INTEGRAL observations of Hercules X-1

Klochkov, D.; Staubert, R.; Постнов, К. А.; Шакура, Н. И.; Santangelo, A.; Tsygankov, Sergey S.; Lutovinov, A.; Kreykenbohm, I.; Wilms, J.

doi:10.1051/0004-6361:20078953

Cited by 71 publications

(49 citation statements)

References 40 publications

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“…While there is no commonly accepted interpretation of the pulse profile dependence on energy and luminosity, we note that similar behavior is observed in Her X-1 (Klochkov et al 2008b), 4U 0115+63, and V 0332+63 (Tsygankov et al 2007). A qualitative interpretation of the pulse profile energy-dependence (based on the results of Mowlavi et al 2006) has been proposed by Tsygankov et al (2007) to explain the variation in V 0332+63 pulse profiles in a purely geometric fashion: if the axis of the magnetic dipole is at an angle to the rotation axis (a necessary condition to observe pulsation), the neutron star itself can eclipse some of the lower parts of the accretion columns, which are thought to primarily emit the harder photons.…”

Section: Timing Resultssupporting

confidence: 74%

Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304−1 measured with INTEGRAL

Malacaria

Klochkov

Santangelo

et al. 2015

A&A

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Context. Be/X-ray binaries show outbursts with peak luminosities up to a few times 10 37 erg/s, during which they can be observed and studied in detail. Most (if not all) Be/X-ray binaries harbor accreting pulsars, whose X-ray spectra in many cases contain cyclotron resonant scattering features related to the magnetic field of the sources. Spectral variations as a function of luminosity and of the rotational phase of the neutron star are observed in many accreting pulsars. Aims. We explore X-ray spectral and timing properties of the Be/X-ray binary GX 304-1 during an outburst episode. Specifically, we investigate the behavior of the cyclotron resonant scattering feature, the continuum spectral parameters, the pulse period, and the energy-and luminosity-resolved pulse profiles. Methods. We analyze the INTEGRAL data from the two JEM-X modules, ISGRI and SPI, covering the 2012 January−February outburst, divided into six observations. We obtain pulse profiles in two energy bands, phase-averaged and phase-resolved spectra for each observation. We combine the luminosity-resolved spectral and timing analysis to probe the accretion geometry and the beaming patterns of the rotating neutron star. Results. We confirm the positive luminosity dependence of the cyclotron line energy in GX 304-1 and report a dependence of the photon index on luminosity. Using a pulse-phase connection technique, we find a pulse period solution valid for the entire outburst. Our pulse-phase resolved analysis shows that the centroid energy of the cyclotron line varies only slightly with pulse phase, while other spectral parameters show more pronounced variations. Our results are consistent with a scenario in which, as the pulsar rotates, we are exploring only a small portion of its beam pattern.

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Section: Timing Resultssupporting

confidence: 74%

Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304−1 measured with INTEGRAL

Malacaria

Klochkov

Santangelo

et al. 2015

A&A

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“…1 of Staubert et al 2007) we display observed values of the pulse phase averaged centroid cyclotron line energy as a function of time, covering the complete history of observations since the discovery of the line in 1976. We combine historical data, as taken from the compilation by Gruber et al (2001, their Tables 2 and 3) for the time before the RXTE era, published values from observations with RXTE and INTEGRAL (Klochkov et al 2006;Staubert et al 2007;Klochkov et al 2008a) and with Suzaku (Enoto et al 2008), as Staubert et al (1983Staubert et al ( , 2013 well as recent values as given in Table 2 (see also Staubert 2013;Fürst et al 2013). For the analysis of the long-term variation of E cyc we exclude values with 35 d phases >0.20 in order to avoid contamination due to a possible third variable, the 35 d phase (a dependence, if any, is very weak for small phases; see Fig.…”

Section: Variation Of E Cyc With Time -Long-term Variationmentioning

confidence: 99%

Long-term change in the cyclotron line energy in Hercules X-1

Staubert

Klochkov

Wilms³

et al. 2014

A&A

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Aims. We investigate the long-term evolution of the Cyclotron Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar Her X-1 and present evidence of a true long-term decrease in the centroid energy E cyc of the cyclotron line in the pulse phase averaged spectra from 1996 to 2012. Methods. Our results are based on repeated observations of Her X-1 by those X-ray observatories capable of measuring clearly beyond the cyclotron line energy of ∼ 40 keV. Results. The historical evolution of the pulse phase averaged CRSF centroid energy E cyc since its discovery in 1976 is characterized by an initial value around 35 keV, an abrupt jump upwards to beyond ∼ 40 keV between 1990 and 1994, and an apparent decay thereafter. Much of this decay, however, was found to be due to an artifact, namely a correlation between E cyc and the X-ray luminosity L x discovered in 2007. In observations after 2006, however, we now find a statistically significant true decrease in the cyclotron line energy. At the same time, the dependence of E cyc on X-ray luminosity is still valid with an increase of ∼ 5% in energy for a factor of two increase in luminosity. A decrease in E cyc by 4.2 keV over the 16 years from 1996 to 2012 can either be modeled by a linear decay, or by a slow decay until 2006 followed by a more abrupt decrease thereafter. Conclusions. We speculate that the physical reason could be connected to a geometric displacement of the cyclotron resonant scattering region in the polar field or to a true physical change in the magnetic field configuration at the polar cap by the continued accretion.

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“…A CRSF is clearly seen in the spectrum around 37 keV, but its energy is strongly variable with pulse phase. In International Gamma-Ray Astrophysics Laboratory (INTEGRAL) data, Klochkov et al (2008) found that it changes from around 30 keV in the off-pulse to over 40 keV during the main peak of the pulse profile. It is now also firmly established that the line energy correlates positively with the X-ray luminosity (Staubert et al 2007;Vasco et al 2011;Staubert 2013).…”

Section: Introductionmentioning

confidence: 99%

The Smooth Cyclotron Line in Her X-1 as Seen With Nuclear Spectroscopic Telescope Array

Fürst

Grefenstette

Staubert

et al. 2013

ApJ

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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 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 d super-orbital period. This paper focuses on the shape and evolution of the hard X-ray spectrum. The broad-band spectra can be fitted with a powerlaw 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 is only changing 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.

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INTEGRAL observations of Hercules X-1

Cited by 71 publications

References 40 publications

Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304−1 measured with INTEGRAL

Luminosity-dependent spectral and timing properties of the accreting pulsar GX 304−1 measured with INTEGRAL

Long-term change in the cyclotron line energy in Hercules X-1

The Smooth Cyclotron Line in Her X-1 as Seen With Nuclear Spectroscopic Telescope Array

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