Wide-field variability search strategies for multi-messenger astronomy

Transcription

1 Wide-field variability search strategies for multi-messenger astronomy Shane L. Larson, P. I. Assistant Professor of Physics Utah State University Old Main Hill Utah State University Logan, UT Sponsored Programs Administrator Kellie Hedin College of Science 0305 Old Main Hill Utah State University Logan, UT

2 Research Description 1 Motivation Gravitational-wave astronomy is an emerging observational discipline that utilizes propagating gravitational disturbances, rather than light, as a probe of astrophysical systems. The modern landscape of gravitational wave astronomy is dominated by laser interferometric observatories, most notably the NASA Laser Interferometer Space Antenna (LISA)[1], and the American LIGO (the largest experiment ever funded by the National Science Foundation)[2]. In today s era where tight budgetary constraints are an important aspect of experimental design and mission evaluation, maximizing the science returns of any mission is crucial to its success. The LISA project is currently undergoing design study reviews to evaluate how cost-saving alterations to the mission infrastructure(such as shorter interferometric armlengths, or alternative orbital configurations) impact the science returns [3] 1. This is not a new endeavour, as many alternative space-based mission designs have been considered in the past, such as OMEGA [4], GEM [5], and DECIGO [6]. Every design for a gravitational wave detector has strengths and weaknesses, but one way to always increase the science return of the instrument is to utilize complementary observational information from other instruments. Observations of astrophysical systems in gravitational waves provide data streams that are highly complementary to traditional electromagnetic observations, encoding information that is difficult to obtain by any other means: precise maps of the dynamical distribution and motion of masses, accurate luminosity distances, and detection of systems that are electromagnetically dim or dark. By contrast electromagnetic observations generally yield spectra which encode information about composition and relative motion with respect to the observer. The science returns, for both traditional electromagnetic telescopes (EMTs) and gravitational wave interferometers (GWIs), are significantly amplified by coincident observation across different spectra what is colloquially known as multi-messenger astronomy. 2 Problem: Coordinating Observations Coordinating multi-messenger observing campaigns is a problem of meshing the distinctly different characteristics of EMTs and GWIs. The nature of EMTs is to be highly directional, with narrow views of the sky that give them high sensitivity to a few sources within their field of view. By comparison, GWIs are highly omnidirectional, sensitive to sources all over the sky, and able to see all sources in their observing band at the same time. Gravitational-wave astronomers analogously liken EMTs to eyes and GWIs to ears in terms of their performance characteristics. The research outlined in this proposal will demonstrate the efficacy of strategies for wide field variability observing campaigns that can be effectively employed for multi-messenger astronomy using gravitational wave interferometers and electromagnetic telescopes. The preliminary data acquired from this project will seed proposals to both NASA and the National Science Foundation for follow-on funding. For multi-messenger observations to be effective, there are two distinct problems that must be addressed: (1) observing campaigns must be coordinated in time, and (2) observing campaigns must cover the appropriate locations on the sky. The first problem is that for many gravitational wave events, it is unclear whether the associated electromagnetic signal will be prompt or delayed. Prompt emission occurs nearly simultaneously with 1 Citations which involve the PI are indicated in bold throughout this proposal. S. L. Larson NASA EPSCoR

