High variability in Vela X-1: giant flares and off states

Kreykenbohm, I.; Wilms, J.; Kretschmar, P.; Torrejón, José M.; Pottschmidt, K.; Hanke, Manfred; Santangelo, A.; Ferrigno, C.; Staubert, R.

doi:10.1051/0004-6361:200809956

Cited by 128 publications

(274 citation statements)

References 78 publications

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“…Later observations failed to detect this low energy feature, revealing only one absorption line at 53 ∼ 55 keV (Orlandini et al 1998;La Barbera et al 2003;Kreykenbohm et al 2008). Kretschmar et al (1997), with HEXE, Kreykenbohm et al (2002) with RXTE, Schanne et al (2007) with INTEGRAL, and Maitra & Paul (2013) and Odaka et al (2013) with Suzaku confirmed the presence of both features, with energies varying between ∼ 23 keV and ∼ 27 keV and between ∼ 45 keV and ∼ 54 keV, respectively.…”

Section: Introductionmentioning

confidence: 96%

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TheSwift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

Parola¹,

Cusumano²,

Segreto³

et al. 2016

Mon. Not. R. Astron. Soc.

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We study the behaviour of the cyclotron resonant scattering feature (CRSF) of the high mass X-ray binary Vela X-1 using the long-term hard X-ray monitoring performed by the Burst Alert Telescope (BAT) on board Swift. High statistics, intensity selected spectra were built along 11 years of BAT survey. While the fundamental line is not revealed, the second harmonic of the CRSF can be clearly detected in all the spectra, at an energy varying between ∼ 53 keV and ∼ 58 keV, directly correlated with the luminosity. We have further investigated the evolution of the CRSF in time, by studying the intensity selected spectra built along four 33-month time intervals along the survey. For the first time we find in this source a secular variation in the CRSF energy: independent of the source luminosity, the CRSF second harmonic energy decreases by ∼ 0.36 keV/year between the first and the third time interval, corresponding to an apparent decay of the magnetic field of ∼ 3 × 10 10 G/year. The intensitycyclotron energy pattern is consistent between the third and the last time intervals. A possible interpretation for this decay could be the settling of an accreted mound that produces either a distortion of the poloidal magnetic field on the polar cap or a geometrical displacement of the line forming region. This hypothesis seems supported by the correspondance between the rate of the line shift per unit accreted mass and the mass accreted on the polar cap per unit area in Vela X-1 and Her X-1, respectively.

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Section: Introductionmentioning

confidence: 96%

“…The mean luminosity of the source is 5 × 10 36 erg s −1 (Fürst et al 2010), and the flux shows a great variability even at short time scales (hours), with the source going from off-states to giant flares up to a few Crab (Kreykenbohm et al 2008).…”

Section: Introductionmentioning

confidence: 99%

TheSwift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

Parola¹,

Cusumano²,

Segreto³

et al. 2016

Mon. Not. R. Astron. Soc.

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“…We will refer to these non-detections as "off-states" or "dips", regardless of the cause of diminished flux (similarly to the definition assumed by Kreykenbohm et al 2008, who studied a subsample of INTEGRAL data of Vela X-1, at early times of the mission).…”

Section: Resultsmentioning

confidence: 99%

“…Located at a distance of 1.9 kpc (Sadakane et al 1985), the orbital period of the system is 8.96 d (van Kerkwijk et al 1995, Kreykenbohm et al 2008, implying that the neutron star orbit is embedded in the companion wind.…”

Section: Introductionmentioning

confidence: 99%

“…The X-ray emission of Vela X-1 is persistent at a level of 10 36 erg s −1 , although variable (within a factor of ∼10), showing rare giant flares together with so-called off-states, that manifest themselves as flux drops lasting a few pulse periods (Kreykenbohm et al 2008, Doroshenko et al 2011. The observed variability is indicative of the presence of massive clumps in the supergiant wind (Nagase et al 1986, Fürst et al 2010).…”

Section: Introductionmentioning

confidence: 99%

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Probing large-scale wind structures in Vela X–1 using off-states with INTEGRAL

Sidoli¹,

Paizis²,

Fürst³

et al. 2014

Monthly Notices of the Royal Astronomical Society

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Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high mass X-ray binaries hosting a supergiant donor. We have analysed in a systematic way ten years of IN-TEGRAL data of Vela X-1 (22-50 keV) and we found that when outside the X-ray eclipse, the source undergoes several luminosity drops where the hard X-rays luminosity goes below ∼3×10 35 erg s −1 , becoming undetected by INTEGRAL. These drops in the X-ray flux are usually referred to as "off-states" in the literature. We have investigated the distribution of these off-states along the Vela X-1 ∼8.9 d orbit, finding that their orbital occurrence displays an asymmetric distribution, with a higher probability to observe an off-state near the pre-eclipse than during the post-eclipse. This asymmetry can be explained by scattering of hard X-rays in a region of ionized wind, able to reduce the source hard X-ray brightness preferentially near eclipse ingress. We associate this ionized large-scale wind structure with the photoionization wake produced by the interaction of the supergiant wind with the X-ray emission from the neutron star. We emphasize that this observational result could be obtained thanks to the accumulation of a decade of INTEGRAL data, with observations covering the whole orbit several times, allowing us to detect an asymmetric pattern in the orbital distribution of off-states in Vela X-1.

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Variability in high‐mass X‐ray binaries

et al. 2019

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Strongly magnetized, accreting neutron stars show periodic and aperiodic variability over a wide range of time scales. By obtaining spectral and timing information on these different time scales, we can have a closer look into the physics of accretion close to the neutron star and the properties of the accreted material. One of the most prominent time scales is the strong pulsation, i.e., the rotation period of the neutron star itself. Over one rotation, our view of the accretion column and the X-ray producing region changes significantly. This allows us to sample different physical conditions within the column but at the same time requires that we have viewing-angle-resolved models to properly describe them. In wind-fed high-mass X-ray binaries, the main source of aperiodic variability is the clumpy stellar wind, which leads to changes in the accretion rate (i.e., luminosity) as well as absorption column. This variability allows us to study the behavior of the accretion column as a function of luminosity, as well as to investigate the structure and physical properties of the wind, which we can compare to winds in isolated stars.

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High variability in Vela X-1: giant flares and off states

Cited by 128 publications

References 78 publications

TheSwift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

TheSwift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

Probing large-scale wind structures in Vela X–1 using off-states with INTEGRAL

Variability in high‐mass X‐ray binaries

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