Engineering topics

Kühnel, M.

Publications and source records attributed to Kühnel, M..

3 records

NASA NTRS2017

The Clumpy Absorber in the High-Mass X-Ray Binary Vela X-1

Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase approximately 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium, and neon whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.

Grinberg, V.↗

NASA NTRS2017

Studying the Accretion Geometry of EXO 2030+375 at Luminosities Close to the Propeller Regime

The Be X-ray binary EXO2030+375 was in an extended low-luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations and optical spectroscopy with the Nordic Optical Telescope (NOT). We present a comprehensive spectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller e ect. The H data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV luminosity of 6:8 1035 erg s-1 (for a distance of 7.1 kpc), are nicely described by standard accreting pulsar models such as an absorbed power law with a high-energy cuto. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phaseresolved spectroscopy we find a peculiar variation of the photon index from 1.5 to 2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 1034 erg s-1 where the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the accretion being stopped by the propeller e ect and we only observe the neutron star surface cooling.

Fürst, F.↗

NASA NTRS2017

A Precessing Be Disc as a Possible Model for Occultation Events in GX 304-1

We report on the RXTE detection of a sudden increase in the absorption column density, NH, during the 2011 May outburst of GX 304−1. The NH increased up to ∼16 × 10(exp 22) atoms cm(exp −2), which is a factor of 3–4 larger than what is usually measured during the outbursts of GX 304−1 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-Proportional Counter Array light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disc around the Be-type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be disc density to be of the order of 10(exp −11)g cm(exp −3). Our model strengthens the idea of inclined Be discs as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.

Kühnel, M.↗