In a study published in the Monthly Notices of the Royal Astronomical Society, LI Chen, ZHANG Guobao and WANG Jiancheng from Yunnan Observatories of the Chinese Academy of Sciences, reported their finding on a rare class of multipeaked type-I X-ray bursts. One quadruple-peaked X-ray burst was found by them for the first time. They also found most of multipeaked bursts are located at the vertex of Color-Color Diagram (CCD). They suggested that the multipeaked bursts in their sample may be ignited at high latitude on the Neutron Star (NS) surface.

Type-I (thermonuclear) X-ray bursts mostly occur in low-mass X-ray binaries (LMXBs). Their light curves usually show a single-peaked profile with a fast rise and exponential decay. The burst X-ray spectrum can be described by a 2-3 keV blackbody, generally interpreted by unstable burning of the accreted fuel on the surface of an accreting NS. Type-I X-ray bursts can be used to identify the compact star as NS in binaries, investigate the composition of burning material and the ways of ignition.

Single-peaked burst can be explained by the thermonuclear flash model which considers continuous energy release, but the mechanism of multipeaked bursts is not clear yet. Recent models about multipeaked bursts generally consider the multi-step release of thermonuclear energy, different burning regions and different ignition latitudes on the surface of NS.

The researchers analyzed all available 16 years Rossi X-ray Timing Explorer (RXTE, a satellite to observe the fast-moving high-energy worlds) data of the LMXB 4U 1636-53. They found 16 multipeaked X-ray bursts which is the largest sample up to now, including 14 double-peaked bursts, one triple-peaked burst and one quadruple-peaked burst. There is a very long "waiting" time between the second and the third peak in this quadruple-peaked burst.

They also found that most of multipeaked bursts are located at the vertex of CCD where the source is in the transition from the hard state to the soft state.

By analyzing the time-resolved spectra and light curves of double-peaked X-ray bursts, researchers found one positive correlation between the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra. The thermal component is believed to originate from the thermal radiation at the inner regions of the accretion disk and/or the surface of the NS. Therefore, the light curve structures of double-peaked bursts may be affected by an enhanced accretion rate or an increased temperature of the NS surface. Assuming the accretion rate is the main factor, their new observational result is consistent with the high-latitude ignition model, which suggests that the double-peaked burst with high peak-flux ratio is likely to occur at the high latitude on the NS surface. 

For the first time, the researchers find the quadruple-peaked burst and investigate the link of ignition latitude with the structure of multipeaked X-ray bursts. They enrich the multipeaked bursts sample and provide interesting observational results of multipeaked bursts.

Contact:

LI Chen

Yunnan Observatories, CAS

lichen@ynao.ac.cn