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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. Source


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. Source


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. Source


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. Source


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. Source

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