The UFFO (Ultra Fast Flash Observatory) Pathfinder: Science and Mission
P. Chen, S. Ahmad, K. Ahn, P. Barrillon, S. Blin-Bondil, S. Brandt, C. Budtz-Jorgensen, A.J. Castro-Tirado, H.S. Choi, Y.J. Choi, P. Connell, S. Dagoret-Campagne, C. De La Taille, C. Eyles, B. Grossan, I. Hermann, M.-H. A. Huang, S. Jeong, A. Jung, J.E. Kim, S.H. Kim, Y.W. Kim, J. Lee, H. Lim, E.V. Linder, T.-C. Liu, Niels Lund, K.W. Min, G.W. Na, J.W. Nam, K. Nam, M.I. Panayuk, I.H. Park, V. Re-Glero, J.M. Rodrigo, G.F. Smoot, Y.D. Suh, S. Svelitov, N. Vedenken, M.-Z Wang, I. Yashin, M.H. Zhao
32 ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , B EIJING
The UFFO (Ultra Fast Flash Observatory) Pathfinder: Science and Mission P. C HEN , S. A HMAD , K. A HN , P. B ARRILLON , S. B LIN - BONDIL , S. B RANDT , C. B UDTZ - JØRGENSEN , A.J. C ASTRO - TIRADO , H.S. C HOI , Y.J. C HOI , P. C ONNELL , S. D AGORET - CAMPAGNE , C. D E LA TAILLE , C. E YLES , B. G ROSSAN , I. H ERMANN , M.-H. A. H UANG , S. J EONG , A. J UNG , J.E. K IM , S.H. K IM , Y.W. K IM , J. L EE , H. L IM , E.V. L INDER , T.-C. L IU , N IELS L UND , K.W. M IN , G.W. N A , J.W. N AM , K. N AM , M.I. P ANAYUK , I.H. P ARK , V. R E-GLERO , J.M. R ODRIGO , G.F. S MOOT , Y.D. S UH , S. S VELITOV , N. V EDENKEN , M.-Z W ANG , I. Y ASHIN , M.H. Z HAO [ THE
UFFO C OLLABORATION ] National Taiwan University, Taipei, Taiwan University of Paris-Sud 11, France Yonsei University, Seoul, Korea National Space Institute, Denmark Instituto de Astrofisica de Andalucia, Consejo Superior de Investigaciones Cientificas, Spain Korea Institute of Industrial Technology, Ansan, Korea Korea Advanced Institute of Science and Technology, Daejeon, Korea University of Valencia, Spain University of California, Berkeley, USA National United University, Miao-Li, Taiwan Ewha Womans University, Seoul, Korea Moscow State University, Moscow, Russia [email protected]; [email protected]
Abstract:
Hundreds of gamma-ray burst (GRB) optical light curves have been measured since the discovery of opti-cal afterglows. However, even after nearly 7 years of operation of the
Swift
Observatory, only a handful of measure-ments have been made soon (within a minute) after the gamma ray signal. This lack of early observations fails to ad-dress burst physics at short time scales associated with prompt emissions and progenitors. Because of this lack of sub-minute data, the characteristics of the rise phase of optical light curve of short-hard type GRB and rapid-rising GRB, which may account for ~30% of all GRB, remain practically unknown. We have developed methods for reaching sub-minute and sub-second timescales in a small spacecraft observatory. Rather than slewing the entire spacecraft to aim the optical instrument at the GRB position, we use rapidly moving mirror to redirect our optical beam. As a first step, we employ motorized slewing mirror telescope (SMT), which can point to the event within 1s, in the UFFO Path-finder GRB Telescope onboard the
Lomonosov satellite to be launched in Nov. 2011. UFFO’s sub-minute measure-ments of the optical emission of dozens of GRB each year will result in a more rigorous test of current internal shock models, probe the extremes of bulk Lorentz factors, provide the first early and detailed measurements of fast-rise GRB optical light curves, and help verify the prospect of GRB as a new standard candle. We will describe the science and the mission of the current UFFO Pathfinder project, and our plan of a full-scale UFFO-100 as the next step.
