A First Look at Cosmic Rays on ACS

Transcription

1 A First Look at Cosmic Rays on ACS Adam Riess June 6, 2002 ABSTRACT We have made an initial study of the characteristics of cosmic ray impacts on the two ACS imaging cameras, HRC and WFC. The fraction of pixels affected by cosmic rays varies from 1.5% to 3% during a 1000 second exposure for both cameras, similar to what was seen on WFPC2 and STIS. The electron deposition due to individual cosmic rays has a well defined cut-off with negligible events of less than 500 electrons and a median of ~1000 electrons. The distribution of the number of pixels affected by a single cosmic ray is strongly peaked at 4 pixels. Distributions of sizes and anisotropies are determined for both cameras, characteristics which can be useful for distinguishing cosmic rays from astrophysical sources in a single image. Introduction If ever a reminder was needed of the degree to which the Earth s atmosphere shields its surface dwellers from deadly cosmic ray radiation, a casual perusal of the cosmic ray (CR) flux on a single ACS dark frame would suffice. Like all HST cameras before it, the ACS HRC and WFC fields of view are heavily peppered by cosmic rays in even the shortest of exposures. For full orbit integrations, approximately 5% of the pixels receive significant charge from cosmic rays via direct deposition or from diffusion from nearby pixels. Great care must be taken in planning and analyzing HST ACS observations to minimize the impact of cosmic rays on science images. For planning Cycle 11, observers were furnished with predictions of the characteristics of CR-impacted frames based on experience from WFPC2 and STIS as well as from ground testing of ACS. Here we provide a first Copyright 1999 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.

2 look at the on-orbit characteristics of cosmic rays on ACS cameras to provide early users of this camera with the most up-to-date information possible to plan their observations. We recommend that those observers who plan to derive useful science from the entire field-of-view of ACS frames (e.g, those searching for rare astrophysical sources rather than those imaging a pre-defined target) review their programs based on the on-orbit characteristics of cosmic rays. Users should also look for further information on cosmic rays to appear in the ACS Data Handbook. In the following sections we will quantify the primary characteristics of the charge deposited on ACS by cosmic rays on HRC and WFC. Fractional Coverage For most users of the HRC and WFC, the most important characteristic of cosmic rays is simply the fraction of pixels they impact. This number provides the basis for assessing the risk that the target(s) in any set of exposures will be compromised. To determine this fraction we totaled the number of pixels more than 5 standard deviations above the median (after accounting for known hot pixels) in 40, 1000-sec dark frames. As seen for the previous CCD s flown on HST, the observed rate of CR impacts on an individual frame varies by a factor of two depending on the proximity of the spacecraft to the confluence of the Earth s magnetic field lines (e.g., the South Atlantic Anomaly). As seen in Figures 1 and 2, the fraction of pixels affected by cosmic rays (in non-saa passages) varies between 1.5% and 3%. This fraction is the same for the WFC and HRC despite their factor of two difference in pixel areas because the census of affected pixels is dominated by charge diffusion, not direct impacts. This fraction is also consistent with what is observed for WFPC2 and for what was anticipated for ACS from ground testing (Clampin et al. 2001). For most science observations, a single CR split (i.e., two exposures) is sufficient to insure measurements of the targets are not compromised by cosmic rays. More consideration is required for survey-type observations with WFC, a bonafide survey instrument. Observers seeking rare or serendipitous objects as well as transients may require that every single WFC pixel in at least one exposure among a set of exposures is free from cosmic ray impacts. For the CR fractions of 1.5% to 3% in 1000 sec, a single ~2400 sec orbit must be broken into 4 exposures (4 CR splits of 500 to 600 sec each) to reduce the number of uncleanable pixels to 1 or less. (It is also recommended that users dither these exposures to remove hot pixels as well.) 2

3 Figure 1: Fraction of pixels impacted (>5 sigma above the median) by cosmic rays in 40 individual 1000 sec dark frames. As seen the fraction varies from 1.5% to 3% over the path of the HST orbit or 15 to 30 pixels per sec. 3

4 Figure 2: Same as Figure 1 for WFC. The number of pixels per second affected by cosmic rays varies from 250 to 500. Cosmic Ray Events Many science observations require a careful consideration of individual cosmic rays events. To either remove cosmic rays events or distinguish them from astrophysical sources users might consider the distributions of observed cosmic ray fluxes, sizes, anisotropies, and the number of attached pixels per event. Before determining the preceding event characteristics, it is necessary to associate or group the affected pixels with individual cosmic ray events. We used a simple association algorithm which considered any two pixels as part of the same event if their fluxes were both above the 5 sigma threshold and their separation was less than 2 pixels (i.e., pixel pairs which were adjoining or diagonal partners). In some cases two distinct events will have become associated by chance proximity, but for most cosmic rays events (~ 4 adjoining pixels and a 2% fractional coverage) less than 1 in 4 events would be spuriously associated. Nevertheless, in the following we must consider that about 25% of the pixels events are likely half as large in flux and size as they appear. Hot pixels were flagged and removed from this analysis. 4

