Dr Karl Remeis Sternwarte And Erlangen Center For Astroparticle Physics

Bamberg, Germany

Dr Karl Remeis Sternwarte And Erlangen Center For Astroparticle Physics

Bamberg, Germany
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Israel G.L.,National institute for astrophysics | Belfiore A.,Istituto di Astrofisica Spaziale e Fisica Cosmica | Stella L.,National institute for astrophysics | Esposito P.,Istituto di Astrofisica Spaziale e Fisica Cosmica | And 24 more authors.
Science | Year: 2017

Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any x-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs.We present observations showing that NGC 5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43 seconds in 2003 to 1.13 seconds in 2014. It has an isotropic peak luminosity of ∼ 1000 times the Eddington limit for a NS at 17.1 megaparsec. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity ≥ 1041 erg second-1) might harbor NSs.


Tomsick J.A.,University of California at Berkeley | Nowak M.A.,Massachusetts Institute of Technology | Parker M.,University of Cambridge | Miller J.M.,University of Michigan | And 24 more authors.
Astrophysical Journal | Year: 2014

The black hole binary Cygnus X-1 was observed in late 2012 with the Nuclear Spectroscopic Telescope Array (NuSTAR) and Suzaku, providing spectral coverage over the ∼1-300 keV range. The source was in the soft state with a multi-temperature blackbody, power law, and reflection components along with absorption from highly ionized material in the system. The high throughput of NuSTAR allows for a very high quality measurement of the complex iron line region as well as the rest of the reflection component. The iron line is clearly broadened and is well described by a relativistic blurring model, providing an opportunity to constrain the black hole spin. Although the spin constraint depends somewhat on which continuum model is used, we obtain a* > 0.83 for all models that provide a good description of the spectrum. However, none of our spectral fits give a disk inclination that is consistent with the most recently reported binary values for Cyg X-1. This may indicate that there is a >13° misalignment between the orbital plane and the inner accretion disk (i.e., a warped accretion disk) or that there is missing physics in the spectral models. © 2014. The American Astronomical Society. All rights reserved.


Fabian A.C.,Institute of Astronomy | Wilkins D.R.,Institute of Astronomy | Miller J.M.,University of Michigan | Reis R.C.,University of Michigan | And 10 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

The spin of Cygnus X-1 is measured by fitting reflection models to Suzaku data covering the energy band 0.9-400keV. The inner radius of the accretion disc is found to lie within 2 gravitational radii (r g=GM/c 2), and a value of is obtained for the dimensionless black hole spin. This agrees with recent measurements using the continuum fitting method by Gou et al. and of the broad iron line by Duro et al. The disc inclination is measured at, which is consistent with the recent optical measurement of the binary system inclination by Orosz et al. of 27°± 08. We pay special attention to the emissivity profile caused by irradiation of the inner disc by the hard power-law source. The X-ray observations and simulations show that the index q of that profile deviates from the commonly used, Newtonian, value of 3 within 3r g, steepening considerably within 2r g, as expected in the strong gravity regime. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Miller J.M.,University of Michigan | Pooley G.G.,University of Cambridge | Fabian A.C.,University of Cambridge | Nowak M.A.,Massachusetts Institute of Technology | And 5 more authors.
Astrophysical Journal | Year: 2012

Models of jet production in black hole systems suggest that the properties of the accretion disk - such as its mass accretion rate, inner radius, and emergent magnetic field - should drive and modulate the production of relativistic jets. Stellar-mass black holes in the "low/hard" state are an excellent laboratory in which to study disk-jet connections, but few coordinated observations are made using spectrometers that can incisively probe the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1 made in the jet-producing low/hard state. Contemporaneous radio monitoring was done using the Arcminute MicroKelvin Array radio telescope. Two important and simple results are obtained: (1) the jet (as traced by radio flux) does not appear to be modulated by changes in the inner radius of the accretion disk and (2) the jet is sensitive to disk properties, including its flux, temperature, and ionization. Some more complex results may reveal aspects of a coupled disk-corona-jet system. A positive correlation between the reflected X-ray flux and radio flux may represent specific support for a plasma ejection model of the corona, wherein the base of a jet produces hard X-ray emission. Within the framework of the plasma ejection model, the spectra suggest a jet base with v/c ≃ 0.3 or the escape velocity for a vertical height of z ≃ 20 GM/c 2 above the black hole. The detailed results of X-ray disk continuum and reflection modeling also suggest a height of z ≃ 20 GM/c 2 for hard X-ray production above a black hole, with a spin in the range 0.6 ≤ a ≤ 0.99. This height agrees with X-ray time lags recently found in Cygnus X-1. The overall picture that emerges from this study is broadly consistent with some jet-focused models for black hole spectral energy distributions in which a relativistic plasma is accelerated at z = 10-100 GM/c 2. We discuss these results in the context of disk-jet connections across the black hole mass scale. © 2012. The American Astronomical Society. All rights reserved.