3 the gravitational wave event. An excellent example being the merger of two neutron stars, a likely candidate for short gamma ray bursts. Coordinated multi-messenger observations are already being pursued between LIGO (a GWI) and SWIFT (an EMT), where triggers from each are used to initiate sources searches by the other [7]. Because gamma-ray bursts are highly beamed, instruments like LIGO will be sensitive to neutron merger events that will not be picked up by burst detectors like SWIFT, and optical transient search triggered by LIGO are being pursued using single telescopes by the LOOCUP project [8]. Delayed emission can be a sophisticated probe of the internal structure of a system, provided the electromagnetic and gravitational wave signals can be correlated with one another. A good example of delayed emission is in the merger of massive black holes in galactic nuclei. The two black holes, when they are proximate to each other are expected to hollow out the center of the circumbinary accretion disk. Once the black holes merger, the gas collapses on timescales related to the size of the hollow and the viscous dynamics of the gas; bright electromagnetic emission from the collapsing gas will follow on timescales of months to years [9]. The second problem is that gravitational wave interferometers have broad sensitivity patterns that cover large swaths of the sky. Ground-based systems like LIGO point via triangulation there are multiple detectors on the surface of the Earth, and the time of flight for signals between the detectors gives the sky location. Most sources observed by space-based detectors like LISA are long lived, and are observable by LISA over a large fraction of the LISA orbit. The modulations of the gravitational wave signal that result from LISA s progression along its orbit can be deconvolved to give directional pointing to the locale of the source. These methods give pointing which is quite poor by astronomical standards, typically on the order of 1 square degree on the sky [10, 11, 12], where as EMTs generally can point to arcsecond accuracy. For a limited number of applications, GWIs can localize to much higher accuracy using specialized geometrical analysis techniques [13][14, 15]. Early, rudimentary studies have shown that LISA observations will benefit from electromagnetic observations [16], but the specifics of how to best maximize the multi-messenger observations has not been effectively explored. The disparity between the localization ability of GWIs and EMTs is well known, and has led to speculation that the best method will employ instruments with large sky coverage, such as the Large Synoptic Survey Telescope [17]. The coordination of covering LISA error boxes on the sky, subject to LSST sky coverage and time coverage has been examined [18], but it is still unlikely that every detected gravitational wave source can be subjected to follow-on observations given the time and demands on an instrument like LSST. Other observational efforts (most notably microlensing surveys) have managed coordinated surveys of meter class instruments(e.g. see excellent reviews provided for the Astro2010 decadal survey, [19, 20]), but only as part of organized campaigns, not as target-of-opportunity observations. 3 Sub-meter telescopes: putting glass on the sky This proposal covers an initial investigation of tapping a previously unused sky-survey resource sub-meter class telescopes, large numbers of which are maintained and operated by universities around the globe. This proposal will begin with the coordination of three existing telescopes at Utah State University (two maintained by the Department of Physics, and another by the College of Engineering), supplemented by observations on remote networks such as SSON[21] and GRAS[22]. This work will provide the groundwork for developing close collaborations with other university observatories over the course of the next year. The use of sub-meter class telescopes addresses the most serious issue with using EMTs to followup gravitational wave observations: the amount of time spend surveying large areas of the sky that do not contain a source of interest. The use of dedicated small instruments is for detection and localization. Larger instruments can be used more effectively and to greater scientific advantage if they can be directed where to point. One of the difficulties of using observatories maintained by universities is that while some univer- S. L. Larson NASA EPSCoR

4 sities have dedicated sub-meter research telescopes, most are in many instances multi-use facilities, being used for classes, public outreach and student research projects. This poses certain difficulties in utilizing these instruments for transient follow-up research including (but not limited to) Scheduling around other demands on the telescope schedule Automation of the facility for computer driven target of opportunity follow-ups Repeatability of instrumentation setup and calibrations when it is subject to repeated alterations to support the multi-use roles. Scheduling issues can be mitigated by increasing the number of telescopes participating in the transient survey program. The number of participating observatories is also crucial to the efficacy of the program to accommodate weather variability as well as daytime/nighttime conditions around the globe. Automation issues are less problematic than they have been in the past, owing to the large number of amateurs who have become involved in astrophotography and photometric science using their own backyard and remote observatories. As a result, automation hardware and the computer software to drive the systems is readily available commercially. The USU Observatory maintained by the Department of Physics (to be used in this research) is already equipped for full automation. The most serious issue for transient identification is the reliable aggregation of data from multiple telescopes when the instrumentation changes on a regular basis. Time-domain variability searches must reliably detect transient effects with low latency (i.e. soon after the observations are made), requiring robust synchronization of data from multiple telescopes. Figure 1 illustrates some of the difficulties faced using early data collected by our research group in June of The survey data shown is looking for long term variability over a wide field by monitoring the open cluster NGC The images clearly show the photometric difference between the uncalibrated images, and most notably, that the field being surveyed is shifted and oriented differently between telescopes. These differences can be corrected by humans, but not with the low latency required for multi-messenger follow-ups. Our primary work during the performance period of this proposal is to develop data exchange protocols, automation scripts and pipelines for synchronizing and collating data from multiple sub-meter aperture telescopes. Schedule and Outcomes The basic outline of the work described in this proposal will proceed in a series of steps designed to: (1) develop our capabilities in observational transient survey work; (2) develop the necessary requirements and protocols for adding telescopes to the network before other university facilities are asked to join the effort; and (3) establish the first generation of data analysis pipelines that will form the foundation for true multi-messenger follow-up identifications once instruments like LISA and LIGO begin making reliable detections. Our plan of work is as follows: Late Summer Data collection from a small group of telescopes to establish the baseline issues that need to be resolved (photometric calibration and image registration) to develop pipeline protocols. Thisworkisalreadyunderwayatalowlevel usingthe20 telescopeattheusuphysics Department and using remote observatories in the GRAS network (see Figure 1). These observations will also help establish a baseline measurement of the population of sky transients as a function of magnitude and sky area. Fall Development of photometric pipelines. Photometric data reduction is a well understood process for single telescopes, and a variety of software tools are available for general scientific use. This phase of the work will build this capability in the PI s group here at USU, which currently does not exist. S. L. Larson NASA EPSCoR

