THE NuSTAR X-RAY SPECTRUM OF HERCULES X-1: A RADIATION-DOMINATED RADIATIVE SHOCK

Wolff, M. T.; Becker, Peter A.; Gottlieb, Amy; Fürst, Felix; Hemphill, Paul; Marcu-Cheatham, Diana M.; Pottschmidt, K.; Schwarm, Fritz-Walter; Wilms, J.; Wood, K. S.

doi:10.3847/0004-637x/831/2/194

Cited by 58 publications

(82 citation statements)

References 39 publications

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“…It is also close the application of their model to the spectrum of Her X-1 (Wolff et al 2016). Farinelli et al (2016) applied an advanced version of the compmag model to data of Cen X-3, 4U 0115+63, and Her X-1 and found smaller electron temperatures of 0.8-3 keV which they explained being due to the inclusion of second order bulk Comptonization in the RTE.…”

Section: Continuum Variation and Modelingmentioning

confidence: 85%

Looking at A 0535+26 at low luminosities with NuSTAR

Ballhausen

Pottschmidt

Fürst

et al. 2017

A&A

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We report on two NuSTAR observations of the high-mass X-ray binary A 0535+26 taken toward the end of its normal 2015 outburst at very low 3-50 keV luminosities of ∼1.4 × 10 36 erg s −1 and ∼5 × 10 35 erg s −1 , which are complemented by nine Swift observations. The data clearly confirm indications seen in earlier data that the source's spectral shape softens as it becomes fainter. The smooth, exponential rollover at high energies present in the first observation evolves to a much more abrupt steepening of the spectrum at 20-30 keV. The continuum evolution can be well described with emission from a magnetized accretion column, modeled using the compmag model modified by an additional Gaussian emission component for the fainter observation. Between the two observations, the optical depth changes from 0.75 ± 0.04 to 0.56−0.04 , the electron temperature remains constant, and there is an indication that the column decreases in radius. Since the energy resolved pulse profiles remain virtually unchanged in shape between the two observations, the emission properties of the accretion column, however, reflect the same accretion regime. This conclusion is also confirmed by our result that the energy of the cyclotron resonant scattering feature (CRSF) at ∼45 keV is independent of the luminosity, implying that the magnetic field in the region in which the observed radiation is produced is the same in both observations. Finally, we also constrain the evolution of the continuum parameters with rotational phase of the neutron star. The width of the CRSF could only be constrained for the brighter observation. Based on Monte-Carlo simulations of CRSF formation in single accretion columns, its pulse phase dependence supports a simplified fan beam emission pattern. The evolution of the CRSF width is very similar to that of the CRSF depth, which is, however, in disagreement with expectations.

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Section: Continuum Variation and Modelingmentioning

confidence: 85%

Looking at A 0535+26 at low luminosities with NuSTAR

Ballhausen

Pottschmidt

Fürst

et al. 2017

A&A

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“…They consider either an ionized partial covering model or an additional broad line at 6.5 keV but are unable to distinguish between these alternatives. Fürst et al (2013) and Wolff et al (2016) fit both a narrow and a broad line to NuSTAR+Suzaku and NuSTAR-only spectra, respectively. Once again the broad line is centered around ∼ 6.5 keV.…”

Section: A11 Her X−1mentioning

confidence: 99%

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

Tzanavaris

Yaqoob

2018

ApJ

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The narrow, neutral Fe Kα fluorescence emission line in X-ray binaries (XRBs) is a powerful probe of the geometry, kinematics and Fe abundance of matter around the accretion flow. In a recent study it has been claimed, using Chandra High-Energy Transmission Grating (HETG) spectra for a sample of XRBs, that the circumnuclear material is consistent with a solar-abundance, uniform, spherical distribution. It was also claimed that the Fe Kα line was unresolved in all cases by the HETG. However, these conclusions were based on ad hoc models that did not attempt to relate the global column density to the Fe Kα line emission. We revisit the sample and test a self-consistent model of a uniform, spherical X-ray reprocessor against HETG spectra from 56 observations of 14 Galactic XRBs. We find that the model is ruled out in 13/14 sources because a variable Fe abundance is required. In 2 sources a spherical distribution is viable but with non-solar Fe abundance. We also applied a solar-abundance Compton-thick reflection model, which can account for the spectra that are inconsistent with a spherical model, but spectra with a broader bandpass are required to better constrain model parameters. We also robustly measured the velocity width of the Fe Kα line and found full width half maximum values up to ∼ 5000 km s −1 . Only in some spectra was the Fe Kα line unresolved by the HETG.

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“…Several studies have, however, been being done on this low accretion rate case by several authors (e.g. Becker & Wolff 2007, Wolff et al 2016, and references therein) since the pioneer works done by Davidson (1973) andBasko &Sunyaev (1975).…”

Section: Summary and Discussionmentioning

confidence: 99%

“…As seen from the top panels of figures 4 and 5, the logarithmic slope of ε against r, ν, is much larger than unity, and thus a single blackbody spectrum is expected in thisṀ range within the simplified arguments in this study. Since establishment of the radiation field is thought to be insufficient behind the shock as in figure 3, however, Comptonization of bremsstrahlung photons and blackbody photons produced around there should be taken into account to consider properties of X-ray spectrum from there, which was already studied by Becker and Wolff (2007) and Wolff et al (2016). We do not go into detail on this lowṀ range here.…”

Section: Properties Of X-ray Spectrummentioning

confidence: 99%

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X-ray emissions from magnetic polar regions of neutron stars

Inoue

2019

Publications of the Astronomical Society of Japan

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Structures of X-ray emitting magnetic polar regions on neutron stars in X-ray pulsars are studied in a range of the accretion rate, 10 17 g s −1 ∼ 10 18 g s −1 . It is shown that a thin but tall, radiation energy dominated, X-ray emitting polar cone appears at each of the polar regions.The height of the polar cone is several times as large as the neutron star radius. The energy gain due to the gravity of the neutron star in the polar cone exceeds the energy loss due to photon diffusion in the azimuthal direction of the cone, and a significant amount of energy is advected to the neutron star surface. Then, the radiation energy carried with the flow should become so large for the radiation pressure to overcome the magnetic pressure at the bottom of the cone. As a result, the matter should expand in the tangential direction along the neutron star surface, dragging the magnetic lines of force, and form a mound-like structure. The advected energy to the bottom of the cone should finally be radiated away from the surface of the polar mound and the matter should be settled on the neutron star surface there. From such configurations, we can expect an X-ray spectrum composed of a multi-color blackbody spectrum from the polar cone region and a quasi-single blackbody spectrum from the polar mound region. These spectral properties agree with observations. A combination of a fairly sharp pencil beam and a broad fan beam is expected from the polar cone region, while a broad pencil beam is expected from the polar mound region. With these X-ray beam properties, basic patterns of pulse profiles of X-ray pulsars can be explained too.

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THE NuSTAR X-RAY SPECTRUM OF HERCULES X-1: A RADIATION-DOMINATED RADIATIVE SHOCK

Cited by 58 publications

References 39 publications

Looking at A 0535+26 at low luminosities with NuSTAR

Looking at A 0535+26 at low luminosities with NuSTAR

New Constraints on the Geometry and Kinematics of Matter Surrounding the Accretion Flow in X-Ray Binaries from Chandra High-energy Transmission Grating X-Ray Spectroscopy

X-ray emissions from magnetic polar regions of neutron stars

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