Calibration of Alignments
of MAXI/GSC
Kosuke Sugimori, Nobuyuki Kawai, Mikio Morii (Tokyo Tech),
Mutsumi Sugizaki, Motoko Serino (RIKEN), and the MAXI team.
[email protected]
http://maxi.riken.jp
One of the important goals of MAXI is to discover X-ray transient objects and inform the world of their positions. For follow-up observations with
other satellites and telescopes, we must reduce the position uncertainties as small as possible. We therefore performed calibration of alignments of
the MAXI/GSC. As a result, we reduced the systematic error in position determination from the initial uncalibrated value of 1.0 degree to 0.2 degree.
Calibration Strategy
Slit
Slat
Collimator
Axis 2:
misalignment around
the axis drawn in the
left figure.
ISS rot. axis
Ideal FoV
ISS rot. axis
ISS rot. axis
Actual FoV
Scan
S
t
a Scan
r
B
S
t
a
r
A
Time [sec]
ISS
rotation
axis
Axis 3:
misalignment
around the moving
axis.
Detection time of all X-ray
sources lags uniformly.
Incident Angle [deg]
Scan with
ISS rotation
Axis 1:
misalignment around
the orbital rotation
axis.
Incident Angle [deg]
MAXI‘s field of view is restricted by the slats
collimators to a narrow and long rectangle, which
scans the sky in the direction perpendicular to the
long dimension. The position of an X-ray source along
the scan direction is determined by the time when
the X-ray source crosses MAXI’s FoV. To determine
the position of the X-ray source perpendicular to
the scan direction, we use the position of the X-ray
image on the detector.
Incident Angle [deg]
Gas Slit Camera
Time [sec]
Time [sec]
Incident angles of X-ray
photons shift uniformly.
Time lags depend on the
incident angles.
Observed
light curve
Incident angle of
X-ray photon
Star A
Light curve of
ideal alignment
Scan
Observed
light curve
Light curve of
ideal alignment
Star B
ISS rotation axis
Position on the
GSC counter
Time
ISS
rotation axis
Time
Results
GRS 1915+105
Cyg X-3, Cyg X-1
Sco X-1
Before
Crab
As a result, we reduced the systematic error in
position determination from the initial uncalibrated
value of 1.0 degree to 0.2 degree.
Upper panels are histograms of the radial distance
between the position of Sco X-1 determined by
MAXI and the actual position.
After
Lower panels are cumulative histograms of upper
histograms.
In September 2009, the attitude of ISS changed greatly because of docking of Space
Shuttle. Therefore the shape of X-ray objects in uncalibrated images are very distorted
(especially Sco X-1).
# of Daily Obs.
0
Radius [deg]
1.0
Systematic error
~ 1.0 deg.
0
Radius [deg]
Reference
Matsuoka, M. et al., PASJ 61 (2009) 999. ,
After
M. Morii, et al. Physica E (2010), doi:10.1016/j.physe.2010.07.031
1.0
Ratio (Cumulative #
of Daily Obs./Total)
Before the calibration (upper panels), images of the objects are somewhat spreading and
their positions shift greatly (green crosses represent actual positions.). But after the
calibration (lower panels), they are improved.
Ratio (Cumulative #
of Daily Obs./Total)
Figures above are images of bright X-ray objects observed by MAXI.
All images are made by using data from September 1, 2009 to September 30, 2009.
# of Daily Obs.
Before
0
Radius [deg]
1.0
Systematic error
~ 0.2 deg.
0
Radius [deg]
1.0