5 Figure 1: Some of our early multi-telescope data looking for variability in open cluster NGC The left and center image were taken by different telescopes. The right image shows the transformed and synchronized data; the left image was rotated 187, stretched in linear scale, and shifted until the images registered. Spring Work will begin on developing basic scripting of data pipelines to automate the synchronization and registration of data from multiple telescopes, and producing automated light curves for transients detected in synchronized data sets. Summer At this point we hope to have well established protocols in place that will enable a true transient survey to be conducted with a blind search by two to three telescopes on a field selected with known variable sources. The expectation is that our experience with this problem will have matured enough that telescope facilities at other universities can be invited to join this effort, allowing the growth and development of a more robust and spatially distributed network that can effectively participate in true multi-messenger searches. We anticipate approaching an initial group of 3-4 universities to begin the development of a full network. Developing a close collaboration with other university observatories will be proposed as part of follow-on funding, and several prospective partners have been identified inside and outside of Utah. It should be noted that the science drivers for wide-field variability searches is not limited to multimessenger astronomy. A network of small telescopes engaged in this work will be capable of detecting other transient events such as exoplanet transits, gamma-ray burst afterglows, novae and cataclysmic variable outbursts, near Earth object surveys, and other sky transients, all suitable science targets for sub-meter class telescopes outfitted with modern sensors. Follow-on funding The work proposed here will lay the framework for future proposals to the yearly NASA ROSES competition. LISA related research, like that proposed here, is supported through the Astrophysics Theory Program (ATP). The natural extension of this work, into more realistic simulations and development of data analysis frameworks for LISA, are all natural subjects for the ROSES/ATP opportunity. Gravitational wave astronomy spans agencies, and the work described here also has applications to ground-based gravitational wave astronomy, notably with regards to LIGO. Extensions of this work S. L. Larson NASA EPSCoR

6 will also be proposed in a CAREER proposal to the National Science Foundation for applications to LIGO. S. L. Larson NASA EPSCoR

7 References [1] P. Bender et al., LISA Pre-Phase A Report, Max-Planck-Institut für Quantenoptic, Garching, second edition, (1998). [2] A. Ambramovici et al., Science, 256, 325 (1992). [3] LISA-light/NGO mission design studies, LISA Working Groups (2011). [4] R. W. Hellings et al., Orbiting Medium Explorer for Gravitational Astrophysics (OMEGA), proposal to NASA Medium Explorer program, (1998) (unpublished). [5] Shane L. Larson, Space-based gravitational wave astrophysics, Ph.D. Thesis, (1999) (unpublished). [6] N. Seto et al., Phys. Rev. Lett. 87, (2001). [7] Abbott et al., Astrophys. J. 715, 1438 (2010). [8] Kanner et al., Class. Quant. Grav. 25, (2008). [9] M. Milosavljevic and E. Phinney, Astrophys. J. 622, L93 (2005). [10] C. Cutler, Phys. Rev. D 57, 7089 (1998). [11] R. Takahashi & N. Seto, Astrophys. J. 575, 1030 (2002). [12] T. A. Moore, and R. W. Hellings, Phys. Rev. D, 65, (2002). [13] Y. Gürsel and M. Tinto, Phys. Rev. D, 40, 3884 (1989). [14] Shane L. Larson and Massimo Tinto, Phys. Rev. D, 70, (2004). [15] Massimo Tinto and Shane L. Larson, Class. Quant. Grav., 22, 531 (2005). [16] A. Cooray, A. Farmer & N. Seto, Astrophys. J. 601, L47 (2004). [17] Large Synoptic Survey Telescope, Science Drivers white paper (retrieved 24 June 2011); lsst. org/lsst/science/overview/ [18] Kocsis et al., Astrophys. J. 684, 870 (2008). [19] B. Gaudi, Astro2010 White Paper (2010); online at sites.nationalacademies.org/bpa/bpa_ [20] A. Gould, Astro2010 White Paper (2010); online at sites.nationalacademies.org/bpa/bpa_ [21] Sierra Stars Observatory Network, [22] Global-Rent-A-Scope Network, S. L. Larson NASA EPSCoR

