Abstract
We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745−248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and ...significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 × 10−3 ph cm−2 s−1. The 3–20 keV spectrum was reproduced successfully by a power-law continuum with narrow and broad (2.7 keV in full width at half maximum) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV, and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick material, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 hr before the RXTE observation with a line flux upper limit of 1.5 × 10−3 ph cm−2 s−1. The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall-back ionized burst wind and an accretion disk, at a distance of ∼60 km from the neutron star. If this interpretation is correct, the results provide new information on nuclear burning processes during thermonuclear X-ray bursts.
X-ray/Hα scaling relationships in stellar flares Kawai, Hiroki; Tsuboi, Yohko; Iwakiri, Wataru B ...
Publications of the Astronomical Society of Japan,
04/2022, Letnik:
74, Številka:
2
Journal Article
Recenzirano
Abstract
We report on the results of our simultaneous observations of three large stellar flares with soft X-rays (SXRs) and an Hα emission line from two binary systems of RS CVn type. The energies ...released in the X-ray and Hα emissions during the flares were 1036–1038 and 1035–1037 erg, respectively. This renders the set of the observations as the first successful simultaneous X-ray/Hα observations of the stellar flares with energies above 1035 erg; although the coverage of the Hα observations was limited, with $\sim\! 10\%$ of the e-folding time in the decay phase of the flares, that of the SXR ones was complete. Combining the obtained physical parameters and those in literature for solar and stellar flares, we obtained a good proportional relation between the emitted energies of X-ray and Hα emissions for a flare energy range of 1029–1038 erg. The ratio of the Hα-line to bolometric X-ray emissions was ∼0.1, where the latter was estimated by converting the observed SXR emission to that in the 0.1–100 keV band according to the best-fitting thin thermal model. We also found that the e-folding times of the SXR and Hα light curves in the decaying phase of a flare are in agreement for a time range of 1–104 s. Even very large stellar flares with energies of six orders of magnitude larger than the most energetic solar flares follow the same scaling relationships with solar and much less energetic stellar flares. This fact suggests that their physical parameters can be estimated on the basis of the known physics of solar and stellar flares.
We present an analysis of the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626−67 observed with Suzaku and Nuclear Spectroscopic Telescope Array (NuSTAR) during a spin-up ...state. The pulsar, which experienced a torque reversal to spin-up in 2008, has a spin period of ∼7.7 s. Comparing the phase-averaged spectra obtained with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape changed between the two observations: the 3-10 keV flux increased by ∼5%, while the 30-60 keV flux decreased significantly by ∼35%. Phase-averaged and phase-resolved spectral analysis shows that the continuum spectrum observed by NuSTAR is well described by an empirical negative and positive power law times exponential continuum with an added broad Gaussian emission component around the spectral peak at ∼20 keV. Taken together with the observed value obtained from the Fermi/gamma-ray burst monitor data, we conclude that the spectral change between the Suzaku and NuSTAR observations was likely caused by an increase in the accretion rate. We also report the possible detection of asymmetry in the profile of the fundamental cyclotron line. Furthermore, we present a study of the energy-resolved pulse profiles using a new relativistic ray tracing code, where we perform a simultaneous fit to the pulse profiles assuming a two-column geometry with a mixed pencil- and fan-beam emission pattern. The resulting pulse profile decompositions enable us to obtain geometrical parameters of accretion columns (inclination, azimuthal and polar angles) and a fiducial set of beam patterns. This information is important to validate the theoretical predictions from radiation transfer in a strong magnetic field.