5 Electron Deposition The flux deposited on the CCD from an individual cosmic ray does not depend on the energy of the cosmic ray but rather the length it travels in the silicon substrate. As a result the deposition distribution has a well-defined minimum with few events of less than 500 electrons (where such low-electron events correspond to CR s which pass through the CCD at a normal angle of incidence). As seen in Figures 3 and 4, the median charge deposited for WFC and HRC is about 1000 electrons, the same as for WFPC2. Because the CR cross section for interaction for most CRs is inversely proportional to the kinetic energy of the CR, lower energy CR events (e.g., less than 1 Mev) will generate even more electron hole pairs (~1 ev losses) than high energy events. A CR event which is stopped in the substrate would essentially have deposited all of its energy in the silicon creating large deposition events. (An exception would be the very rare CR which has only a few ev s of energy left after crossing the 10 s of MeV s of shielding.) A useful characteristic of the deposition distribution is its well-defined minimum; e.g., multi-pixel events in a 500 sec broad-band exposures which have an apparent magnitude of 25th or fainter are unlikely to be caused by cosmic rays. Such information can help with the removal of false positives from searches for faint transients (e.g, high-redshift SNe). 5

6 HRC Figure 3: Electron deposition by cosmic rays on HRC. As seen, a minimum deposition of ~500 e is seen corresponding to CR s with normal incidence. The median deposition is ~1000 e. WFC Figure 4: Same as Figure 4 for WFC. 6

7 Attached pixels Using the same association algorithm discussed above we made histograms of the number of pixels associated with individual cosmic rays. As seen in Figures 5 and 6 for HRC and WFC the salient features are a strong peak in the distribution function at 4 to 5 pixels. On the smaller side there is a sharp decline in events. Although a few events are seen which encompass only one pixel, examination of these events indicate that at least some and maybe all of these sources are actually transient hot pixels or unstable pixels which can appear hot in one exposure (with no charge diffusion) and normal in the next. Such pixels are very rare but do exist. There is a long tail in the direction towards increasing numbers of attached pixels. Some of these are likely due to two individual events associated by their chance superposition, but more are from oblique incidence cosmic rays which skim the surface of the CCD leaving a long trail (which is wider near the surface). Unfortunately the number of attached pixels is not a very useful characteristic to distinguish CR s from unresolved astrophysical sources. 7

8 Figure 5: Distribution of the number of pixels associated with a single CR event. Some bias exists for events > 6 pixels which may be composed of two events with chance superposition. Figure 6: Same as Figure 5 for WFC. 8

9 Cosmic Ray Shapes and Sizes To determine the size and anisotropy of CR events we measured the moments of the individual CR event flux distributions with equations 1 through 8. The distribution of cosmic ray sizes are shown in Figures 7 and 8. For the relations used, the cosmic ray size corresponds to it gaussian standard deviation or the full-width at half maximum divided by 2.2. The size distribution for both chips peaks near 0.4 (or 0.9 as a FWHM). This is considerably less (i.e., sharper) than that of the critically sampled PSF. In fact very few CR s are as large as an unresolved source (and those of large size are long and narrow) rendering this measure useful for discriminating CR s from astrophysical sources. In Figures 9 and 10 we show the distribution of cosmic ray event anisotropies. For the relation used, an anisotropy of 0 corresponds to perfect symmetry. For both the HRC and WFC CR many events are relatively symmetric due to the charge diffusion function, though oblique incidence trails or small size events (i.e., 2 adjacent pixels) does stretch the distribution to large anisotropies. This characteristic is not terribly useful for distinguishing cosmic rays from astrophysical sources. 9

10 First Moments ( 1)I 0 = S i ( 2)I x = S i x i i ( 3)I y = S i y i i I 0 I 0 Second Moments ( 4)I xx S i ( x i I x ) 2 = i ( 5)I yy S i ( y i I y ) 2 = i I 0 I 0 ( 6)I xy = S i ( x i I x ) ( y i I y ) i I 0 ( 7)Size = ( I xx + I yy ) ( 8)anisotropy = ( I xx I yy ) 2 + 4I 2 xy ( I xx + I yy ) 2 10

11 Acknowledgements We would like to express our thanks to the ACS IDT, the ACS Group, Ron Gilliland, and Stefano Casertano for many helpful discussions References Clampin, M.., et al. 2001, ACS Instrument Handbook, Version 1.0 (Baltimore: STScI) 11

12 Figure 7: Histogram of the sizes of individual cosmic rays events on WFC as defined by equation 7. Size is equivalent to the Gaussian standard deviation or the FWHM divided by 2.2 Figure 8: Same as Figure 7 for HRC 12

13 Figure 9: Histogram of the anisotropies of individual cosmic rays events on HRC. An anisotropy of zero corresponds to perfect symmtry Figure 10: Same as Figure 9 for WFC. 13