TO: Distribution

FROM: Vega team:

Martin Zombeck, sec'y, Jeremy Drake, Marco Barbera, Yousaf Butt, F. Rick Harnden,

Brad Wargelin

SUBJECT: Vega calibration observations

DATE: 12/23/99

The following is a preliminary report on the analysis of the Vega observations.

1.0 Introduction

The purpose of the Vega calibration observations was to determine the UV susceptibility of HRC-I and HRC-S. In order to accomplish this Vega was positioned at the center of HRC-I and at four locations on HRC-S in order to sample the four different thicknesses of HRC-S' UVIS. The geometrical layout of the UVIS's can be found at http://hea-www.harvard.edu/HRC/overview/overview.html#focalplane.

Vega (a Lyr; A0V; U=0.02, B= 0.03, V= 0.03; a(2000) = 18h 36m 56.2s , b(2000) = +38o 47' 01") is not an X-ray emitter as demonstrated by its non-detection by the ROSAT PSPC. It is, however, a bright UV emitter and is a suitable object to test the UV response of the HRC detectors. On day 301 of 1999 (October 28, 1999) five approximately 3 ks observations of Vega were carried out in order to determine the HRC detectors' UV susceptibility. These observations are summarized in Table 1.

Table 1. Summary of Vega observations.
OBSIDStart Time* Exp. TimeDetector UVIS SegmentCount Rate**
1415301:15:14:093306 HRC-IHRC-I< 1.4 x 10-3

and > 1.2 x 10-5

1416301:16:17:293178 HRC-SSeg0 thick Al0.17 +/- 0.01
1417301:17:20:493477 HRC-SSeg0 thin Al240 +/-0.4
1418301:18:24:093291 HRC-SSeg+1 thin Al475 +/-0.7
1419301:19:27:293265 HRC-SSeg+1 thick Al ***

* 28 October 1999

** Corrected for dead time losses. *** Chandra was mis-pointed and Vega was not within the FOV.

The properties of the UVIS's are shown in Table 2.

Table 2. Properties of the UVIS's.
Detector UVIS Segment Polyimide (A)Aluminum (A)
HRC-IHRC-I5520 +/- 193 763 +/- 28
HRC-SSeg0 thick Al2750 +/- 118 793 +/- 18
HRC-SSeg0 thin Al2750 +/- 142 307 +/- 30
HRC-SSeg+1 thin Al2090 +/- 140 304 +/- 20
HRC-SSeg+1 thick Al 2125 +/- 1621966 +/- 64



2.0 The Results

OBSID 1415 (HRC-I)

Table 1 gives the start and exposure times for the observation. Vega was approximately on-axis. There was 1 event at the expected location of Vega and an unknown source about 5 arc minutes off-axis. Using the technique of Kraft, et al. in Determination of Confidence Limits for Experiments with Low Numbers of Counts, we obtain an upper limit for the Vega count rate of 1.4 x 10-3 evt s-1 (2s) and a lower limit of 1.2 x 10-5 evt s-1 (2s ). The total primary science rate (1415time.gif or 1415time.ps)for the observation was 45 evt s-1 and the valid event rate was 45 evt s-1. The pulse height distribution (1415pha.gif or 1415pha.ps)for the entire detector shows a broad spectrum with a peak at approximately channel 80. There is one source (1415regions.gif or 1415regions.ps) visible in the field with 34 net counts about 5 arc minutes off-axis: a(2000) = 18h 36m 59s.1 d(2000) = 38o 42' 7" with a count rate of 1.0 +/- 0.2 x 10-2 evt s-1. This source was seen by ROSAT; an optical counterpart could not be found after a cursory search through the SIMBAD catalog.

OBSID 1416 (HRC-S)

Table 1 gives the start and exposure times for the observation. Vega was placed on the thick Al part of the central segment (Seg 0) of the HRC-S UVIS and was approximately on-axis. The Vega event rate was 0.17 evt s-1 (1416regions.gif or 1416regions.ps). The total primary science rate (1416time.gif or 1416time.ps) for the observation was 50 evt s-1 and the valid event rate was 50 evt s-1. The pulse height distribution (1416pha.gif or 1416pha.ps) shows a peak at approximately channel 95.

