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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. 1

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

  2. 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)

  3. 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)

  4. 4

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

  5. 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)

  6. 6

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

  7. 7

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

  8. 8

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

  9. 9

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

  10. 10

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

  11. 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)

  12. 12

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

  13. 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)

  14. 14

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

  15. 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)

  16. 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)

  17. 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)

  18. 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)

  19. 19

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

  20. 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)

  21. 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)

  22. 22

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

  23. 23

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

  24. 24

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

  25. 25

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

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

Correspondence to Hsiang-Kuang Chang.

Ethics declarations

Competing interests

Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. 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, H., King, S., Liang, J. et al. Occultation of X-rays from Scorpius X-1 by small trans-neptunian objects. Nature 442, 660–663 (2006). https://doi.org/10.1038/nature04941

Download citation

Further reading


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.