Using a distant X-ray source to detect small Transneptunian Objects (TNOs)
[translated and adapted from Sterne und Weltraum 10/2006 by Gareth Kelly]
A Taiwanese astronomy team has used an unusual technique to detect very small TNOs. They
used measurements from the satellite ROSSI X-ray Timing Explorer of the brightest X-ray
object in the sky, Scorpius X-1, which is a pulsar (a neutron star). The team used 90 hours of
data obtained between 1996 and 2002. Computer searches of the satellite’s data revealed 58
breaks in the X-rays which they were able to interpret as the signal being blocked by TNOs, with
a diameter of between 20 and 100 metres and an orbital radius of between 30 and 50
Astronomical Units*. They hope to develop the technique to estimate the total number of small
TNOs. Up till now, the smallest TNO directly imaged by the Hubble Space Telescope, is about
30 km across.
* An Astronomical Unit (AU) is the mean orbital radius of the Earth = 149  1011 m.
Exercise 1
(a)
Determine the ratio,  
observer
(b)
 TNO
, of the angular sizes of a TNO and X-1.
 X-1
object
angular size, 
Can the X-ray source be regarded as being a point, relative to the size of the TNO?
Use the following data:
Diameter of TNO, DTNO = 100 m; Diameter of X-1 pulsar, DX-1 = 20 km.
180  3600
Distance to TNO, dTNO = 43 AU; distance to X-1 = 28 kpc [1 pc=
AU]

Exercise 2
Depending on the position of the Earth in its orbit over the 6 years of the observations, the
angular speed of the Earth relative to the TNO is sometimes added and sometimes subtracted.
What is the maximum blocking time for the signals which could be expected from a spherical
TNO with a 100 m diameter?
The orbital period of the TNO can be obtained using Kepler’s third law in the form:
d 2  P3 , where d is the orbital radius in AUs, as above; P is the orbital period in years.
Exercise 3
Electromagnetic waves moving past an object are diffracted. Because of this a bright spot is
observed in the middle of the shadow of the object, observed from a long way away. The spot’s
radius, r , depends upon the wavelength of the light and the distance of the TNO according to
the equation:
2 
r
dTNO
4 2
How large is the bright spot in the X-ray region, with  = 06 nm and in visible light with
 = 500 nm. Does the TNO actually cast a shadow, as seen from the Earth, in both cases?
Ref: for the original paper on which the SuW article was based: http://www.nature.com/nature/journal/v442/n7103/full/nature04941.html
Examples of data from occultation events
58 dip events with random probability lower
than 10-3 were found
Ref: for the original paper on which the SuW article was based: http://www.nature.com/nature/journal/v442/n7103/full/nature04941.html