Universe  ID: 11804

RXTE Data Link Pulsar Pulses with a QPO

A quasi-periodic oscillation, or QPO, is a flicker of X-ray light from an astronomical object that hovers around certain frequencies. The X-rays are thought to be emitted near the inner edge of an accretion disk where gas falls onto a compact object such as a white dwarf, neutron star (also known as a pulsar) or black hole.

For pulsars like SAX J1808.43658 (SAX J1808 for short), gas channeled onto the neutron star’s magnetic poles creates hot spots that are a strong source of X-rays. The object rotates 401 times a second, and as its hot spots wheel into view from Earth, spacecraft like NASA’s Rossi X-ray Timing Explorer (RXTE) detect strong pulses. RXTE also detects a QPO flickering between 300 and 700 times a second.

For the first time, RXTE observations have shown that the pulses and the QPO have a direct relationship, providing insight into the inner structure of the accretion disk. The pulses from the hot spots are twice as bright when the QPO frequency matches or is faster than the pulsar’s spin, and its brightness dims by the same amount when the QPO fluctuates more slowly than the pulsar’s rotation. RXTE observed these changes during outbursts in 2002, 2005 and 2008.

This result strongly suggests that the QPO is a region of especially hot gas at the inner edge of the accretion disk and that its fluctuations trace its orbital motion. When the QPO orbits more slowly than the neutron star’s spin, the flow of matter onto the pulsar becomes inhibited by the pulsar’s magnetic field. During an outburst, the inner edge of the disk is forced closer to the pulsar, resulting in a faster-moving QPO and compression of the magnetic field. When the QPO matches or bests the pulsar’s 401 hertz spin, the flow of matter onto the neutron star is enhanced, with more gas reaching the magnetic poles, which produce brighter pulses. During these episodes, matter may also flow directly onto the pulsar's equatorial regions (lateral accretion).

Credits

Michael Lentz (USRA): Lead Animator
Scott Wiessinger (USRA): Producer
Francis Reddy (Syneren Technologies): Writer
Please give credit for this item to:
NASA's Goddard Space Flight Center