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| Figure 1: Projected timeline response for PAIRITEL and Swift bursts. During Sept 2004, we found real-world trigger-to-image times of ∼1.1 (10 deg slew) and ∼1.9 minutes (50 degree slew from non-ToO target). Click the image for larger version. |
Two broad scientific aims drive the GRB Afterglow Key Project: probing the early universe and constraining the history of dust obscured star formation. While some inroads have been made on both these fronts using GRBs, the Swift experiment is expected to deliver 1 - 3 new localizations per week; currently we enjoy about one good localization per month. This order of magnitude increase in the GRB rate will be accompanied by unprecedented precision: almost all Swift X-ray-derived localizations will be better than 8 arcmin in diameter (comparable to the field of view of PAIRITEL), and most will be localized to the arcsecond level in a few minutes; burst positions are relayed via dedicated internet socket alerts which will hook directly into our queue schedule software. Figure 1 below shows the projected timeline.
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| Figure 1: Projected timeline response for PAIRITEL and Swift bursts. During Sept 2004, we found real-world trigger-to-image times of ∼1.1 (10 deg slew) and ∼1.9 minutes (50 degree slew from non-ToO target). Click the image for larger version. |
Swift on-board instruments will observe the early afterglows of GRBs at optical wavelengths (< 6000 Å), but have no capabilities at infrared wavelengths. Functionally, this implies that dust-extinguished afterglows and bursts from redshifts beyond z ≤ 4.5 cannot be studied by Swift at long wavelengths. The detection of IR afterglows must be generated from the ground. Bright IR and weak optical afterglows are indicative of dust-extincted bursts. IR or optical dropouts are indicative of high-redshift bursts (Swift optical imaging will be immediately public). For bursts at high redshift, due to time dilation, GRB afterglows remain bright in the IR, approximately Ks ≅ 17 mag for a typical afterglow in the first few hours at redshifts up to z ∼ 15. See figure 2.
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| Figure 2: PAIRITEL J & Ks sensitivty to high redshift gamma-ray bursts, extrapolated from the the best studied early afterglows at lower redshift: 990123. Even at redshifts of z ∼ 10, a burst 30 times fainter than GRB 990123 and dimmed by 1 magnitude of visual extinction would be detectable by PAIRITEL in the first 10 minutes. |
Given the current estimates about the rate of high redshift GRB production, we can expect 1 - 10 Swift bursts per year to originate from z > 8. The need to quickly observe and identify high redshift candidates for spectroscopic follow-up at Magellan or MMT drives the need for systematic automated follow-up and robust software to reduce images and compare against 2MASS catalogs. Ultimately, PAIRITEL could be the gateway to place meaningful limits on the age of reionization and the optical depth to Thompson scattering in the early universe.
Every burst localized by Swift or HETE-II, younger than three days and accessible from Mount Hopkins, will be observed in the GRB key project. Though the precise strategy will need to be honed with experience, nominally we will devote 3 hours of observing to bursts less than 1 day old and 1 hour to bursts between 2 -- 3 days old. This translates into roughly 1 -- 4 hours per night of observing. In one year we will gather a uniform sample of 70 GRB afterglows simultaneously at J, H, Ks. We summarize the main products and drivers for this key project:
Currently, supernova light curves are followed by the CfA supernova group in the optical during the dark parts of the month, and to a lesser extent in the IR during bright time. With the advent of automatic SN searches, many more SNe are being discovered early in their evolution, before they reach maximum brightness. At these times, it is critical to be able to follow them on a daily basis, in both the optical and in the IR. Yet on the 1.2-m, breaks in the data-taking occur because of the current instrument changeover system. In addition, IR observations are often limited, because of the more infrequent scheduling of IR instruments around the world. With dedicated IR capabilities, during dark or gray time, daily coverage at all wavelengths visible from the ground would be possible. There are few well-sampled, high-quality SNe light curves at IR wavelengths, and so the discovery space with this unique channel looks promisingly large. It is known that there are distinct differences between IR and optical SN Ia light curves and some clear advantages to the IR. Data over a range of optical filters show that the relation between the decline rate of SN Ia after maximum and peak absolute luminosity becomes flatter with increasing wavelength which suggests that SNe Ia can also be precise distance indicators in the IR. As a bonus, the absorption by dust decreases into the red so that this always-uncertain correction is small. For core-collapse SNe, IR photometry can provide further probes of circumstellar interaction and the formation of dust. SN IR light curves in nearby galaxies are bright, typically K < 15 mag, in the first several weeks. If current discovery rates continue, there will be 15 - 20 SNe per year bright enough to observe with the 1.3-m, with 3 - 4 available on any given night. Total exposure time for each SN would be 20 minutes for each SN (including slew overhead). This amounts to roughly 1 - 1.5 hours per night for the SN key project.
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| Figure 3: Showing the features of the PAIRITEL SN Ia light curve survey. |
Connecting the Local Universe to the Distant Universe (PI: Pahre): We propose to observe science-selected Spitzer fields, reaching 3.5 magnitudes fainter than 2MASS (1 - 1.5 hours per field). The FOVs of Spitzer and the 2MASS instrument on the 1.3 m are well matched. Since the launch of Spitzer, the IR community finally has access to deep mid-IR (3.5 - 160 micron) imaging with excellent spatial resolution (5 x 5 ; 1.5 pixels at 6.5 micron). A typical 500 second integration at 3.6 micron observation will reach 4 μJy (5 σ), comparable in flux density to Sloan Digital Sky Survey (SDSS) images (R = 22.5 mag equals 3 μJy), but significantly deeper than the publicly available near-IR observations from the 2MASS survey (K = 15 mag equals 640 μJy). By providing relatively deep integrations we will bridge the sensitivity gap been optical and mid-IR imaging in large areas of the sky. SDSS has been a tremendous success by providing 5-color photometry over some 6000 deg2 of the North Galactic cap. An infrared survey could leverage strongly against the SDSS database. For example, JHK colors allow photometric redshifts to be obtained to greatly improved accuracy; near-IR colors allow the selection of z > 7 quasars (while SDSS is limited to z < 6.5), so reaching well into the recombination epoch. By devoting 3 hours per night on average to this key project, we will be able to survey 0.36 deg2/night in 10 minute exposures (i.e., 18 fields per night). This should reach K = 17 (5 σ) or slightly better. With 100 photometric nights/year on the Ridge, about 36 deg2/year could be surveyed. The arguments for a wide field IR survey are broad. UKIDSS plans to cover 3000 x 4000 deg2 to K = 18.4 in their `Large Area Survey' using UKIRT. We cannot match this sensitivity, but a K = 17 - 17.5 survey still covers much of the same science, and we will deliberately cover the parts of SDSS that UKIDSS does not cover, making the two surveys complementary.
Last modified: Mon Oct 18 23:08:20 MST 2004