Occultation of XRays from Scorpius X1 by Small TransNeptunian Objects 2006, Nature Letters

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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

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Kuiper Belt Object
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Cumulative sky density vs. apparent red
magnitude of KBOs (Bernstein et al. 2004)
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Conceptual illustration of KBO detection
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• 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
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Why X-ray occultation?
faster photometry less diffraction
better chance to detect
shorter events
smaller bodies
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.
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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.
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Optical occultation survey may be able to detect
objects down to km size (e.g. the Taiwan-America
Occultation Survey, TAOS).
Direct imaging is possible only for larger ones
(e.g. Trujillo et al. 2001).
Models predict a turnover in the power law at
small sizes (e.g. Kenyon et al. 2004).
A pains-taking HST observation reports 3
detections (Bernstein et al. 2004).
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Summary gt The discovery of X-ray occultation by
small TNOs opens up a new window in this
field. gt 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. gt
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.