Klochkov D.,University of Tübingen | Doroshenko V.,University of Tübingen | Santangelo A.,University of Tübingen | Staubert R.,University of Tübingen | And 10 more authors.
Astronomy and Astrophysics | Year: 2012

Context. X-ray spectra of many accreting pulsars exhibit significant variations as a function of flux and thus of mass accretion rate. In some of these pulsars, the centroid energy of the cyclotron line(s), which characterizes the magnetic field strength at the site of the X-ray emission, has been found to vary systematically with flux. Aims. GX 304-1 is a recently established cyclotron line source with a line energy around 50 keV. Since 2009, the pulsar shows regular outbursts with the peak flux exceeding one Crab. We analyze the INTEGRAL observations of the source during its outburst in January-February 2012. Methods. The observations covered almost the entire outburst, allowing us to measure the source's broad-band X-ray spectrum at different flux levels. We report on the variations in the spectral parameters with luminosity and focus on the variations in the cyclotron line. Results. The centroid energy of the line is found to be positively correlated with the luminosity. We interpret this result as a manifestation of the local sub-Eddington (sub-critical) accretion regime operating in the source. © 2012 ESO.


Hemphill P.B.,University of California at San Diego | Rothschild R.E.,University of California at San Diego | Markowitz A.,University of California at San Diego | Markowitz A.,Dr Karl Remeis Sternwarte And Erlangen Center For Astroparticle Physics | And 4 more authors.
Astrophysical Journal | Year: 2014

We present results from the first Suzaku observation of the high-mass X-ray binary 4U 1538-522. The broadband spectral coverage of Suzaku allows for a detailed spectral analysis, characterizing the cyclotron resonance scattering feature at 23.0 ± 0.4 keV and the iron Kα line at 6.426 ± 0.008 keV, as well as placing limits on the strengths of the iron Kβ line and the iron K edge. We track the evolution of the spectral parameters both in time and in luminosity, notably finding a significant positive correlation between cyclotron line energy and luminosity. A dip and spike in the light curve is shown to be associated with an order-of-magnitude increase in column density along the line of sight, as well as significant variation in the underlying continuum, implying the accretion of a overdense region of a clumpy stellar wind. We also present a phase-resolved analysis, with most spectral parameters of interest showing significant variation with phase. Notably, both the cyclotron line energy and the iron Kα line intensity vary significantly with phase, with the iron line intensity significantly out of phase with the pulse profile. We discuss the implications of these findings in the context of recent work in the areas of accretion column physics and cyclotron resonance scattering feature formation. © 2014. The American Astronomical Society. All rights reserved.


Suchy S.,University of California at San Diego | Furst F.,Dr Karl Remeis Sternwarte And Erlangen Center For Astroparticle Physics | Pottschmidt K.,University of Maryland Baltimore County | Pottschmidt K.,NASA | And 5 more authors.
Astrophysical Journal | Year: 2012

We present the analysis of two Suzaku observations of GX301-2 at two orbital phases after the periastron passage. Variations in the column density of the line-of-sight absorber are observed, consistent with accretion from a clumpy wind. In addition to a cyclotron resonance scattering feature (CRSF), multiple fluorescence emission lines were detected in both observations. The variations in the pulse profiles and the CRSF throughout the pulse phase have a signature of a magnetic dipole field. Using a simple dipole model we calculated the expected magnetic field values for different pulse phases and were able to extract a set of geometrical angles, loosely constraining the dipole geometry in the neutron star. From the variation of the CRSF width and energy, we found a geometrical solution for the dipole, making the inclination consistent with previously published values. © 2012. The American Astronomical Society. All rights reserved.