This paper presents a data processing algorithm with machine learning for polarization extraction and event selection applied to photoelectron track images taken with X-ray polarimeters. The method ...uses a convolutional neural network (CNN) classification to predict the azimuthal angle and 2-D position of the initial photoelectron emission from a 2-D track image projected along the X-ray incident direction. Two CNN models are demonstrated with data sets generated by a Monte Carlo simulation: one has a commonly used loss function calculated by the cross entropy and the other has an additional loss term to penalize nonuniformity for an unpolarized modulation curve based on the H-test, which is used for periodic signal search in X-ray/γ-ray astronomy. The modulation curve calculated by the former model with unpolarized data has several irregular features, which can be canceled out by unfolding the angular response or simulating the detector rotation. On the other hand, the latter model can predict a flat modulation curve with a residual systematic modulation down to ≲1%. Both models show almost the same modulation factors and position accuracy of less than 2 pixel (or 240μm) for all four test energies of 2.7, 4.5, 6.4, and 8.0 keV. In addition, event selection is performed based on probabilities from the CNN to maximize the polarization sensitivity considering a trade-off between the modulation factor and signal acceptance. The developed method with machine learning improves the polarization sensitivity by 10%–20%, compared to that determined with the image moment method developed previously.
We present a data processing algorithm for angular reconstruction and event selection applied to 2-D photoelectron track images from X-ray polarimeters. The method reconstructs the initial emission ...angle of a photoelectron from the initial portion of the track, which is obtained by continuously cutting a track until the image moments or number of pixels fall below tunable thresholds. In addition, event selection which rejects round tracks quantified with eccentricity and circularity is performed so that polarimetry sensitivity considering a trade-off between the modulation factor and signal acceptance is maximized. The modulation factors with applying track selection are 26.6±0.4, 46.1±0.4, 62.3±0.4, and 61.8±0.3% at 2.7, 4.5, 6.4, and 8.0 keV, respectively, using the same data previously analyzed by Iwakiri et al. (2016), where the corresponding numbers are 26.9±0.4, 43.4±0.4, 54.4±0.3, and 59.1±0.3%. The method improves polarimeter sensitivity by 5%–10% at the high energy end of the band previously presented (Iwakiri et al. 2016).
Abstract
We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image, ...and Neutron Star Interior Composition Explorer Mission. This energetic GRB was located relatively nearby (
z
= 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (
b
= 4.°3) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography, we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (≳10 kpc). We present analysis of the light curves and spectra at X-ray and UV–optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at
T
0
+ 4.5 ks than that from any previous GRB observed by Swift. In its rest frame, GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10
4
long GRBs were as energetic as GRB 221009A; such a large
E
γ
,iso
implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ≲1 per 1000 yr—making this a truly remarkable opportunity unlikely to be repeated in our lifetime.
The 2.1-s anomalous X-ray pulsar 1E 1547.0−5408 exhibited an X-ray outburst on 2009 January 22, emitting a large number of short bursts. The wide-band all-sky monitor (WAM) on-board Suzaku detected ...at least 254 bursts in the 160 keV–6.2 MeV band over the period of January 22 00:57–17:02 UT from the direction of 1E 1547.0−5408. One of these bursts, which occurred at 06:45:13, produced the brightest fluence in the 0.5–6.2 MeV range, with an averaged 0.16–6.2 MeV flux and extrapolated 25 keV–2 MeV fluence of about 1 × 10−5 erg cm−2 s−1 and about 3 × 10−4 erg cm−2, respectively. After pile-up corrections, the time-resolved WAM spectra of this burst were well-fitted in the 0.16–6.2 MeV range by two-component models; specifically, a blackbody plus an optically thin thermal bremsstrahlung or a combination of a blackbody and a power-law component with an exponential cut-off. These results are compared with previous works reporting the persistent emission and weaker short bursts followed by the same outburst.
