16 research outputs found
Looking at A 0535+26 at low luminosities with NuSTAR
We report on two NuSTAR observations of the HMXB A 0535+26 taken toward the
end of its normal 2015 outburst at very low keV luminosities of
erg/s and erg/s which are
complemented by 9 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
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 to
, 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
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 in
disagreement with expectations.Comment: 14 pages, 11 figures, 3 tables, accepted for publication in A&
Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR
We present an analysis of the spectral shape and pulse profile of the
accretion-powered pulsar 4U 1626-67 observed with Suzaku and 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 NPEX continuum with an added broad
Gaussian emission component around the spectral peak at 20 keV. Taken together
with the observed Pdot value obtained from Fermi/GBM, we conclude that the
spectral change between the Suzaku and NuSTAR observations was likely caused by
an increase of 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.Comment: 19 pages, 14 figures, Accepted for publication in ApJ on May 5, 201
Be X-ray Binary Outburst Zoo II
We have continued our recently started systematic study of Be X-ray binary (BeXRB) outbursts. Specifically, we are developing a catalogue of outbursts including their basic properties based on nearly all available X-ray all-sky-monitors. These properties are derived by fitting asymmetric Gaussians to the outburst lightcurves. This model describes most of the outbursts covered by our preliminary catalogue well; only 13% of all datasets show more complex outburst shapes. Analyzing the basic properties, we reveal a strong correlation between the outburst length and the reached peak flux. As an example, we discuss possible models describing the observed correlation in EXO 2030+375
The Goodness of Simultaneous Fits in ISIS
In a previous work, we introduced a tool for analyzing multiple datasets simultaneously, which has been implemented into ISIS. This tool was used to fit many spectra of X-ray binaries. However, the large number of degrees of freedom and individual datasets raise an issue about a good measure for a simultaneous fit quality. We present three ways to check the goodness of these fits: we investigate the goodness of each fit in all datasets, we define a combined goodness exploiting the logical structure of a simultaneous fit, and we stack the fit residuals of all datasets to detect weak features. These tools are applied to all RXTE-spectra from GRO 1008−57, revealing calibration features that are not detected significantly in any single spectrum. Stacking the residuals from the best-fit model for the Vela X-1 and XTE J1859+083 data evidences fluorescent emission lines that would have gone undetected otherwise.M. Kühnel was supported by the Bundesministerium für Wirtschaft und Technologie under Deutsches Zentrum für Luft- und Raumfahrt grants 50OR1113 and 50OR1207
GRO J1008 57: a laboratory for accretion physics
We present timing and spectroscopic results of three outbursts of the transient high mass X-ray binary GRO J1008−57 in 2005, 2007, and 2011. The orbital parameters from the literature are not in agreement with the measured pulse arrival times. We therefore updated the orbital solution, specifically the orbital period and the time of periastron passage, using pointed observations with RXTE, Swift, and Suzaku. We confirmed our results with an analysis of RXTE-ASM lightcurves. We show that GRO J1008−57’s outbursts occur mostly at the same orbital phase and therefore make predictions of outbursts and, thus, scheduled observations possible.
The X-ray spectrum of GRO J1008−57 during an outburst can be well described by a cutoff power law with an additional black body at energies below 10 keV. We found that the same spectral model describes GRO J1008−57 during the rise and the decline of the outburst at fluxes changing by two orders of magnitude. In particular, the photon index of the power law and the black body flux show a correlation with the total X-ray flux. Other parameters such as the black body temperature and the folding energy are independent of flux and remain the same over all analyzed outbursts
The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE - Minimizing cyclotron line biases
We present an analysis of X-ray spectra of the high-mass X-ray binary 4U 0115+634 as observed with Suzaku and RXTE in 2011 July, during the fading phase of a giant X-ray outburst. We used a continuum model consisting of an absorbed cutoff power law and an ad hoc Gaussian emission feature centered around 8.5 keV, which we attribute to cyclotron emission. Our results are consistent with a fundamental cyclotron absorption line centered at ∼10.2 keV for all observed flux ranges. At the same time we rule out significant influence of the 8.5 kev Gaussian on the parameters of the cyclotron resonant scattering feature, which are not consistent with the cyclotron line energies or the depths of previously reported flux-dependent descriptions. We also show that some continuum models can lead to artificial line-like residuals in the analyzed spectra, which are then misinterpreted as unphysically strong cyclotron lines. Specifically, our results do not support the existence of a previously claimed additional cyclotron feature at ∼15 keV. Apart from these features, we find for the first time evidence for a He-like Fe XXV emission line at ∼6.7 keV and weak H-like Fe XXVI emission close to ∼7.0 keV.We acknowledge funding by the European Space Agency under contract number C4000115860/15/NL/IB, by the Bundesministerium für Wirtschaft und Technologie under Deutsches Zentrum für Luft- und Raumfahrt grants 50OR0808, 50OR0905, 50OR1113, and 50OR1207, and by the Deutscher Akademischer Austauschdienst. MTW is supported by the NASA Astrophysical Data Analysis Program and the Chief of Naval Research. VG is supported through the Margarethe von Wrangell fellowship by the ESF and the Ministry of Science, Research and the Arts Baden-Württemberg. SMN and JMT acknowledge Spanish Ministerio de Ciencia, Tecnología e Innovación (MICINN) through the grant ESP2016-76683-C3-1-R and ESP2017-85691-P, respectively