Keywords:
Gamma Ray Burst, Importance of GRB Prompt Signal
Much progress has been made in Gamma Ray Burst (GRB) science since the launch of the Swift observatory [1] 7 years ago. The observations from Swift did not, however, produce a simple picture of GRB, but rather documented the richness and complexity of this phe-nomenon. Just a few years ago, GRBs were believed to be of only two types, distinguished only by their gamma-X emission: a shorter, hard spectrum burst, with duration of gamma-X emission less than two seconds, and a longer, soft burst. After 476 observations by
Swift
Burst Alert Telescope (BAT) made between Dec. 2004 and Dec. 2009 (BAT2 Catalog) [2] and its follow-up UV-optical observation by the
Swift
UV-Optical Telescope (UVOT), a huge variation in optical light curves has been observed, especially in the early rise time. Figure 1 shows a sample of the optical light curve measurements made soon after GRB triggers. There appear to be distinct classes of fast-rising ( t peak < 10 s) and slow-rising bursts [1]. UTHOR
ET AL . PAPER S HORT T ITLE Additionally, the optical light curves are complex, with decays, plateaus, changes in slope, and other features that are not yet understood. Panaitescu and Vestrand [3] claim that the optical luminosity distribution of the fast-rising bursts at ~10 s is quite narrow, and has promise as a kind of "standard candle" which would make GRBs useful as a cosmological probe of the very high redshift universe. In order to move this possible trend to the status of a refined tool, a larger sample of such objects is required, and in particular, better resolution is required at early times. Are there more features in the early light curve that are missed by the sparse sampling? Does any feature of the rise correlate with the luminosity or a particular aspect of the physics? How many bursts are misclassified because the rapid rise was missed? The need for earlier measurements (faster optical response after the initial gamma-ray burst) is clear and compelling. The challenge is particularly acute for short-hard GRB observations, which have few early measurements. What is the shape of the rise? Is the shape homogeneous? The physical origin of this type of burst remains an outstand-ing mystery, so any hint as to its origin would be ex-tremely valuable. Because of the short time scale for the gamma-X light curves and the lower bolometric luminos-ity, these bursts are believed to originate from the merger of compact objects. Is there any prompt UV-optical emis-sion from such events? What would we see if we ob-served more of these events in the sub-minute or sub-second regime? Are there ultra-short events on the accre-tion disk dynamical timescale of compact objects (that are beamed so we can see them)? Early observations would answer these questions and open a new window probing compact object structure, populations, and evo-lution. Figure 1. GRB optical light curve rise time and shape [3] for the Fast-Rising class (top) and the Decay class (bot-tom). In GRB 080319B, extraordinary, bright, variable optical emission, which peaked at the visual magnitude of 5.3, has been observed while the prompt gamma-ray emission was still active. This observation clearly shows that there can exist a prompt optical emission component that tracks the gamma-ray light curve [4]. This further moti-vates us to push for the observation of optical emissions in the GRB prompt signals. In addition to providing the first early and detailed meas-urements of fast-rise GRB optical light curves and help-ing verify the prospect of GRB as a new standard candle, other science potentials from systematic observations of GRB prompt signals include a more rigorous test of cur-rent internal shock models and the probing of the ex-tremes of bulk Lorentz factors. A New Approach: Steering the Op-tical Path, not the Spacecraft