We present a data processing algorithm for angular reconstruction and event selection applied to 2-D photoelectron track images from X-ray polarimeters. The method reconstructs the initial emission ...angle of a photoelectron from the initial portion of the track, which is obtained by continuously cutting a track until the image moments or number of pixels fall below tunable thresholds. In addition, event selection which rejects round tracks quantified with eccentricity and circularity is performed so that polarimetry sensitivity considering a trade-off between the modulation factor and signal acceptance is maximized. The modulation factors with applying track selection are 26.6 ± 0.4, 46.1 ± 0.4, 62.3 ± 0.4, and 61.8 ± 0.3% at 2.7, 4.5, 6.4, and 8.0 keV, respectively, using the same data previously analyzed by Iwakiri et al. (2016), where the corresponding numbers are 26.9±0.4, 43.4±0.4, 54.4±0.3, and 59.1 ± 0.3%. The method improves polarimeter sensitivity by 5%–10% at the high energy end of the band previously presented (Iwakiri et al. 2016).
2022PASJ...74..477K We report on the results of our simultaneous observations of three large
stellar flares with soft X-rays (SXRs) and an H$\mathrm{\alpha}$ emission line
from two binary systems of ...RS CVn type. The energies released in the X-ray and
H$\mathrm{\alpha}$ emissions during the flares were $10^{36}$--$10^{38}$ and
$10^{35}$--$10^{37}$ erg, respectively. It renders the set of the observations
as the first successful simultaneous X-ray/H$\mathrm{\alpha}$ observations of
the stellar flares with energies above $10^{35}$ erg; although the coverage of
the H$\mathrm{\alpha}$ observations of the stellar flares with energies above
$10^{35}$ erg; although the coverage of the H$\mathrm{\alpha}$ observations was
limited, with $\sim$10\% of the $e$-folding time in the decay phase of the
flares, that of the SXR ones was complete. Combining the obtained physical
parameters and those in literature for solar and stellar flares, we obtained a
good proportional relation between the emitted energies of X-ray and
H$\mathrm{\alpha}$ emissions for a flare energy range of $10^{29}$--$10^{38}$
erg. The ratio of the H$\mathrm{\alpha}$-line to bolometric X-ray emissions was
$\sim$0.1, where the latter was estimated by converting the observed SXR
emission to that in the 0.1--100 keV band according to the best-fitting thin
thermal model. We also found that the $e$-folding times of the SXR and
H$\mathrm{\alpha}$ light curves in the decaying phase of a flare are in
agreement for a time range of $1$--$10^4$~s. Even very large stellar flares
with energies of six orders of magnitude larger than the most energetic solar
flares follow the same scaling relationships with solar and much less energetic
stellar flares. This fact suggests that their physical parameters can be
estimated on the basis of the known physics of solar and stellar flares.
We discover an unidentified strong emission feature in the X-ray spectrum of EXO 1745\(-\)248 obtained by RXTE at 40 hr after the peak of a superburst. The structure was centered at 6.6 keV and ...significantly broadened with a large equivalent width of 4.3 keV, corresponding to a line photon flux of 4.7 \(\times\) 10\(^{-3}\) ph cm\(^{-2}\) s\(^{-1}\). The 3-20 keV spectrum was reproduced successfully by a power law continuum with narrow and broad (2.7 keV in FWHM) Gaussian emission components. Alternatively, the feature can be described by four narrow Gaussians, centered at 5.5 keV, 6.5 keV, 7.5 keV and 8.6 keV. Considering the strength and shape of the feature, it is unlikely to have originated from reflection of the continuum X-rays by some optically thick materials, such as an accretion disk. Moreover, the intensity of the emission structure decreased significantly with an exponential time scale of 1 hr. The feature was not detected in an INTEGRAL observation performed 10 h before the RXTE observation with a line flux upper limit of 1.5 \(\times\) 10\(^{-3}\) ph cm\(^{-2}\) s\(^{-1}\). The observed emission structure is consistent with gravitationally redshifted charge exchange emission from Ti, Cr, Fe, and Co. We suggest that the emission results from a charge exchange interaction between a highly metal-enriched fall back ionized burst wind and an accretion disk, at a distance of \(\sim\)60 km from the neutron star. If this interpretation is correct, the results provide new information on the understanding of nuclear burning processes during thermonuclear X-ray bursts.
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