x-ray occultation of scorpius x-1 by small trans-neptunian objects liang, jau-shian institute of...
Post on 20-Dec-2015
214 views
TRANSCRIPT
X-ray occultation of Scorpius X-1 by small trans-neptunian objects
Liang, Jau-shian
Institute of Physics, NTHU
2006/04/27
outline
• Introduction(1). trans-neptunian objects(2). the Idea of Occultation Survey(3). detection limit(4). Sco X-1
• Data• Searching method
(1). Introduction(2). Results
• Further analysis(1). size distribution(2). event rate
trans-neptunian objects (TNOs)
• TNOs are objects in the outskirt of our solar system beyond the orbit of Neptune.
• The TNOs were first discovered in 1992.
Distance (AU)
Dia
met
er o
f T
NO
(k
m)
diffraction limit(Optical)
mR= 20 25 30
angular size of a target star
D ⊙
/ 100 pc
D ⊙
/ 1000 pcpencil-b
eam surve
y
detection limit
Sco X-1
• the brightest persistent X-ray source in the sky
• low-mass X-ray binary system• Z-track source• X-ray quasi-periodic oscillator 6-20 Hz : 1%–5% amplitude (rms)
~45 Hz : ~1% amplitude (rms) ~800 Hz : 0.9% and 1.2% amplitude (rms) ~1100 Hz : 0.6%–1.2% amplitude (rms)
• RA,DEC : 16 19 55.07, -15 38 24.8 (J2000)• Distance: 2.8 0.3kpc• Orbital period: 18.9 hr• Inclination angle: 40o 6o
• Luminosity: 2.3 × 1038 ergs s-1
• Mass: 1.4M⊙ and 0.42M⊙
• About 6o to the north of the eciptic
Data
• RXTE/PCA archival data of Sco X-1
• span over 7 years from 1996 to 2002
• total exposure time : ~ 322 ksec
• count rate ~ 105 counts/sec
• S/N ~ 10 (for a millisecond bin)
Rossi X-Ray Timing Explorer (RXTE): December 1995 -
Altitude: 580 kmorbital period: 90 minutesinclination: 23 degreesInstruments: Proportional Counter Array(PCA), High Energy X-ray Timing Experiment (HEXTE), All-Sky Monitor (ASM)
The Proportional Counter Array(PCA)
The instrumental properties are: • Detectors: 5 proportional counters • Collecting area: 6500 square cm• Time resolution: 1 microsec • Energy range: 2 - 60 keV• Spatial resolution: collimator with 1 degree FWHM• Sensitivity: 0.1 mCrab
searching method
• derived the difference between the photon counts of the time bin and mean counts in units of the standard deviation
• The mean counts and standard deviation are derived in a time window running with the time bin.
Size distribution
• R = Vxte-tno * T
R: diameter
Vxte-tno: relative projected speed on the sky
T: duration of event
)()( tnoxtetnoxte VVV
earthtnoearthtno ddVV //
xteV
)( xteVtnoV
)( tnoV
sun
direction of Sco X-1
AUdtno 43
power law index: 3.4 +- 0.5
The numerical simulations of coagulation models for planet formation with collisional cascade predict a power index of 3.5 – 4.0 for TNOs smaller than 0.1 – 1 km, of 4.0 – 4.5 for those larger than 10 – 100 km, and a turnover size between 1 and 30 km10,12.
the size distribution: power law
Estimate of occultation rate
total solid angle
v: velocity, : distance
The main belt asteroids
Based on the SDSS result, there are about 670000 objects with D>1 km.With D0 = 5 km, b1=3, b2=1.3, we have 1= 1.4 X 107 km-1 and 2 = 9.4 X 105 km-1.
Trans-Neptunian Objects
b = 3and estimated total number of TNOs larger than 100km being about 3.8 X 104,we have = 1.1 X 1011 km-1
For v = 25 km/s, min = 30 AU
In a 322-ksec data, the expected number of occultation is about 12.
Our detection of 58 events is more than that number but not by orders of magnitudes.