Dr. Karl Remeis Sternwarte and ECAP

Bamberg, Germany

Dr. Karl Remeis Sternwarte and ECAP

Bamberg, Germany
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Fornasini F.M.,University of California at Berkeley | Fornasini F.M.,Harvard - Smithsonian Center for Astrophysics | Tomsick J.A.,University of California at Berkeley | Bachetti M.,National institute for astrophysics | And 7 more authors.
Astrophysical Journal | Year: 2017

IGR J18214-1318, a Galactic source discovered by the International Gamma-Ray Astrophysics Laboratory, is a high-mass X-ray binary (HMXB) with a supergiant O-type stellar donor. We report on the XMM-Newton and NuSTAR observations that were undertaken to determine the nature of the compact object in this system. This source exhibits high levels of aperiodic variability, but no periodic pulsations are detected with a 90% confidence upper limit of 2% fractional rms between 0.00003-88 Hz, a frequency range that includes the typical pulse periods of neutron stars (NSs) in HMXBs (0.1-103 s). Although the lack of pulsations prevents us from definitively identifying the compact object in IGR J18214-1318, the presence of an exponential cutoff with e-folding energy ≲30 keV in its 0.3-79 keV spectrum strongly suggests that the compact object is an NS. The X-ray spectrum also shows a Fe Kα emission line and a soft excess, which can be accounted for by either a partial-covering absorber with NH ≈ 1023cm-2, which could be due to the inhomogeneous supergiant wind, or a blackbody component with kT = 1.74-0.05 +0.04 keV and RBB ≈ 0.3 km, which may originate from NS hot spots. Although neither explanation for the soft excess can be excluded, the former is more consistent with the properties observed in other supergiant HMXBs. We compare IGR J18214-1318 to other HMXBs that lack pulsations or have long pulsation periods beyond the range covered by our observations. © 2017. The American Astronomical Society. All rights reserved.


Rothschild R.E.,University of California at San Diego | Kuhnel M.,Dr. Karl Remeis Sternwarte and ECAP | Pottschmidt K.,University of Maryland Baltimore County | Pottschmidt K.,NASA | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2017

The Rossi X-ray Timing Explorer (RXTE) observed four outbursts of the accreting Xray binary transient source, GX 304-1 in 2010 and 2011. We present results of detailed 3-100 keV spectral analysis of 69 separate observations, and report a positive correlation between cyclotron line parameters, as well as other spectral parameters, with power-law flux. The cyclotron line energy, width and depth versus flux, and thus luminosity, correlations show a flattening of the relationships with increasing luminosity, which are well described by quasi-spherical or disc accretion that yield the surface magnetic field to be ~5 × 1012 Gauss. Since HEXTE (High Energy X-ray Timing Experiment) cluster A was fixed aligned with the Proportional Counter Array field of view and cluster B was fixed viewing a background region 1.° 5 off of the source direction during these observations near the end of the RXTE mission, the cluster A background was estimated from cluster B events using HEXTEBACKEST. This made possible the detection of the ~55 keV cyclotron line and an accurate measurement of the continuum. Correlations of all spectral parameters with the primary 2-10 keV power-law flux reveal it to be the primary driver of the spectral shape. The accretion is found to be in the collisionless shock braking regime. © 2016 The Authors.


Bachetti M.,Toulouse 1 University Capitole | Bachetti M.,CNRS Institute for research in astrophysics and planetology | Harrison F.A.,Cahill Center for Astronomy and Astrophysics | Cook R.,Cahill Center for Astronomy and Astrophysics | And 22 more authors.
Astrophysical Journal | Year: 2015

Timing of high-count-rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count rates rather than for timing analysis of bright objects. The instrumental dead time per event is relatively long (∼2.5 msec) and varies event-to-event by a few percent. The most obvious effect is a distortion of the white noise level in the power density spectrum (PDS) that cannot be easily modeled with standard techniques due to the variable nature of the dead time. In this paper, we show that it is possible to exploit the presence of two completely independent focal planes and use the cospectrum, the real part of the cross PDS, to obtain a good proxy of the white-noise-subtracted PDS. Thereafter, one can use a Monte Carlo approach to estimate the remaining effects of dead time, namely, a frequency-dependent modulation of the variance and a frequency-independent drop of the sensitivity to variability. In this way, most of the standard timing analysis can be performed, albeit with a sacrifice in signal-to-noise ratio relative to what would be achieved using more standard techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1, and GRS 1915+105. © 2015. The American Astronomical Society. All rights reserved.


Furst F.,California Institute of Technology | Pottschmidt K.,University of Maryland Baltimore County | Pottschmidt K.,NASA | Wilms J.,Dr. Karl Remeis Sternwarte and ECAP | And 14 more authors.
Astrophysical Journal Letters | Year: 2014

We present a spectral analysis of three simultaneous Nuclear Spectroscopy Telescope Array and Swift/XRT observations of the transient Be-neutron star binary KS 1947+300 taken during its outburst in 2013/2014. These broadband observations were supported by Swift/XRT monitoring snapshots every three days, which we use to study the evolution of the spectrum over the outburst. We find strong changes of the power-law photon index, which shows a weak trend of softening with increasing X-ray flux. The neutron star shows very strong pulsations with a period of P ≈ 18.8 s. The 0.8-79 keV broadband spectrum can be described by a power law with an exponential cutoff and a blackbody component at low energies. During the second observation we detect a cyclotron resonant scattering feature at 12.5 keV, which is absent in the phase-averaged spectra of observations 1 and 3. Pulse phase-resolved spectroscopy reveals that the strength of the feature changes strongly with pulse phase and is most prominent during the broad minimum of the pulse profile. At the same phases the line also becomes visible in the first and third observation at the same energy. This discovery implies that KS 1947+300 has a magnetic field strength of B ≈ 1.1 × 1012(1 + z) G, which is at the lower end of known cyclotron line sources. © 2014. The American Astronomical Society. All rights reserved.


Furst F.,California Institute of Technology | Pottschmidt K.,University of Maryland Baltimore County | Pottschmidt K.,NASA | Wilms J.,Dr. Karl Remeis Sternwarte and ECAP | And 15 more authors.
Astrophysical Journal | Year: 2014

We present NuSTAR observations of Vela X-1, a persistent, yet highly variable, neutron star high-mass X-ray binary (HMXB). Two observations were taken at similar orbital phases but separated by nearly a year. They show very different 3-79 keV flux levels as well as strong variability during each observation, covering almost one order of magnitude in flux. These observations allow, for the first time ever, investigations on kilo-second time-scales of how the centroid energies of cyclotron resonant scattering features (CRSFs) depend on flux for a persistent HMXB. We find that the line energy of the harmonic CRSF is correlated with flux, as expected in the sub-critical accretion regime. We argue that Vela X-1 has a very narrow accretion column with a radius of around 0.4 km that sustains a Coulomb interaction dominated shock at the observed luminosities of L x ∼ 3 × 1036 erg s -1. Besides the prominent harmonic line at 55 keV the fundamental line around 25 keV is clearly detected. We find that the strengths of the two CRSFs are anti-correlated, which we explain by photon spawning. This anti-correlation is a possible explanation for the debate about the existence of the fundamental line. The ratio of the line energies is variable with time and deviates significantly from 2.0, also a possible consequence of photon spawning, which changes the shape of the line. During the second observation, Vela X-1 showed a short off-state in which the power-law softened and a cut-off was no longer measurable. It is likely that the source switched to a different accretion regime at these low mass accretion rates, explaining the drastic change in spectral shape. © 2014. The American Astronomical Society. All rights reserved.


Bellm E.C.,California Institute of Technology | Furst F.,California Institute of Technology | Pottschmidt K.,University of Maryland Baltimore County | Pottschmidt K.,NASA | And 12 more authors.
Astrophysical Journal | Year: 2014

GRO J1008-57 is a high-mass X-ray binary for which several claims of a cyclotron resonance scattering feature near 80 keV have been reported. We use NuSTAR, Suzaku, and Swift data from its giant outburst of 2012 November to confirm the existence of the 80 keV feature and perform the most sensitive search to date for cyclotron scattering features at lower energies. We find evidence for a 78 keV line in the NuSTAR and Suzaku data at >4σ significance, confirming the detection using Suzaku alone by Yamamoto et al. A search of both the phase-averaged and phase-resolved data rules out a fundamental at lower energies with optical depth larger than 5% of the 78 keV line. These results indicate that GRO J1008-57 has a magnetic field of 6.7 × 1012(1 + z) G, the highest among known accreting pulsars. © 2014. The American Astronomical Society. All rights reserved..


Miyasaka H.,California Institute of Technology | Bachetti M.,Toulouse 1 University Capitole | Bachetti M.,CNRS Institute for research in astrophysics and planetology | Harrison F.A.,California Institute of Technology | And 20 more authors.
Astrophysical Journal | Year: 2013

The Nuclear Spectroscopic Telescope Array hard X-ray telescope observed the transient Be/X-ray binary GS 0834-430 during its 2012 outburst - the first active state of this system observed in the past 19 yr. We performed timing and spectral analysis and measured the X-ray spectrum between 3-79 keV with high statistical significance. We find the phase-averaged spectrum to be consistent with that observed in many other magnetized, accreting pulsars. We fail to detect cyclotron resonance scattering features that would allow us to constrain the pulsar's magnetic field in either phase-averaged or phase-resolved spectra. Timing analysis shows a clearly detected pulse period of ∼12.29 s in all energy bands. The pulse profiles show a strong, energy-dependent hard phase lag of up to 0.3 cycles in phase, or about 4 s. Such dramatic energy-dependent lags in the pulse profile have never before been reported in high-mass X-ray binary pulsars. Previously reported lags have been significantly smaller in phase and restricted to low energies (E < 10 keV). We investigate the possible mechanisms that might produce this energy-dependent pulse phase shift. We find the most likely explanation for this effect is a complex beam geometry. © 2013. The American Astronomical Society. All rights reserved.


