Instruments: HRC

The HRC continues to function smoothly with no major problems or anomalies. Analysis of in-flight and laboratory calibration data is on-going. Regular observations of AR Lac indicate a noticeable decrease in the gain of the MCPs (microchannel plates) in both detectors at the position of best focus. The magnitude of the droop is not sufficient to make any significant difference in the performance or operation of the flight instrument. There are no plans to raise the MCP voltage to increase the gain for at least several more years. The shutter failure described in the previous Newsletter has been traced to a failure in a relay. Because of concern that the shutter could get stuck in the "IN" position, all future use of the HRC shutters has been discontinued.

Regular use of the HRC door has been discontinued as well because it is controlled by the same type of relay. Because the HRC sits at the telescope focus during radiation belt passages, this has increased the radiation dose received by the HRC. The HRC is an intrinsically radiation hard detector, and there has been no indication that this increased dose has had any effect on the flight instrument. The primary concern about this enhanced radiation dose is longterm degradation of the polyimide in the UVIS. Such a degradation would show up as an increased UV sensitivity long before it was noticeable in the X-ray band. Regular calibration/monitoring observations of Vega have found no change in the UV sensitivity of either the HRC-I or HRC-S. A study of the feasibility of using the HRC anti-coincidence shield as a replacement for the EPHIN to monitor the particle environment has been completed. It has been determined that the anti-co rates can be used to safe the instruments prior to entry into the radiation belt. Plans are currently being drawn up to implement this when the EPHIN can no longer monitor the particle background.

The HRC has been used to make a wide range of scientific investigations over the past year. Here we present preliminary results from two of them: an HRC survey of BL Lacs to search for extended emission due to coronal gas, and HRC monitoring of the X-ray binary population of M31 and detection of an X-ray counterpart to M31*.

Rita Sambruna and Davide Donato of George Mason University performed X-ray imaging observations of 6 higher-redshift BL Lacs with the HRC to study the gaseous environment around the nucleus. The sample extends an AO1 sample to higher luminosities in order to probe the circumnuclear environs as a function of core power and distance. The science goal of this investigation is to confirm the presence of diffuse hot halos around the cores of BL Lacs, testing unification models which unify these sources with FRI galaxies.

The presence of this halo can be assessed fitting the surface brightness profile of each source using an analytical model of the instrument Point Spread Function (PSF) with or without an additional beta model, necessary to describe the diffuse soft X-ray emission. The significance of this additional model can be determined using an F-test.

The shape of the instrument PSF is obtained fitting the surface brightness profile of different point-like sources with a 9-parameters analytical formula. To model the PSF of a point source, we considered three candidates: an observed star (AR Lac), a simulated BL Lacertae object (using Chart and MARX), and an observed distant BL Lacertae object. The results obtained using the third candidate are the best to fit their observations.

Figures 8 and 9 show the HRC-I raw image and the radial profile, respectively, of one of the most promising targets, 1E 1415.6+2557. The dashed line is the instrument PSF, and the dot-dashed line the beta model. The continuous thick line is the total model, while the thin dotted line is the background. The residual of the best-fit model are shown in the bottom panel. The beta model is detected with high (> 99.9 %) significance. The core radius for this source is ~4" (corresponding to 12.6 kpc) and the beta parameter is 0.45.

Figure 8: The HRC image of the BL Lac 1E 1415.6+2557.

Figure 9: The radial profile of 1E 1415.6+2557. The dashed line is the instrument PSF, and the dot-dashed line the beta model. The continuous thick line is the total model, while the thin dotted line is the background. The residuals of the best-fit model are shown in the bottom panel.

Michael Garcia and co-workers continue to monitor the X-ray point source population of the nearby galaxy M31. The HRC has been used to resolve what is believed to be the X-ray counterpart of M31* from the diffuse emission and point sources near the center of the galaxy. A 47 ks HRC-I image of the M31 nucleus, with 1/8" pixels, is shown in Figure 10. The source at the top is N1, the brighter source in the center is the super-soft source SSS. The position of M31* is marked with a small heavy circle (0.1' radius = 1s position error) in the center. The source outlined with the dashed circle is clearly consistent with the position of M31*. There are 13 counts within this dashed circle. The approximate locations of the diffuse double nucleus P1/P2 are indicated. ACIS observations are consistent with the apparent flux from M31*, but do not appear to resolve it from the nearby bright sources and diffuse surrounding emission.

Figure 10: The HRC-I image of the center of M31. See text for discussion.