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Title: Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?

Abstract

The accreting millisecond X-ray pulsar SAX J1808.4−3658 shows a peculiar orbital evolution that proceeds at a very fast pace. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves and which physical processes (such as mass loss or spin–orbit coupling) are occurring in the binary. It has also been suggested that, when in quiescence, SAX J1808.4−3658 turns on as a radio pulsar, a circumstance that might provide a link between accreting millisecond pulsars and black-widow (BW) radio pulsars. In this work, we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in 2014 August and an X-ray study of the 2015 outburst with Chandra , Swift XRT, and INTEGRAL . In quiescence, we detect no radio pulsations and place the strongest limit to date on the pulsed radio flux density of any accreting millisecond pulsar. We also find that the orbit of SAX J1808.4−3658 continues evolving at a fast pace. We compare the orbital evolution of SAX J1808.4−3658 to that of several other accreting and nonaccreting binaries, including BWs, redbacks, cataclysmic variables, black holes, and neutron stars in low-mass X-raymore » binaries. We discuss two possible scenarios: either the neutron star has a large moment of inertia and is ablating the donor, generating mass loss with an efficiency of 40%, or the donor star has a strong magnetic field of at least 1 kG and is undergoing quasi-cyclic variations due to spin–orbit coupling.« less

Authors:
;  [1]; ;  [2];  [3]; ; ;  [4];  [5]
  1. Leiden Observatory, Leiden University, Neils Bohrweg 2, 2333 CA, Leiden (Netherlands)
  2. ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7900 AA, Dwingeloo (Netherlands)
  3. SRON-National Institute for Space Research, Sorbonnelaan 2, NL-3584 CA, Utrecht (Netherlands)
  4. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam (Netherlands)
  5. University of Southampton, School of Physics and Astronomy, Southampton SO17 1BJ (United Kingdom)
Publication Date:
OSTI Identifier:
22663559
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 841; 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; BLACK HOLES; FLUX DENSITY; GHZ RANGE; L-S COUPLING; MAGNETIC FIELDS; MASS TRANSFER; MOMENT OF INERTIA; NEUTRON STARS; NEUTRONS; ORBITS; PULSARS; PULSATIONS; ROTATION; SMALL ANGLE SCATTERING; STELLAR WINDS; TELESCOPES; X RADIATION; X-RAY DIFFRACTION

Citation Formats

Patruno, Alessandro, King, Andrew R., Jaodand, Amruta, Hessels, Jason W. T., Kuiper, Lucien, Bult, Peter, Wijnands, Rudy, Van der Klis, Michiel, and Knigge, Christian. Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA6F5B.
Patruno, Alessandro, King, Andrew R., Jaodand, Amruta, Hessels, Jason W. T., Kuiper, Lucien, Bult, Peter, Wijnands, Rudy, Van der Klis, Michiel, & Knigge, Christian. Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?. United States. doi:10.3847/1538-4357/AA6F5B.
Patruno, Alessandro, King, Andrew R., Jaodand, Amruta, Hessels, Jason W. T., Kuiper, Lucien, Bult, Peter, Wijnands, Rudy, Van der Klis, Michiel, and Knigge, Christian. Thu . "Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?". United States. doi:10.3847/1538-4357/AA6F5B.
@article{osti_22663559,
title = {Radio Pulse Search and X-Ray Monitoring of SAX J1808.4−3658: What Causes Its Orbital Evolution?},
author = {Patruno, Alessandro and King, Andrew R. and Jaodand, Amruta and Hessels, Jason W. T. and Kuiper, Lucien and Bult, Peter and Wijnands, Rudy and Van der Klis, Michiel and Knigge, Christian},
abstractNote = {The accreting millisecond X-ray pulsar SAX J1808.4−3658 shows a peculiar orbital evolution that proceeds at a very fast pace. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves and which physical processes (such as mass loss or spin–orbit coupling) are occurring in the binary. It has also been suggested that, when in quiescence, SAX J1808.4−3658 turns on as a radio pulsar, a circumstance that might provide a link between accreting millisecond pulsars and black-widow (BW) radio pulsars. In this work, we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in 2014 August and an X-ray study of the 2015 outburst with Chandra , Swift XRT, and INTEGRAL . In quiescence, we detect no radio pulsations and place the strongest limit to date on the pulsed radio flux density of any accreting millisecond pulsar. We also find that the orbit of SAX J1808.4−3658 continues evolving at a fast pace. We compare the orbital evolution of SAX J1808.4−3658 to that of several other accreting and nonaccreting binaries, including BWs, redbacks, cataclysmic variables, black holes, and neutron stars in low-mass X-ray binaries. We discuss two possible scenarios: either the neutron star has a large moment of inertia and is ablating the donor, generating mass loss with an efficiency of 40%, or the donor star has a strong magnetic field of at least 1 kG and is undergoing quasi-cyclic variations due to spin–orbit coupling.},
doi = {10.3847/1538-4357/AA6F5B},
journal = {Astrophysical Journal},
number = 2,
volume = 841,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}
  • We report on time-resolved optical imaging of the X-ray binary SAX J1808.4-3658 during its quiescent state and 2008 outburst. The binary, containing an accretion-powered millisecond pulsar, has a large sinusoidal-like modulation in its quiescent optical emission. We employ a Markov chain Monte Carlo technique to fit our multi-band light curve data in quiescence with an irradiated star model, and derive a tight constraint of 50{sup +6}{sub -5} deg on the inclination angle i of the binary system. The pulsar and its companion are constrained to have masses of 0.97{sub -0.22}{sup +0.31} M{sub Sun} and 0.04{sub -0.01}{sup +0.02} M{sub Sun} (bothmore » 1{sigma} ranges), respectively. The dependence of these results on the measurements of the companion's projected radial velocity is discussed. We also find that the accretion disk had nearly constant optical fluxes over a {approx}500 day period in the quiescent state our data covered, but started brightening 1.5 months before the 2008 outburst. Variations in modulation during the outburst were detected in our four observations made 7-12 days after the start of the outburst, and a sinusoidal-like modulation with 0.2 mag amplitude changed to have a smaller amplitude of 0.1 mag. The modulation variations are discussed. We estimate the albedo of the companion during its quiescence and the outburst, which was approximately 0 and 0.8 (for isotropic emission), respectively. This large difference probably provides additional evidence that the neutron star in the binary turns on as a radio pulsar in quiescence.« less
  • 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
  • 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
  • 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
  • We study the relation between the 300-700 Hz upper kHz quasi-periodic oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi X-ray Timing Explorer. We find that the pulse amplitude systematically changes by a factor of ∼2 when the upper kHz QPO frequency passes through 401 Hz: it halves when the QPO moves to above the spin frequency and doubles again on the way back. This establishes for the first time the existence of a direct effect of kHz QPOs on the millisecond pulsations and provides amore » new clue to the origin of the upper kHz QPO. We discuss several scenarios and conclude that while more complex explanations can not formally be excluded, our result strongly suggests that the QPO is produced by azimuthal motion at the inner edge of the accretion disk, most likely orbital motion. Depending on whether this azimuthal motion is faster or slower than the spin, the plasma then interacts differently with the neutron-star magnetic field. The most straightforward interpretation involves magnetospheric centrifugal inhibition of the accretion flow that sets in when the upper kHz QPO becomes slower than the spin.« less