In the
Swift observatory, the entire spacecraft slews to point its UV-Optical Telescope (UVOT) at the GRB position after the Burst Alert Telescope (BAT) [7] identi-fies the onset of the event. The histogram of burst event as a function of response time falls off below 100 s, with an almost complete cutoff at 60 s. Due to its finite mis-sion lifetime,
Swift cannot be expected to significantly increase the number of sub-minute response events. To circumvent this challenge, our approach is to redirect the optical path of the incoming GRB beam instead of the entire spacecraft. One exciting prospect is to invoke a micro electrical mechanical system (MEMS) to build a mirror array (MMA), where each pixel of mirror is con-trolled by a nano-fabricated micro-motor and therefore no massive mechanical motion is required. Such a MMA can redirect the telescope beam to a target within one msec. The technology is reasonably mature. The Research Cen-ter for MEMS Space Telescope (RCMST) at Ewha Women’s University, Korea, a key institution in the UFFO Collaboration, has successfully invoked this ap-proach in two recent satellite missions, KAMTEL and MTEL [5]. The challenge, however, is the uniformity of the MEMS array for large area mirrors. While further R&D is required, we believe that this would become the future paradigm for flash or transient observation tele-scopes. UFFO-Pathfinder
With the aforementioned science motivations in mind, the Taiwan and Korea members of the present UFFO Col-laboration initiated the POET (Prompt Observation of Energetic Transients) satellite project in 2008, but it was later aborted. Afterwards, the UFFO-Pathfinder was proposed in 2009 in response to an opportunity afforded by available space aboard the
Lomonosov
Space Mission, scheduled for launch in Nov. 2011. UFFO is a multina-tional project that involves scientists from Denmark, France, Korea, Russia, Spain, Taiwan, and the U.S. [6]. The
Lomonosov
UNIVSERSAT spacecraft will carry several instruments, including the TUS air shower cos- ND I NTERNATIONAL C OSMIC R AY C ONFERENCE , B EIJING mic ray experiment. Through collaboration with Moscow State University, the UFFO Pathfinder proposal was granted for X-ray triggered observations of GRB through an optical telescope. UFFO-Pathfinder consists of two major components: the Slewing Mirror Telescope (SMT) and the UFFO Burst Alert and Trigger Telescope (UBAT). We briefly de-scribe their characteristics below.
Slewing Mirror Telescope
Because of the time constraint for meeting the launching schedule, the UFFO Collaboration has decided not to employ the MEMS mirror array but a semi-conventional approach for the UFFO Pathfinder slewing mirror tele-scope (SMT). It is a Ritchey-Chrétien telescope with motorized gimbal-mounted mirror 10 cm in diameter, with a field of view of 17 ×
17 arcmin. Other specifications of SMT are given in Table 1. Under the Lomonosov mass constraint, we roughly evenly divide the allotted total mass of 21.5 kg to UFFO’s two key components: SMT and UBAT. With only 11.5 kg in weight, SMT can redi-rect the incoming GRB light path within 1s.
Telescope Ritchey-Chrétien + motorized mirror plate Aperture 10 cm diameter F-number 11.4 Detector and / Operation Intensified CCD with MCP/ Photon Counting Field of View 17 x 17 arcmin Detection Ele-ment 256 x 256 pixels Telescope PSF 1 arcsec @ 350 nm Pixel Scale 4 arcsec Wavelength Range 200 nm – 650 nm Sensitivity 17.5 mag/10 s exposure 18.7 mag/100 s exposure (5 sigma, open filter) Bright Limit mv = 6 mag Data taking start time after trigger+location < 1 s Data Rate 1 GB/day Mass, Power consumption, Size 11.5 kg, 10W, 30cm (W) x 20cm (H) x 62cm (L) Table 1. Specifications of UFFO Pathfinder SMT.