Rothschild R.,University of California at San Diego | Markowitz A.,University of California at San Diego | Hemphill P.,University of California at San Diego | Caballero I.,SAP | And 6 more authors.
Astrophysical Journal | Year: 2013

We have analyzed three observations of the high-mass X-ray binary A 0535+26 performed by the Rossi X-Ray Timing Explorer (RXTE) three, five, and six months after the last outburst in 2011 February. We detect pulsations only in the second observation. The 3-20 keV spectra can be fit equally well with either an absorbed power law or absorbed thermal bremsstrahlung model. Reanalysis of two earlier RXTE observations made 4 yr after the 1994 outburst, original BeppoSAX observations 2 yr later, reanalysis of four EXOSAT observations made 2 yr after the last 1984 outburst, and a recent XMM-Newton observation in 2012 reveal a stacked, quiescent flux level decreasing from 2 to <1 × 10 -11 erg cm-2 s-1 over 6.5 yr after outburst. The detection of pulsations during half of the quiescent observations would imply that accretion onto the magnetic poles of the neutron star continues despite the fact that the circumstellar disk may no longer be present. The accretion could come from material built up at the corotation radius or from an isotropic stellar wind. © 2013. The American Astronomical Society. All rights reserved.


Furst F.,California Institute of Technology | Grefenstette B.W.,California Institute of Technology | Staubert R.,University of Tübingen | Tomsick J.A.,University of California at Berkeley | And 19 more authors.
Astrophysical Journal | Year: 2013

Her X-1, one of the brightest and best studied X-ray binaries, shows a cyclotron resonant scattering feature (CRSF) near 37 keV. This makes it an ideal target for a detailed study with the Nuclear Spectroscopic Telescope Array (NuSTAR), taking advantage of its excellent hard X-ray spectral resolution. We observed Her X-1 three times, coordinated with Suzaku, during one of the high flux intervals of its 35 day superorbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broadband spectra can be fitted with a power law with a high-energy cutoff, an iron line, and a CRSF. We find that the CRSF has a very smooth and symmetric shape in all observations and at all pulse phases. We compare the residuals of a line with a Gaussian optical-depth profile to a Lorentzian optical-depth profile and find no significant differences, strongly constraining the very smooth shape of the line. Even though the line energy changes dramatically with pulse phase, we find that its smooth shape does not. Additionally, our data show that the continuum only changes marginally between the three observations. These changes can be explained with varying amounts of Thomson scattering in the hot corona of the accretion disk. The average, luminosity-corrected CRSF energy is lower than in past observations and follows a secular decline. The excellent data quality of NuSTAR provides the best constraint on the CRSF energy to date. © 2013. The American Astronomical Society. All rights reserved..


Grinberg V.,Massachusetts Institute of Technology | Leutenegger M.A.,University of Maryland Baltimore County | Leutenegger M.A.,NASA | Hell N.,Dr. Karl Remeis Sternwarte and ECAP | And 12 more authors.
Astronomy and Astrophysics | Year: 2015

Binary systems with an accreting compact object offer a unique opportunity to investigate the strong, clumpy, line-driven winds of early-type supergiants by using the compact object's X-rays to probe the wind structure. We analyze the two-component wind of HDE 226868, the O9.7Iab giant companion of the black hole Cyg X-1, using 4.77 Ms Rossi X-ray Timing Explorer (RXTE) observations of the system taken over the course of 16 years. Absorption changes strongly over the 5.6 d binary orbit, but also shows a large scatter at a given orbital phase, especially at superior conjunction. The orbital variability is most prominent when the black hole is in the hard X-ray state. Our data are poorer for the intermediate and soft state, but show signs for orbital variability of the absorption column in the intermediate state. We quantitatively compare the data in the hard state to a toy model of a focussed Castor-Abbott-Klein wind: as it does not incorporate clumping, the model does not describe the observations well. A qualitative comparison to a simplified simulation of clumpy winds with spherical clumps shows good agreement in the distribution of the equivalent hydrogen column density for models with a porosity length on the order of the stellar radius at inferior conjunction; we conjecture that the deviations between data and model at superior conjunction could either be due to lack of a focussed wind component in the model or to a more complicated clump structure. © ESO, 2015.

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