MAXI Gas Slit Camera (GSC) T. Mihara, M. Sugizaki, M. Kohama, and behalf of MAXI Team [email protected] http://maxi.riken.jp.
MAXI Gas Slit Camera (GSC) T. Mihara, M. Sugizaki, M. Kohama, and behalf of MAXI Team
MAXIGSC utilizes twelve large-area proportional counters, a slit and slats collimator. The energy range is 2-30 keV. The FOVs is two set of 160 x 3 degrees. One is toward forward direction of the ISS and the other is the zenithal direction. Two FOV arches scan the sky in every 92 minutes with the ISS rotation. The one-dimensional position-resolution of the PC resolves the X-ray sources within the narrow FOV. Thus the position resolution is determined by the slats collimator and a combination of the slit and the position resolution of the detector. These make the point spread function of 1.5 x 1.5 degrees. The room background is 1.2 × 10−4 c/s/cm2/keV, which is almost the same level with Ginga/LAC. The 5σ sensitivity is 15 mCrab/day, which is improved with the sqrt (t) law and reach the 5σ confusion limit of 0.5 mCrab in 900 days, if the particle-background estimation is accurate enough.
Gas Slit Camera
Each horizon/zenith module embodies the three camera units to cover a wide rectangular FOV of 160 deg x1.5 deg (FWHM) with an almost equal geometrical area of 10--15 cm2. It scans the whole sky according to the ISS rotation coupled with the orbital motion of 92-minute period. Each module the areas of 10 deg from the FOV edges to both the rotation poles are not covered because these directions are interfered with the ISS structures.
The two FOVs of the horizon and the zenith modules scan the sky twice per orbit at 0 and 21.5 minutes in 92-minute orbital period. However, the actual observation operation in orbit is limited in the low particle-background area to protect the counters form the heavy particle irradiation. It reduces the observation duty cycle to about 50%. Still the two FOVs can cover the whole sky.
GSC proportional counters employ resistive carbon fibers with a diameter of 10 micro meters for anode wires. The higher resistive anodes are preferable for the better position resolution because the thermal noise on the readout signal is inversely proportional to the anode resistance.
Each GSC counter has 12 preamplifiers and following signal readout chains for six position-sensitive anodes processed into the both ends (left and right), and two signal chains for veto detectors, which results in 14 analog signals in total.
The front-end electronics boards are built in the back of the proportional counter. The electronic boards include the high-voltage power supply, HK (House-Keeping) electronics and their connectors. The HK circuit monitors temperatures at eight points in the camera (HV box, preamplifiers, gas cell, etc.) as well as the HV values. The HV-power supply with low-power consumption was manufactured by Meisei Electric Co.Ltd.
The energy linearity at 1650 V with K-lines. The data was taken with GSC 0 and anode 0, at off-anode point. The residuals are from expedient function of a - b * exp(c E). With the Xe L-edge jump at 4.70 keV.
Examples of energy spectra of element K-lines (Ti, Cr, Fe, Zn, Se, Y in the energy order). The HV is 1650V.
The point spread function (PSF) of the GSC is determined by the angular response of the slit-and-slat collimator and the position response of the position-sensitive gas counter along the anode wire.
The collimator is designed to have an angular resolution of 1.0--1.5 deg in FWHM, depending on the X-ray incident angle.
The position resolution of the gas counter depends on the X-ray energy, which varies from 1 to 4 mm within the 2--30 keV energy band.
Each GSC gas counter embodies six carbon-anode cells for X-ray detection and the surrounding veto cells for anti-coincidence to remove the cosmic-ray events. The efficiency of the background reduction depends on the LD threshold of the read-out signals. The amplifier gains and LD levels of all the read-out signals are tuned during the initial verification operation.
The background rates of all the 12 GSC counters are found to have similar time variation during the entire observation-operation period. They exhibit clear anti-correlations with the geomagnetic COR in orbit.
Cutoff Rigidity (GV)
Latitude <｜ ｜
RBM count-rate maps of horizon (RBM-H) and zenith (RBM-Z).
High Voltage Operation
Exposure Map and Sensitivity
We started full GSC operation with twelve counter units, named as GSC 0, ..., GSC 9, GSC A, GSC B, on August 15, 2009. The high voltages of all the twelve gas counters were set at the nominal 1650 V. They are reduced to 0 V when the ISS passes through heavy particle background area defined by an on-board Radiation-Zone (RZ) map. In the beginning of the mission, we set the RZ map only at the SAA.
An exposure map on the sky is calculated from the counter HV-operation history, the detector effective area (as a function of a photon incident angle), and information of the ISS orbit, attitude, and configuration parameters (the solar paddles, the space-shuttle).
The status of the GSC response for the nominal HV of 1650 V are shown. The responses functions for the reduced HV of 1550 V are being developed.
On September 8, 2009, the analog power on the GSC 6 counter was suddenly down. It was followed by the power down of another counter, GSC 9, on September 14. The data indicated that a carbon-anode wire in each of these counters was fractured. It was also suggested that the location fractured on the carbon wire had a large amount of discharges occurred repeatedly. We then changed the counter operation strategy so that any risks to cause potential damages on the carbon-anode wire such as discharge and heavy irradiation should be avoided. The high-voltage reduction at the high latitude above 40deg was employed since September 23. Two noisy counters, GSC A and GSC B, which are considered to be heavily damaged, were also stopped.
On March 26, 2010, the analog power on the GSC 3 counter was down, which were considered to suffer a carbon-wire fracture by discharge again. Since then, the operation voltage was reduced to 1550 V, once the counter had a discharge.
As the results of these limited HV operation, the effective
observation efficiency is reduced to about 40%. The sensitivity is also expected to be reduced. The GSC 3, one of the three counters with the fractured carbon wire, started operation with the normal wires on June 21, 2010.
Exposure maps of 1-orbit, 1-day and 27 days on Jan. 2, 2010.
GSC covers approximately 85% of the whole sky for one orbit except for the orbits including the SAA passage, and 95% for a day. The daily map has uncovered area for the Sun direction, the solar-paddle shadow, and the rotation pole that drift on the sky according to the precession of the ISS orbit (70d). We can achieve the 100% full coverage in every three weeks.
An actual exposure time for an arbitrary direction on the sky was typically 4000 cm2 sper day. It is about one third of the initial expectation because the HV operation at the high latitude above 40deg was stopped and four counters out of 12 were disabled in
these period. The 5σ source sensitivity is expected to become 15 mCrab/day.
(left) Light curves of three energy bands of the Crab.
(right) Spectrum of the Crab taken on August 15, 2009 and the best-fit power-law model with an interstellar absorption, folded by the response matrix obtained by the response builder. Backgrounds are extracted from events in the adjacent sky region. The spectrum is open daily in the web for several sources.
Red 2-4 keV,
Green 4-8 keV,
Blue 8-20 keV.
The background spectra including CXB of
MAXI/GSC, RXTE/RXTE, Ginga/LAC.
The GSC all-sky X-ray image taken for the first year. The diameter of point source images roughly represents the relative brightness according to the PSF spread. More than a hundred of discrete X-ray sources over the whole sky are easily seen, as well as unresolved Galactic ridge emission along the Galactic plane.
The HV-on-time of GSC 2 camera.
The on-time is about 40% since Sep. 23 2009.