OBSID 1417 (HRC-S)

Table 1 gives the start and exposure times for the observation. Vega was placed on the thin Al part of the central segment (Seg 0) of the HRC-S UVIS and was approximately 10 arc minutes off-axis. The observed Vega event rate was 138 evt s-1. The total primary science rate (1417time.gif or 1417time.ps) for the observation was saturated at 184 evt s-1 and the the valid event rate was 320 evt s-1. Thus the dead-time corrected image rate is (320/184) x 138 = 240 evt s-1 . The pulse height distribution shows a peak at approximately channel 80.

Three faint "ghosts" can be seen in the image (1417regions.gif or 1417regions.ps), labeled B, C, and D with count rates of about 0.3%, 0.3%, and 0.1% of the Vega image, respectively. C and D have pulse height distributions (1417pha.gif or 1417pha.ps) similar to Vega but the distribution of B is broader, appearing to be a composite of peaked distribution slightly higher in pulse height than the Vega distribution and a background-like component. We are investigating the cause of these "ghosts".

OBSID 1418 (HRC-S)

Table 1 gives the start and exposure times for the observation. Vega was placed on the thin Al part of the wing segment (Seg +1) of the HRC-S UVIS and was approximately 20 arc minutes off-axis. The observed Vega event rate was 159 evt s-1. The total primary science rate (1418time.gif or 1418time.ps) for the observation was saturated at 184 evt s-1 and the valid event rate was 550 evt s-1. Thus the dead-time corrected image rate is (550/184) x 159 = 475 evt s-1 . The pulse height distribution shows a peak at approximately channel 95.

Three faint "ghosts" can be seen in the image (1418regions.gif or 1418regions.ps), labeled B, C, and D (actually 2 small images) with count rates of about 1%, 0.5%, and 0.02% of the Vega image, respectively. B and C have pulse height distributions (1418pha.gif or 1418pha.ps) similar to Vega but the distribution of D, while peaked, has a peak pulse height about half that of Vega. We are investigating the cause of these "ghosts".

OBSID 1419 (HRC-S)

Table 1 gives the start and exposure times for the observation. Vega was to be placed on the thick Al part of the wing segment (Seg +1) of the HRC-S UVIS but this was not accomplished. The total primary science rate (1419time.gif or 1419time.ps) for the observation was 70 evt s-1 and the valid event rate was 70 evt s-1.

Although there is no main image, three faint "ghosts" can be seen in the image (1419regions.gif or 1419regions.ps), labeled B, C, and D with count rates of 2%, 1%, and 0.5% of the total rate, respectively. All the pulse height distributions (1419pha.gif or 1419pha.ps) are peaked with the peak pulse heights slightly higher than channel 100. We are investigating the cause of these "ghosts".

3.0 Comparison of Observed Count Rates with the Model Predictions

The expected count rates for the 5 Vega observations were calculated by convolving the Vega UV-visible spectrum with the modeled effective areas for the various regions of the detectors and making a correction for vignetting (0.85 @ 10 arc minutes, 0.70 @ 20 arc minutes).

Table 3 provides a summary of the results and provides links to plots of the effective areas and to comprehensive reports of the modeling calculations.

Table 3. A Comparison of Observed Count Rates for Vega with Model Predictions.
OBSIDDetector UVIS SegmentObs. Count Rate* Pred. Count Rate***Eff. Area

Model

Model

Report

1415HRC-IHRC-I < 1.4 x 10-3

and > 1.2 x 10-5


7.4 x 10-4
hrci_uv uv
1416HRC-SSeg0 thick Al 0.17 +/- 0.010.9uv0thick uv0thick
1417HRC-SSeg0 thin Al 240 +/-0.4391uv0thin uv0thin
1418HRC-SSeg+1 thin Al 475 +/-0.71960uv1thin uv1thin
1419HRC-SSeg+1 thick Al **1.4 x 10-6 uv1thick uv1thick

* Corrected for dead time losses. ** Chandra was mis-pointed and Vega was not within the FOV.

*** Corrected for vignetting

4.0 Conclusions

Given the crudeness of the UV-visible detector response models, the agreement (factors of 2 - 4) between predicted and observed rates is remarkable. We still need to explain the "ghosts" but as far as the purpose of the observations is concerned we have demonstrated that the detectors pass the "Vega test". Although we failed to test the thick Al part of Seg 1, our expectations are that we would have obtained a null result, since the predicted rate is so low. The ROSAT HRI Vega rate was 0.1 evt s-1. The mirror area was almost identical to the HRMA and the detector also had a CsI photocathode. Our HRC-I UVIS shield has reduced the UV-Visible light contamination from Vega by a factor of at least 75 compared to ROSAT.