Solar System Minnows – X-ray occultation of Scorpius X-1 by small Trans-Neptunian Objects 張祥光 梁昭賢、吳秉憲、林峻哲、丘政倫、葉倫文、吳宇智、劉志原 Department of Physics and Institute of Astronomy National Tsing Hua University 金升光 Institute of Astronomy and Astrophysics, Academia Sinica

The 26th IAU General Assembly, Session 2, Prague, Aug 24, 2006

古柏帶 (Kuiper Belt) 古柏帶物體 (Kuiper Belt Objects, KBOs) 奧特雲 (Oort Cloud) Trans-Neptunian region Trans-Neptunian Objects (TNOs) proposed by Leonard (1930), Edgeworth (1943, 1949), Oort (1950), Kuiper (1951) So, sometimes EKB, EKBOs.

IAUC 5611 ( ) D. Jewitt & J. Luu The first TNO (1992 QB1) was discovered with the UH 2.2-m telescope on top Mauna Kea on August 30, Nature 362, 730 (1993)

Eris (2003 UB313)

(Cooray 2006, Nature 442, 640)

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 photometry less diffraction shorter eventssmaller 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 10 5 cps. * Detection of msec time-scale occultation is possible. * The RXTE/PCA data used in this search spans over 7 years from 1996 to The total exposure is ksec.

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

What are they??? - Instrumental? (see Chang et al. 2007, MNRAS, (astro-ph/ )) - 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? The Fresnel scale : F = (λd / 2) 1/2 For λ=0.3 nm (4 keV) F ~ AU ~ ,000 AU ~ kpc Most likely, occultation by objects within about 1000 AU.

Diffraction Pattern (mono-chromatic)

Light curves of the 12 probable non-instrumental events

Direct imaging is possible only for larger ones (e.g. Trujillo et al. 2001). Optical occultation survey may be able to detect objects down to km size (e.g. the Taiwan- America Occultation Survey, TAOS). Models predict a turnover in the power law at small sizes (e.g. Kenyon et al. 2004).

Thank you! 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.  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. Acknowledgement : 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.

(Roques et al., 2006, AJ 132, 819) EXPLORATION OF THE KUIPER BELT BY HIGH-PRECISION PHOTOMETRIC STELLAR OCCULTATIONS: FIRST RESULTS Roques, F.; Doressoundiram, A.; Dhillon, V.; Marsh, T.; Bickerton, S.; Kavelaars, J. J.; Moncuquet, M.; Auvergne, M.; Belskaya, I.; Chevreton, M.; Colas, F.; Fernandez, A.; Fitzsimmons, A.; Lecacheux, J.; Mousis, O.; Pau, S.; Peixinho, N.; Tozzi, G. P.

EXPLORATION OF THE KUIPER BELT BY HIGH-PRECISION PHOTOMETRIC STELLAR OCCULTATIONS: FIRST RESULTS Roques, F.; Doressoundiram, A.; Dhillon, V.; Marsh, T.; Bickerton, S.; Kavelaars, J. J.; Moncuquet, M.; Auvergne, M.; Belskaya, I.; Chevreton, M.; Colas, F.; Fernandez, A.; Fitzsimmons, A.; Lecacheux, J.; Mousis, O.; Pau, S.; Peixinho, N.; Tozzi, G. P. (Roques et al., 2006, AJ 132, 819)

Angular Scale of TNO X-ray Occultation Sco X-1 (at 2.8 kpc): 1 mas = 1 1kpc or kpc = 4.2  10 8 km 1  as = 420, kpc binary orbit: 18.9 hrs ~ 2,500,000 km Pluto = 2300 km = 0.1 arcsec (in diameter) TNO23 m = 1  as (at Pluto distance)