Order in the Chaos: Spin-up and Spin-down during the 2002 Outburst of SAX J1808.4-3658

Burderi, L.; Salvo, T. Di; Menna, M. T.; Riggio, A.; Papitto, A.

doi:10.1086/510666

Cited by 74 publications

(138 citation statements)

References 13 publications

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“…The main type of phase shift is connected with so-called timing noise which appears as irregular pulsephase jumps on timescales from hours to months (e.g., Burderi et al 2006;Hartman et al 2008). The amplitude of pulse-phase offsets usually correlated with Xray flux (e.g., Papitto et al 2007;Reggio et al 2008;Patruno et al 2009a,b) though sudden strong jumps in phase are also observed (e.g., Burderi et al 2006;Hartman et al 2008Hartman et al , 2009). This phenomenon is not completely understood yet.…”

Section: Discussion Of Phase Shifts In Accreting Millisecond Pulsars mentioning

confidence: 99%

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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Recent measurements of the surface magnetic fields of classical T Tauri stars (CTTSs) and magnetic cataclysmic variables show that their magnetic fields have a complex structure. Investigation of accretion onto such stars requires global three-dimensional (3D) magnetohydrodynamic (MHD) simulations, where the complexity of simulations strongly increases with each higher-order multipole. Previously, we were able to model disc accretion onto stars with magnetic fields described by a superposition of dipole and quadrupole moments. However, in some stars, like CTTS V2129 Oph and BP Tau, the octupolar component is significant and it was necessary to include the next octupolar component. Here, we show results of global 3D MHD simulations of accretion onto stars with superposition of the dipole and octupole fields, where we vary the ratio between components. Simulations show that if octupolar field strongly dominates at the disc-magnetosphere boundary, then matter flows into the ring-like octupolar poles, forming ring-shape spots at the surface of the star above and below equator. The light-curves are complex and may have two peaks per period. In case where the dipole field dominates, matter accretes in two ordered funnel streams towards poles, however the polar spots are meridionally-elongated due to the action of the octupolar component. In the case when the fields are of similar strengths, both, polar and belt-like spots are present. In many cases the light-curves show the evidence of complex fields, excluding the cases of small inclinations angles, where sinusoidal light-curve is observed and 'hides' the information about the field complexity.We also propose new mechanisms of phase shift in stars with complex magnetic fields. We suggest that the phase shifts can be connected with: (1) temporal variation of the star's intrinsic magnetic field and subsequent redistribution of main magnetic poles; (2) variation of the accretion rate, which causes the disc to interact with the magnetic fields associated with different magnetic moments. We use our model to demonstrate these phase shift mechanisms, and we discuss possible applications of these mechanisms to accreting millisecond pulsars and young stars.

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Section: Discussion Of Phase Shifts In Accreting Millisecond Pulsars mentioning

confidence: 99%

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

2012

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“…For the second harmonic that is less affected by phase noise (Burderi et al 2006;Riggio et al 2008), we repeated the fitting procedure using these phase delays. In the constant spin frequency derivative case, we obtainedν = 1.45(16) × 10 −13 Hz s −1 with a χ 2 r = 1.74(238.5/137).…”

Section: Second Harmonicmentioning

confidence: 99%

Timing of the accreting millisecond pulsar IGR J17511-3057

Riggio

Papitto

Burderi

et al. 2011

A&A

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Context. Timing analysis of accretion-powered millisecond pulsars (AMPs) is a powerful tool for probing the physics of compact objects. The recently discovered IGR J17511-3057 was the twelfth discovered of the 13 AMPs known. The Rossi XTE satellite provided an extensive coverage of the 25 days-long observation of the source outburst. Aims. Our goal is to investigate the complex interaction between the neutron star magnetic field and the accretion disk by determining the angular momentum exchange between them. The presence of a millisecond coherent flux modulation allows us to investigate this interaction from the study of pulse arrival times. To separate the neutron star proper spin frequency variations from other effects, a precise set of orbital ephemeris is mandatory. Methods. Using timing techniques, we analysed the pulse phase delays by fitting differential corrections to the orbital parameters. To remove the effects of pulse phase fluctuations, we applied the timing technique that had been already successfully applied to the case of an another AMP, XTE J1807-294. Results. We report a precise set of orbital ephemeris. We demonstrate that the companion star is a main-sequence star. We find pulse phase delay fluctuations on the first harmonic with a characteristic amplitude of about 0.05, similar to those also observed for the AMP XTE J1814-338. For the second time, an AMP shows a third harmonic detected during the entire outburst. The first harmonic phase delays exhibit a puzzling behaviour, while the second harmonic phase delays clearly spin-up. The third harmonic also shows a spin-up, although not highly significant (3σ c.l.). The presence of a fourth harmonic is also reported. If we assume that the second harmonic is a good tracer of the spin frequency of the neutron star, we infer a mean spin frequency derivative for this source of 1.65(18) × 10 −13 Hz s −1 . Conclusions. To interpret the pulse phase delays of the four harmonics, we apply the disk threading model, but obtain different and incompatibleṀ estimates for each harmonic. In particular, the phase delays of the first harmonic are heavily affected by phase noise, and consequently, on the basis of these data, it is not possible to derive a reliable estimate ofṀ. The second harmonic gives aṀ consistent with the flux assuming that the source is at a distance of 6.3 kpc. The third harmonic gives a lowerṀ value, with respect to the first and second harmonic, and this would reduce the distance estimate to 3.6 kpc.

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“…In the case of SAX J1808.4-3658 the distance to the source is known and is about 3.5 kpc [11]; therefore we can check if also in this case there is a discrepancy between the mass accretion rate inferred from the timing results and the observed X-ray luminosity. We still find a discrepancy, but, in this case, the mass accretion rate inferred from timing is only a factor of 2 larger than the X-ray luminosity, since this is about 1 × 10 37 ergs/s (see [10]). The spin-down observed at the end of the outburst can be explained, for instance, by a threading of the accretion disk by the neutron star magnetic field outside the accretion radius.…”

Section: Sax J18084-3658mentioning

confidence: 58%

“…We have performed a timing analysis of the 2002 outburst, which lasted about 40 days from October 15 to November 26, one of the most extensively covered by RXTE observations (see details in [10]). In this case, the pulse profile shows the presence of a significant first harmonic, and we therefore studied both the phase delays of the fundamental and the phase delays of the harmonic as a function of time.…”

Section: Sax J18084-3658mentioning

confidence: 99%

Order in the chaos? The strange case of accreting millisecond pulsars

Salvo¹,

Burderi²,

Riggio³

et al. 2008

AIP Conference Proceedings

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Abstract. We review recent results from the X-ray timing of accreting millisecond pulsars in LMXBs. This is the first time a timing analysis is performed on accreting millisecond pulsars, and for the first time we can obtain information on the behavior of a very fast pulsar subject to accretion torques. We find both spin-up and spin-down behaviors, from which, using available models for the accretion torques, we derive information on the mass accretion rate and magnetic field of the neutron star in these systems. We also find that the phase delays behavior as a function of time in these sources is sometimes quite complex and difficult to interpret, since phase shifts, most probably driven by variations of the X-ray flux, are sometimes present.

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Order in the Chaos: Spin-up and Spin-down during the 2002 Outburst of SAX J1808.4-3658

Cited by 74 publications

References 13 publications

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Accretion onto stars with octupole magnetic fields: Matter flow, hot spots and phase shifts

Timing of the accreting millisecond pulsar IGR J17511-3057

Order in the chaos? The strange case of accreting millisecond pulsars

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