8 SHANE L. LARSON Curriculum Vita Department of Physics, M/C 4415 Phone: Utah State University Logan, UT WWW: Appointments 1. Assistant Professor of Physics, Utah State University: 06/ present. 2. Assistant Professor of Physics, Weber State University: 08/ /2008. Professional Preparation 1. Senior Postdoctoral Scholar, Center for Gravitational Wave Physics & Institute for Gravitational Physics and Geometry, Penn State (L. S. Finn): 07/ / Postdoctoral Scholar, California Institute of Technology (T. A. Prince): 07/ / NASA EPSCoR Postdoctoral Research Associate, Jet Propulsion Laboratory/Montana State University (R. W. Hellings): 07/ / Ph. D. Physics, Montana State University (1999) 5. M. S. Physics, Montana State University (1994) 6. B. S. Physics (with High Scholarship), Oregon State University (1991) Selected Publications 1. The LISA gravitational wave foreground: a study of double white dwarfs, Ashley J. Ruiter, Krzysztof Belczynski, Matthew Benacquista, Shane L. Larson and Gabriel Williams, Astrophysical Journal 717, (2010). 2. Integrated Sachs-Wolfe Effect for Gravitational Radiation, Pablo Laguna, Shane L. Larson, David Spergel and Nicolas Yunes, Astrophysical Journal Letters 715, L12 (2010). 3. Constraining the black hole mass spectrum with gravitational wave observations I: the error kernel, Danny C. Jacobs, Joseph E. Plowman, Ronald W. Hellings, Sachiko Tsuruta and Shane L. Larson, MNRAS, 401, 2706 (2010) 4. Detecting a Stochastic Gravitational-Wave Background: The Overlap Reduction Function, Lee Samuel Finn, Shane L. Larson and Joseph D. Romano, Physical Review D, 79, (2009) 5. Selection effects in resolving Galactic binaries with LISA, M. J. Benacquista, Shane L. Larson and B. E. Taylor, Class. Quant. Grav. 24, S513 (2007) 6. Gravitational wave bursts from the Galactic massive black hole, C. Hopman, M. Freitag and Shane L. Larson, MNRAS 378, 129 (2007). 7. Observing IMBH-IMBH binary coalescences via gravitational radiation, J. M. Fregeau, Shane L. Larson, M. C. Miller, R. O Shaughnessy, and F. A. Rasio, ApJ 646, L135 (2006) 8. Semi-relativistic approximation to gravitational radiation from encounters with non-spinning black holes, J. R. Gair, D. Kennefick and Shane L. Larson, Phys. Rev. D 72, (2005). 9. Constraining supermassive black hole systems using pulsar timing: application to 3C66B, F. A. Jenet, A. Lommen, Shane L. Larson and L. Wen, ApJ 606, 799 (2004) 10. LISA data analysis: source identification and subtraction, Neil J. Cornish and Shane L. Larson, Phys. Rev. D 67, (2003) 11. Unequal arm space-borne gravitational wave interferometers,shane L. Larson, Ronald W. Hellings and William A. Hiscock, Phys. Rev. D 66, (2002) 12. Space missions to detect the cosmic gravitational-wave background, Neil J. Cornish and Shane L. Larson, Classical and Quantum Gravity 18, 3473 (2001) 13. Low frequency gravitational waves from binary white dwarf MACHOs, William A. Hiscock, Shane L. Larson, Joshua Routzahn, and Ben Kulick, Astrophysical Journal 540, L5 (2000) S. L. Larson NASA EPSCoR