Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects


Since the discovery1 of the trans-neptunian objects (TNOs) in 1992, nearly one thousand new members have been added to our Solar System2,3, several of which are as big as—or even larger than—Pluto4,5. The properties of the population of TNOs, such as the size distribution and the total number, are valuable information for understanding the formation of the Solar System, but direct observation is only possible for larger objects with diameters above several tens of kilometres. Smaller objects, which are expected to be more abundant, might be found when they occult background stars6,7,8,9,10, but hitherto there have been no definite detections. Here we report the discovery of such occultation events at millisecond timescales in the X-ray light curve of Scorpius X-1. The estimated sizes of these occulting TNOs are ≤100 m. Their abundance is in line with an extrapolation of the distribution2 of sizes of larger TNOs.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: Deviation distributions of the Sco X-1 RXTE/PCA light curve.
Figure 2: RXTE/PCA light curves of some example dip events.
Figure 3: The differential size distribution of TNOs.

Similar content being viewed by others


  1. Jewitt, D. C. & Luu, J. X. Discovery of the candidate Kuiper belt object 1992 QB1 . Nature 362, 730–732 (1993)

    Article  ADS  Google Scholar 

  2. Trujillo, C. A., Jewitt, D. C. & Luu, J. X. Properties of the trans-Neptunian belt: Statistics from the Canada-France-Hawaii telescope survey. Astron. J. 122, 457–473 (2001)

    Article  ADS  Google Scholar 

  3. Luu, J. X. & Jewitt, D. C. Kuiper belt objects: relics from the accretion disk of the sun. Annu. Rev. Astron. Astrophys. 40, 63–101 (2002)

    Article  ADS  Google Scholar 

  4. Brown, M. E., Trujillo, C. A. & Rabinowitz, D. 2003 EL_61, 2003 UB_313, and 2005 FY_9. IAU Circ. 8577 (2005)

  5. Bertoldi, F., Altenhoff, W., Weiss, A., Mentn, K. M. & Thum, C. The trans-neptunian object UB313 is larger than Pluto. Nature 439, 563–564 (2006)

    Article  ADS  CAS  Google Scholar 

  6. Bailey, M. E. Can ‘invisible’ bodies be observed in the Solar System? Nature 259, 290–291 (1976)

    Article  ADS  Google Scholar 

  7. Brown, M. J. I. & Webster, R. L. Occultations by Kuiper belt objects. Mon. Not. R. Astron. Soc. 289, 783–786 (1997)

    Article  ADS  Google Scholar 

  8. Cooray, A. & Farmer, A. J. Occultation searches for Kuiper belt objects. Astrophys. J. 587, L125–L128 (2003)

    Article  ADS  Google Scholar 

  9. Alcock, C. et al. TAOS: The Taiwanese-American Occultation Survey. Earth Moon Planet 92, 459–464 (2003)

    Article  ADS  Google Scholar 

  10. Roques, F. et al. A search for small Kuiper belt objects by stellar occultations. Astrophys. J. 594, L63–L66 (2003)

    Article  ADS  Google Scholar 

  11. Giacconi, R., Gursky, H., Paolini, F. & Rossi, B. Evidence for X-rays from sources outside the solar system. Phys. Rev. Lett. 9, 439–443 (1962)

    Article  ADS  Google Scholar 

  12. Bradt, H. V., Rothschild, R. E. & Swank, J. H. X-ray timing explorer mission. Astron. Astrophys. Suppl. Ser. 97, 355–360 (1993)

    ADS  CAS  Google Scholar 

  13. Jahoda, K. et al. Peak luminosities of bursts from GRO J1744–28 measured with the RXTE PCA. Nucl. Phys. B 69, 210–215 (1998)

    Article  Google Scholar 

  14. Middleditch, J. & Priedhorsky, W. C. Discovery of rapid quasi-periodic oscillations in Scorpius X-1. Astrophys. J. 306, 230–237 (1986)

    Article  ADS  CAS  Google Scholar 

  15. Scargle, J. D. et al. The quasi-periodic oscillations and very low frequency noise of Scorpius X-1 as transient chaos: a dripping handrail? Astrophys. J. 411, L91–L94 (1993)

    Article  ADS  CAS  Google Scholar 

  16. van der Klis, M., Wijnands, R., Horne, K. & Chen, W. Kilohertz quasi-periodic oscillation peak separation is not constant in Scorpius X-1. Astrophys. J. 481, L97–L100 (1997)

    Article  ADS  Google Scholar 

  17. Gierlinski, M. & Zdziarski, A. A. Discovery of powerful millisecond X-ray flares from Cygnus X-1. Mon. Not. R. Astron. Soc. 343, L84–L88 (2003)

    Article  ADS  Google Scholar 

  18. Uttley, P., McHardy, I. M. & Vaughan, S. Non-linear X-ray variability in X-ray binaries and active galaxies. Mon. Not. R. Astron. Soc. 359, 345–362 (2005)

    Article  ADS  Google Scholar 

  19. Steeghs, D. & Casares, J. The mass donor of Scorpius X-1 revealed. Astrophys. J. 568, 273–278 (2002)

    Article  ADS  CAS  Google Scholar 

  20. Bradshaw, C. F., Geldzahler, B. J. & Fomalont, E. B. The X-ray spectral changes of Scorpius X-1. Astrophys. J. 592, 486–497 (2003)

    Article  ADS  Google Scholar 

  21. Barnard, R., Church, M. J. & Balucinska-Church, M. Physical changes during Z-track movement in Sco X-1. Astron. Astrophys. 405, 237–247 (2003)

    Article  ADS  Google Scholar 

  22. Bradshaw, C. F., Fomalont, E. B. & Geldzahler, B. J. High resolution parallax measurements of Scorpius X-1. Astrophys. J. 512, L121–L124 (1999)

    Article  ADS  Google Scholar 

  23. Kenyon, S. J. Planet formation in the outer solar system. Publ. Astron. Soc. Pacif. 114, 265–283 (2002)

    Article  ADS  Google Scholar 

  24. Kenyon, S. J. & Bromley, B. C. The size distribution of Kuiper belt objects. Astrophys. J. 128, 1916–1926 (2004)

    ADS  Google Scholar 

  25. Bernstein, G. L. et al. The size distribution of trans-Neptunian bodies. Astrophys. J. 128, 1364–1390 (2004)

    Google Scholar 

Download references


We thank A. Cooray for comments that significantly improved this paper. We also benefited from discussions with R. Taam, Y.-H. Chu, W. Becker and C. Alcock. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center, and of the JPL HORIZON online Solar System data and ephemeris computation service. This work was supported by the National Science Council of the Republic of China and Academia Sinica in Taipei.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Hsiang-Kuang Chang.

Ethics declarations

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

Supplementary Table 1

The log of all the data employed. (XLS 46 kb)

Supplementary Table 2

Individual information of all the events. (XLS 29 kb)

Supplementary Figure 1

Light curves of all the events. (PDF 425 kb)

Supplementary Discussion

Occultation-rate estimate of asteroids and TNOs. (PDF 102 kb)

Rights and permissions

Reprints and permissions

About this article

Cite this article

Chang, HK., King, SK., Liang, JS. et al. Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects. Nature 442, 660–663 (2006).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing