dependent in the 4–8 keV band, with a nondetection when the source displays low fluxes but a 5
σ
detection during high fluxes, in which case we obtain a polarization fraction of 0.043 ± 0.008 and a polarization angle of 52.°6 ± 5.°4. This confirms a previous marginal detection with OSO-8 in the 1970s and marks Sco X-1 as the second astrophysical source with a significant polarization measurement in the keV band. The measured polarization angle is in line with the jet orientation of the source on the sky plane (54°), which is supposedly the symmetry axis of the system. Combining previous spectral analysis, our measurements suggest that an optically thin corona is located in the transition layer under the highest accretion rates, and disfavor the extended accretion disk corona model.
The Medium Energy X-ray telescope (ME) is one of the three main telescopes on board the
Insight
hard X-ray modulation telescope (
Insight-
HXMT) astronomy satellite. ME contains 1728 pixels of Si-PIN ...detectors sensitive in 5–30 keV with a total geometrical area of 952 cm
2
. The application specific integrated circuit (ASIC) chip, VA32TA6, is used to achieve low power consumption and low readout noise. The collimators define three kinds of field of views (FOVs) for the telescope, 1°×4°, 4°×4°, and blocked ones. Combination of such FOVs can be used to estimate the in-orbit X-ray and particle background components. The energy resolution of ME is ~3 keV at 17.8 keV (FWHM) and the time resolution is 255 μs. In this paper, we introduce the design and performance of ME.
PDF
