Long-term X-ray variability of ultraluminous X-ray sources

Lupin Chun Che Lin, Chin Ping Hu, Albert K.H. Kong, David Chien Chang Yen, Jumpei Takata, Yi Chou

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


Long-term X-ray modulations on time-scales from tens to hundreds of days have been widely studied for X-ray binaries located in the Milky Way and the Magellanic Clouds. For other nearby galaxies, only the most luminous X-ray sources can be monitored with dedicated observations. We here present the first systematic study of long-term X-ray variability of four ultraluminous X-ray sources (ESO 243-49 HLX-1, Holmberg IX X-1, M81 X-6, and NGC 5408 X-1) monitored with Swift. By using various dynamic techniques to analyse their light curves, we find several interesting low-frequency quasi-periodicities. Although the periodic signals may not represent any stable orbital modulations, these detections reveal that such long-term regular patterns may be related to superorbital periods and structure of the accretion discs. In particular, we show that the outburst recurrence time of ESO 243-49 HLX-1 varies over time and suggest that it may not be the orbital period. Instead, it may be due to some kinds of precession, and the true binary period is expected to be much shorter.

Original languageEnglish
Pages (from-to)1644-1657
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 1 Dec 2015


  • Methods: data analysis
  • X-rays: binaries
  • X-rays: individual: ESO 243-49 HLX-1
  • X-rays: individual: Holmberg IX X-1
  • X-rays: individual: M81 X-6
  • X-rays: individual: NGC 5408 X-1


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