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.
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.
|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.
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.
|FIGURE 5: Rotational modulation of O VIII, 18.97 Å line converted to velocity space as a function of phase.