This animation illustrates the direct relationship between a pulsar's X-ray pulses and its quasi-periodic oscillation (QPO), an X-ray source originating near the inner edge of an accretion disk and shown here as a white patch. When the QPO orbits more slowly than the pulsar’s spin, the magnetic field of the neutron star inhibits some gas from flowing onto hot spots at its magnetic poles, dimming the X-ray pulses. 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 pulsar's magnetic field. When the QPO matches or bests the pulsar’s spin, more gas streams onto the magnetic poles, and the pulses brighten. Gas may even flow directly onto the pulsar's equatorial region. This pulse-QPO relationship was first observed in a system called SAX J1808 by NASA's Rossi X-ray Timing Explorer.
Credit: NASA's Goddard Space Flight Center
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).