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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.

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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.


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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.

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Correspondence to Hsiang-Kuang Chang.

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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)

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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).

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