Occultation of X-Rays from Scorpius X-1 by Small Trans-Neptunian Objects (2006, Nature Letters)

Hsiang-Kuang Chang, Sun-Kun King, Jau-Shian Liang, Ping-Shien Wu, Lupin Chun-Che Lin,& Jeng-Lun Chiu1

Kuiper Belt Object

[ Conceptual illustration of KBO detection ]

Optical occultation search:

TAOS – CfA, NCU, ASIAA

French – Roques et al. Paris Observatory

Australian – Georgevits et al. UNSW, AAO

Canadian – Kavelaars et al. NRC, McMaster U

(Whipple – CfA, JPL)

No definite detections so far……

Why X-ray occultation?

faster photometryless diffraction

shorter events smaller bodies better chance to detect

* The background X-ray source needs to be bright enough.

* Sco X-1 is the brightest in the Sky.

* RXTE/PCA has the largest effective area.

* The typical PCA count rate of Sco X-1 is 105 cps. * Detection of msec time-scale occultation is possible.

* The RXTE/PCA data used in this search spans over 7 years from 1996 to 2002. The total exposure is 564.3 ksec.

* In the 564-ksec data, 107 events were found.

* What are they? Instrumental? Intrinsic variation of Sco X-1? Objects moving around Sco X-1? Objects in the interstellar space moving across the line of sight? The main belt asteroids?

* Assuming a distance of 43 AU and a circular orbital with a minimum inclination, the corresponding size of these occulting TNOs can be estimated from its speed relative to the RXTE spacecraft at each event epoch.

Direct imaging is possible only for larger ones (e.g. Trujillo et al. 2001).

A pains-taking HST observation reports 3 detections (Bernstein et al. 2004).

Models predict a turnover in the power law at small sizes (e.g. Kenyon et al. 2004).

Optical occultation survey may be able to detect objects down to km size (e.g. the Taiwan-America Occultation Survey, TAOS).

Summary

> The discovery of X-ray occultation by small TNOs opens up a new window in this field.

> The first result indicates a small turnover radius (less than about 100 meters) in the size distribution of TNOs, or, a second component at small sizes. The total mass in the trans-Neptunian region can be several earth masses, much more than previously thought.

> More detailed analysis and more observations will be very rewarding in deriving the size and spatial distribution of TNOs and in resolving the X-ray emitting region of Sco X-1.