UFFO Burst Alert and Trigger Telescope
The UFFO Burst Alert & Trigger telescope (UBAT) will be similar to the Swift BAT X-gamma trigger camera, using a coded mask aperture camera scheme for good position detection for transients and wide field of view. Figure 2. A rendering of the opto-mechanics of the UFFO Pathfinder slewing mirror telescope (SMT). However, in order to respond over a wider energy range, making the camera more sensitive to broad-band hard sources including GRB, a design including the LYSO crystal and 64 (8x8) MAPMT as the detector is invoked, resulting in a sensitive energy range of 5-200 keV. With a mass constraint of 10 kg for UBAT, we use a detection area of 191 cm . The resulting sensitivity is 310 mCrab in 10 s at 5 σ . The specifications of UBAT are given in Table 2 and a 3D view of it is shown in figure 3. Figure 4 shows the integrated UFFO system. Figure 3. A rendering of the UFFO Burst Alert and Trig-ger Telescope (UBAT) Figure 4. The integrated UFFO Pathfinder. The SMT enclosure is not shown in this drawing. Expected Results and Impact
In order to estimate our event rate, we examined the fluence distribution of Swift BAT GRB that triggered
Swift
UVOT observations during the first 7 years of
UTHOR
ET AL . PAPER S HORT T ITLE Table 2. Key parameters of UFFO Burst Alert and Trigger Telescope (UBAT).
Swift operation. This is a more conservative number than the total rate of
Swift
BAT GRB. Scaling by our esti-mated sensitivity to that of
Swift
BAT, we find that we will still receive an expected ~43 GRB triggers for SMT per year from UBAT. Of these, we expect ~2 short-hard triggers per year. The actual number of SMT observa-tions that we accomplish should cover about the same number, unless our orbit has significantly more restric-tions than that of Swift. It is likely that some reduction in these numbers could result from our inability to point away from the galactic plane, unlike Swift. Because short bursts are hard, and because our detectors have more low-energy response than BAT, some reduction in the short-hard rate may result. Barring significant malfunctions, UFFO will provide sub-minute UV-optical measurements for dozens of GRB observations within the first year of operation, with about 9 detections. These optical measurements will be the first ever under 10 s after the gamma ray trigger, and will make up the first ever large-sample, systematic survey of optical emission in the sub-minute regime. We note that there is an exciting synergy with the TUS instrument. In the case that a GRB produces neutrino or other cosmic ray signals, extensive air showers are ex-pected. Our experiment would detect the source GRB event in X-gamma and UV-optical photons, while the TUS instrument would detect the particle shower, meas-uring the source position and arrival times of the particles with 0.1° and 10 µs accuracy, respectively. Such meas-urements would enable the first measurements of neu-trino masses from GRB emission, and would serve as an exciting new measure of photon and particle dispersion relations, of great interest to fundamental particle phys-ics, cosmology and relativity tests. Future Prospect: UFFO-100
While still busy with the preparation of UFFO-Pathfinder, the UFFO Collaboration has been exploring its next step, a more ambitious project: UFFO-100, based on the same design principle but with larger total mass of 100 kg (thus the name UFFO-100). This would afford a 30 cm aperture slewing telescope and a 1024 cm CZT X-ray camera. The goal is to finally integrate the MMA technology with the motorized slewing mirror and to add an IR-sensitive cam-era to detect the distinguished bursts. The key compo-nents and the dimensions of UFFO-100 are shown in figure 5. We expect to launch UFFO-100 in 2015 by the Soyuz Launcher.
Figure 5. A rendering of the UFFO-100 GRB Telescope Summary
The UFFO Pathfinder has now entered the final stage of preparation before it is launched in Nov. 2011. We ea-gerly look forward to its exciting GRB findings and the proof-of-principle for this new approach to future GRB telescopes. [1] N. Gehrels et al., ApJ, 2004, :1005-1020. [2] T. Sakamoto et al., “The Second Swift BAT Gamma-Ray Burst Catalog”, arXiv:1104.4689, accepted for pub-lication in ApJS. [3] A. Panaitescu and W. Vestrand, MNRAS, 2008, : 497-504. [4] J. L. Racusin et al., Nature, 2008, :183-188. [5] I. H. Park et al., Optics Express, 2008, (25): 20249. [6] I. H. Park et al. [UFFO Collaboration], “The UFFO Pathfinder”, arXiv:0912.0773. [7] S. D. Barthelmy, Proc. SPIE, 2004, :175. Mass of the camera 10 kg Energy range 5 – 200 keV Telescope PSF ≤
17 arcmin Source position accuracy ≤