Hemphill P.B.,University of California at San Diego | Rothschild R.E.,University of California at San Diego | Caballero I.,SAP | Pottschmidt K.,University of Maryland Baltimore County | And 4 more authors.
Astrophysical Journal | Year: 2013

We present a spectral and timing analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL) observations of two high-mass X-ray binaries, 4U 1538-522 and 4U 1907+09. Our timing measurements for 4U 1538-522 find the pulse period to have exhibited a spin-up trend until approximately 2009, after which there is evidence for a torque reversal, with the source beginning to spin down to the most recently measured period of 525.407 ± 0.001 s. The most recent INTEGRAL observations of 4U 1907+09 are not found to yield statistically significant pulse periods due to the significantly lower flux from the source compared with 4U 1538-522. A spectral model consisting of a power-law continuum with an exponential cutoff and modified by two cyclotron resonance scattering features is found to fit both sources well, with the cyclotron scattering features detected at ∼22 and ∼49 keV for 4U 1538-522 and at ∼18 and ∼36 keV for 4U 1907+09. The spectral parameters of 4U 1538-522 are generally not found to vary significantly with flux and there is little to no variation across the torque reversal. Examining our results in conjunction with previous work, we find no evidence for a correlation between cyclotron line energy and luminosity for 4U 1538-522. 4U 1907+09 shows evidence for a positive correlation between cyclotron line energy and luminosity, which would make it the fourth, and lowest luminosity, cyclotron line source to exhibit this relationship. © 2013. The American Astronomical Society. All rights reserved.


Hemphill P.B.,University of California at San Diego | Rothschild R.E.,University of California at San Diego | Furst F.,California Institute of Technology | Grinberg V.,Massachusetts Institute of Technology | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016

We have performed a full time and luminosity-resolved spectral analysis of the high-mass X-ray binary 4U 1538-522 using the available RXTE, INTEGRAL, and Suzaku data, examining both phase-averaged and pulse-phase-constrained data sets and focusing on the behaviour of the cyclotron resonance scattering feature (CRSF). No statistically significant trend between the energy of the CRSF and luminosity is observed in the combined data set. However, the CRSF energy appears to have increased by ~1.5 keV in the ~8.5 yr between the RXTE and Suzaku measurements, with Monte Carlo simulations finding the Suzaku measurement 4.6σ above the RXTE points. Interestingly, the increased Suzaku CRSF energy is much more significant and robust in the pulse-phase-constrained spectra from the peak of the main pulse, suggesting a change that is limited to a single magnetic pole. The seven years of RXTE measurements do not show any strongly significant evolution with time on their own. We discuss the significance of the CRSF's behaviour with respect to luminosity and time in the context of historical observations of this source as well as recent observational and theoretical work concerning the neutron star accretion column, and suggest some mechanisms by which the observed change over time could occur. © 2016 The Authors.


Rivers E.,University of California at San Diego | Markowitz A.,University of California at San Diego | Pottschmidt K.,NASA | Pottschmidt K.,University of Maryland Baltimore County | And 7 more authors.
Astrophysical Journal | Year: 2010

We present results from two observations of the wind-accreting X-ray pulsar 4U 1907+09 using the Suzaku Observatory. The broadband time-averaged spectrum allows us to examine the continuum emission of the source and the cyclotron resonance scattering feature at ∼ 19 keV. Additionally, using the narrow CCD response of Suzaku near 6 keV allows us to study in detail the Fe K bandpass and to quantify the Fe Kβ line for this source for the first time. The source is absorbed by fully covering material along the line of sight with a column density of N H ∼ 2 × 1022 cm -2, consistent with a wind-accreting geometry, and a high Fe abundance ∼ (3-4 times solar). Time- and phase-resolved analyses allow us to study variations in the source spectrum. In particular, dips found in the 2006 observation which are consistent with earlier observations occur in the hard X-ray bandpass, implying a variation of the whole continuum rather than occultation by intervening material, while a dip near the end of the 2007 observation occurs mainly in the lower energies implying an increase in along the line of sight, perhaps indicating clumpiness in the stellar wind. © 2010. The American Astronomical Society. All rights reserved.

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