Operations
Because of continued exposure, the MCP (microchannel
plate) gain in the region of best focus in both the HRC-I and HRC-S
has dropped significantly since launch. It is likely that the high
voltage on the MCPs will be raised later this year to offset this gain
drop. The only operational anomaly during the past year was the
failure of one of the shutters to insert and block zeroth order during
an HRC-S/LETG observation of the Crab Nebula. The cause of this
failure is being investigated, but at the present time the use of the
shutters is not available to GOs. The impact of this limitation on
science observations with the HRC-S/LETG is minimal as these shutters
have only been used for observations of the Crab.
Finally, we have been investigating the possibility of using the HRC anti-coincidence shield as a backup to the EPHIN particle detector. The lifetime of the EPHIN is uncertain, and it is critical that the particle environment of Chandra is monitored for safe operation of both focal plane detector systems. Contingency plans are currently being developed to use the HRC anticoincidence shield as the primary particle monitor in the event of a serious problem with the EPHIN.
Calibration
Work has been continuing in the HRC laboratory in our effort to map out the
imaging properties of the HRC-S detector in fine detail. This campaign
of measurements was motivated by small discrepancies between
theoretically predicted and actual detected LETG features. (See last
year's Chandra Newsletter.) These discrepancies are most likely
related to two factors: inadequate statistics in generating HRC degap
parameters and/or
limitations of the three tap algorithm currently used to adjust event
positions. (See the website "http://hea-www.harvard.edu/HRC/calib/hrci_cal.html#flightdegap"
for details.) In an effort to
better understand this problem and develop an improved event
positioning algorithm, the HRC IPI is conducting a series of
measurements on the laboratory Proof of Concept (POC) HRC-S. Each of
the three POC HRC-S segments is being illuminated in their entirety
and a small region of one of the segments will be illuminated to great
depth. The purpose of these measurements is to reduce statistics as a
cause of uncertainty in the functional form of the HRC degap, and to
more generally evaluate the three tap algorithm in small detail. Given
the fine pixel scale of the HRC, achieving 3 σ statistics even on a
small area of the detector is daunting; the degap function is
generated for each amplifier/tap which is 256 HRC pixels on a
side. Several square HRC taps would entail collecting ~ 5 x
108 events
in that region, a process that would take a run time of ~ 5 million
seconds. The HRC POC detector is now instrumented with Danahar
multiple axes motion control with sub-micron precision and readout to
facilitate these measurements. A second set of experiments will use
these capabilities to map out the HRC spatial imaging properties on a
sub-micron level. These measurements will be accomplished by moving
the HRC POC behind a multi-pinhole mask. The real and reconstructed
positions of the pinholes will be used to measure and parameterize the
HRC positioning algorithms on a very fine scale. The results of these
measurements will then be applied to the event processing algorithm of
the flight HRC detectors.
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| FIGURE 4: X-ray sources detected by the HRC-I overlaid on a 2MASS J-band image of the Sigma Orionis cluster. Left: the full HRC field. with the central 4 arcmins as outlined and shown in the right panel. The small horizontal marks on the bottom of each figure indicate 3" (left) and 30" (right). |
Sigma Orionis Cluster
In November 2002 a team lead by Scott Wolk (CfA)
used the HRC-I to observe the Sigma Orionis cluster for ~100 ks. This
cluster is unique because it is relatively dust free for its
(approximately) 3 million year age. The HRC-I was used because a major
goal of the observation was to detect brown dwarfs which are common in
this field. Brown dwarfs are fairly soft X-ray sources peaking below
500 eV. The HRC provided good sensitivity to soft X-rays across the
entire 30' x 30' field. To date, they have detected 195 sources in the
field. Two views of the positions of the X-ray points overlaid onto a
2MASS J-band image are shown in Figure 4. We have optical photometry
of 172 of these sources. About 5 have colors consistent with those of
brown dwarfs. Other findings include the discovery of X-rays from 4 of
the 5 components of the Sigma Orionis multiple itself. Correlation
with our spectroscopic data is ongoing.
AB Doradus
The team of G.A.J. Hussain (ESTEC/ESA), N. Brickhouse
(CFA), A.K. Dupree (CFA) used the HRC-S/LETG to probe the stellar
coronae of rapidly rotating stars. Rapidly rotating solar-
type stars display signs of activity that are typically over two
orders of magnitude greater than those observed on the Sun. AB Dor
(K0V, Prot = 0.51 days, vesin i = 90 km/s) is one of the brightest
examples of the ultra-fast rotators, a class of very active single
stars that have recently arrived onto the main sequence. LETG/HRC-S
monitored AB Dor continuously on 2002 December 11 over 88 ksec (1.98
rotation cycles). No large flares were observed over this period. The
88 ksec exposure was divided into eight quarter-phase bins and the
positions of line centroids in the strongest spectral lines were
measured. The strongest line (O VIII, 18.97 Ã
) can be centroided to
the greatest degree of precision and shows evidence of rotational
modulation that repeats from one cycle to the next. When converted to
velocityspace, this corresponds to a cyclic variation with an
amplitude of approximately 30 km s-1 as shown in Figure 5. This result
strongly suggests that AB Dor s quiescent coronal emission is
concentrated in a compact region near the surface at high latitudes,
(as emission from low latitudes would cause a larger amplitude
variation and no modulation would be detected from a diffuse
corona). It is well established that surface magnetic activity in
these active rapid rotators is concentrated near the poles and high
latitudes. These results support the emerging view that active coronae
are also preferentially located at high latitudes.
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| FIGURE 5: Rotational modulation of O VIII, 18.97 Å line converted to velocity space as a function of phase. |