arXiv subjects

Tenyo Kawamura

Publications and source records attributed to Tenyo Kawamura.

5 recordsLinked to original sources

arXiv2024

Polarized X-rays Correlated with Short--Timescale Variability of Cygnus X-1

We systematically investigate the variability of polarized X-rays on a timescale of a few seconds in the low/hard state of the black hole binary Cygnus X-1. The correlation between polarization degrees and angles with X-Ray intensity was analyzed using data collected by the Imaging X-ray Polarimetry Explorer (IXPE) in June 2022. Given that X-Ray variability in the low/hard state of Cygnus X-1 is non-periodic, flux peaks were aggregated to suppress statistical fluctuations. We divided the temporal profiles of these aggregated flux peaks into seven time segments and evaluated the polarization for each segment. The results reveal that the polarization degree was 4.6\%$\pm$1.2 and 5.3\%$\pm$1.2 before and after the peak, respectively, but decreased to 3.4\%$\pm$1.1 and 2.7\%$\pm$1.1 in the segments including and immediately following the peak. Furthermore, the polarization angle exhibited a slight shift from approximately 30$^{\circ}$ to $\sim$40$^{\circ}$ before and after the peak. These findings suggest that the accretion disk contracts with increasing X-Ray luminosity, and the closer proximity of the X-Ray emitting gas to the black hole may lead to reduced polarization.

astro-ph.HE↗

arXiv2023

The origin of long soft lags and the nature of the hard-intermediate state in black hole binaries

Fast variability of the X-ray corona in black hole binaries can produce a soft lag by reverberation, where the reprocessed thermalized disc photons lag behind the illuminating hard X-rays. This lag is small, and systematically decreases with increasing mass accretion rate towards the hard-soft transition, consistent with a decreasing truncation radius between the thin disc and X-ray hot inner flow. However, the soft lag suddenly increases dramatically just before the spectrum becomes disc-dominated (hard-intermediate state). Interpreting this as reverberation requires that the X-ray source distance from the disc increases dramatically, potentially consistent with switching to X-rays produced in the radio jet. However, this change in lag behaviour occurs without any clear change in hard X-ray spectrum, and before the plasmoid ejection event which might produce such a source (soft-intermediate state). Instead, we show how the soft lag can be interpreted in the context of propagation lags from mass accretion rate fluctuations. These normally produce hard lags, as the model has radial stratification, with fluctuations from larger radii modulating the harder spectra produced at smaller radii. However, all that is required to switch the sign is that the hottest Comptonized emission has seed photons which allow it to extend down in energy below the softer emission from the slower variable turbulent region from the inner edge of the disc. Our model connects the timing change to the spectral change, and gives a smooth transition of the X-ray source properties from the bright hard state to the disc-dominated states.

astro-ph.HE↗

arXiv2022

MAXI J1820+070 X-ray spectral-timing reveals the nature of the accretion flow in black hole binaries

Black hole X-ray binaries display significant stochastic variability on short time-scales (0.01-100 seconds), with a complex pattern of lags in correlated variability seen in different energy bands. This behaviour is generally interpreted in a model where slow fluctuations stirred up at large radii propagate down through the accretion flow, modulating faster fluctuations generated at smaller radii. Coupling this scenario with radially-stratified emission opens the way to measure the propagation time-scale from data, allowing direct tests of the accretion flow structure. We previously developed a model based on this picture and showed that it could fit NICER (0.5-10 keV) data from the brightest recent black hole transient, MAXI J1820+070. However, here we show it fails when extrapolated to higher energy variability data from Insight-HXMT. We extend our model so that the spectrum emitted at each radius changes shape in response to fluctuations (pivoting) rather than just changing normalisation. This gives the strong suppression of fractional variability as a function of energy seen in the data. The derived propagation time-scale is slower than predicted by a magnetically arrested disc (MAD), despite this system showing a strong jet. Our new model jointly fits the spectrum and variability up to 50 keV, though still cannot match all the data above this. Nonetheless, the good fit for 3-40 keV means the QPO can most easily be explained as an extrinsic modulation of the flow, such as produced in Lense-Thirring precession, rather than arising in an additional spectral-timing component such as the jet.

astro-ph.HE↗

arXiv2022

A full spectral-timing model to map the accretion flow in black hole binaries: the low/hard state of MAXI J1820+070

The nature and geometry of the accretion flow in the low/hard state of black hole binaries is currently controversial. While most properties are generally explained in the truncated disc/hot inner flow model, the detection of a broad residual around the iron line argues for strong relativistic effects from an untruncated disc. Since spectral fitting alone is somewhat degenerate, we combine it with the additional information in the fast X-ray variability and perform a full spectral-timing analysis for NICER and NuSTAR data on a bright low/hard state of MAXI J1820+070. We model the variability with propagating mass accretion rate fluctuations by combining two separate current insights: that the hot flow is spectrally inhomogeneous, and that there is a discontinuous jump in viscous time-scale between the hot flow and variable disc. Our model naturally gives the double-humped shape of the power spectra, and the increasing high-frequency variability with energy in the second hump. Including reflection and reprocessing from a disc truncated at a few tens of gravitational radii quantitatively reproduces the switch in the lag-frequency spectra, from hard lagging soft at low frequencies (propagation through the variable flow) to the soft lagging hard at the high frequencies (reverberation from the hard X-ray continuum illuminating the disc). The viscous time-scale of the hot flow is derived from the model, and we show how this can be used to observationally test ideas about the origin of the jet.

astro-ph.HE↗

arXiv2020

Development of a Low-noise Front-end ASIC for CdTe Detectors

We present our latest ASIC, which is used for the readout of Cadmium Telluride double-sided strip detectors (CdTe DSDs) and high spectroscopic imaging. It is implemented in a 0.35 um CMOS technology (X-Fab XH035), consists of 64 readout channels, and has a function that performs simultaneous AD conversion for each channel. The equivalent noise charge of 54.9 e- +/- 11.3 e- (rms) is measured without connecting the ASIC to any detectors. From the spectroscopy measurements using a CdTe single-sided strip detector, the energy resolution of 1.12 keV (FWHM) is obtained at 13.9 keV, and photons within the energy from 6.4 keV to 122.1 keV are detected. Based on the experimental results, we propose a new low-noise readout architecture making use of a slew-rate limited mode at the shaper followed by a peak detector circuit.

physics.ins-det↗