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Title: Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658

Abstract

We report the discovery of a 1-5 Hz X-ray flaring phenomenon observed at >30 mCrab near peak luminosity in the 2008 and 2011 outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 in observations with the Rossi X-ray Timing Explorer. In each of the two outbursts this high luminosity flaring is seen for ∼3 continuous days and switches on and off on a timescale of 1-2 hr. The flaring can be seen directly in the light curve, where it shows sharp spikes of emission at quasi-regular separation. In the power spectrum it produces a broad noise component, which peaks at 1-5 Hz. The total 0.05-10 Hz variability has a fractional rms amplitude of 20%-45%, well in excess of the 8%-12% rms broadband noise usually seen in power spectra of SAX J1808.4-3658. We perform a detailed timing analysis of the flaring and study its relation to the 401 Hz pulsations. We find that the pulse amplitude varies proportionally with source flux through all phases of the flaring, indicating that the flaring is likely due to mass density variations created at or outside the magnetospheric boundary. We suggest that this 1-5 Hz flaring is a high mass accretion rate version of themore » 0.5-2 Hz flaring which is known to occur at low luminosity (<13 mCrab), late in the tail of outbursts of SAX J1808.4-3658. We propose the dead-disk instability, previously suggested as the mechanism for the 0.5-2 Hz flaring, as a likely mechanism for the high luminosity flaring reported here.« less

Authors:
;  [1]
  1. Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam (Netherlands)
Publication Date:
OSTI Identifier:
22365731
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 789; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; DENSITY; EMISSION; INSTABILITY; LUMINOSITY; MASS; NEUTRONS; PULSARS; SMALL ANGLE SCATTERING; SPECTRA; STARS; VISIBLE RADIATION; X RADIATION; X-RAY DIFFRACTION

Citation Formats

Bult, Peter, and Van der Klis, Michiel, E-mail: p.m.bult@uva.nl. Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658. United States: N. p., 2014. Web. doi:10.1088/0004-637X/789/2/99.
Bult, Peter, & Van der Klis, Michiel, E-mail: p.m.bult@uva.nl. Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658. United States. doi:10.1088/0004-637X/789/2/99.
Bult, Peter, and Van der Klis, Michiel, E-mail: p.m.bult@uva.nl. Thu . "Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658". United States. doi:10.1088/0004-637X/789/2/99.
@article{osti_22365731,
title = {Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658},
author = {Bult, Peter and Van der Klis, Michiel, E-mail: p.m.bult@uva.nl},
abstractNote = {We report the discovery of a 1-5 Hz X-ray flaring phenomenon observed at >30 mCrab near peak luminosity in the 2008 and 2011 outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 in observations with the Rossi X-ray Timing Explorer. In each of the two outbursts this high luminosity flaring is seen for ∼3 continuous days and switches on and off on a timescale of 1-2 hr. The flaring can be seen directly in the light curve, where it shows sharp spikes of emission at quasi-regular separation. In the power spectrum it produces a broad noise component, which peaks at 1-5 Hz. The total 0.05-10 Hz variability has a fractional rms amplitude of 20%-45%, well in excess of the 8%-12% rms broadband noise usually seen in power spectra of SAX J1808.4-3658. We perform a detailed timing analysis of the flaring and study its relation to the 401 Hz pulsations. We find that the pulse amplitude varies proportionally with source flux through all phases of the flaring, indicating that the flaring is likely due to mass density variations created at or outside the magnetospheric boundary. We suggest that this 1-5 Hz flaring is a high mass accretion rate version of the 0.5-2 Hz flaring which is known to occur at low luminosity (<13 mCrab), late in the tail of outbursts of SAX J1808.4-3658. We propose the dead-disk instability, previously suggested as the mechanism for the 0.5-2 Hz flaring, as a likely mechanism for the high luminosity flaring reported here.},
doi = {10.1088/0004-637X/789/2/99},
journal = {Astrophysical Journal},
number = 2,
volume = 789,
place = {United States},
year = {Thu Jul 10 00:00:00 EDT 2014},
month = {Thu Jul 10 00:00:00 EDT 2014}
}
  • We present a simultaneous periodic and aperiodic timing study of the accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts of the source and for the first time provide a full and systematic investigation of the enigmatic phenomenon of the 1 Hz flares observed during the final stages of some of the outbursts. We show that links between pulsations and 1 Hz flares might exist, and suggest that they are related with hydrodynamic disk instabilities that are triggered close to the disk-magnetosphere boundary layer when the system is entering the propeller regime.
  • The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5 {+-} 1.2) x 10{sup -18} Hz s{sup -1}, which is consistent with the previously determined spin derivative. The spin down occurs mostly during quiescence, and is most likely due to the magnetic dipole torque from a B = 1.5 x 10{sup 8} G dipolar field at themore » neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80 {+-} 0.06) x 10{sup -12} s s{sup -1}, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.« less
  • In this Letter we present a new analysis of the torques acting on the accreting millisecond X-ray pulsars SAX J1808.4-3658 and XTE J1814-338, and show how our results can be used to constrain theoretical models of the spin evolution. In particular, we find upper limits on any spin-up/down phase of XTE J1814-338 of |{nu}-dot| < or approx. 1.5x10{sup -14} Hzs{sup -1} at 95% confidence level. We examine the possibility that a gravitational wave torque may be acting in these systems and suggest that a more likely scenario is that both systems are close to spin equilibrium, as set by themore » disk/magnetosphere interaction.« less
  • The pulse shapes detected during multiple outbursts of SAX J1808.4-3658 are analyzed in order to constrain the neutron star's mass and radius. We use a hot-spot model with a small scattered-light component to jointly fit data from two different epochs, under the restriction that the star's mass and radius and the binary's inclination do not change from epoch to epoch. All other parameters describing the spot location, emissivity, and relative fractions of blackbody to Comptonized radiation are allowed to vary with time. The joint fit of data from the 1998 'slow decay' and the 2002 'end of outburst maximum' epochsmore » using the constraint i < 90 deg. leads to the 3{sigma} confidence constraint on the neutron star mass 0.8 M{sub sun} < M < 1.7 M{sub sun} and equatorial radius 5 km < R < 13 km. Inclinations as low as 41 deg. are allowed. The best-fit models with M>1.0 M{sub sun} from joint fits of the 1998 data with data from other epochs of the 2002 and 2005 outbursts also fall within the same 3{sigma} confidence region. This 3{sigma} confidence region allows a wide variety of hadronic equations of state, in contrast with an earlier analysis by Leahy et al. of only the 1998 outburst data that only allowed for extremely small stars.« less
  • The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this Letter, we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-Ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baselinemore » of 13 years. We find that the expansion of the 2 hr orbit is accelerating at a rate of P-double dot{sub b} approx. = 1.6 x 10{sup -20} s s{sup -2} and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (|{nu}-dot| < or approx. 4 x 10{sup -13} Hz s{sup -1}) and the long-term spin-down remains stable over 13 years with {nu}-dot approx. = -10{sup -15} Hz s{sup -1}.« less