ANTARES: Example Science Use Cases

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1 ANTARES: Example Science Use Cases 1 Introduction The design of the antares system is meant to be flexible enough to accommodate filters that can distinguish almost any object of interest to time-domain astronomy, subject to the limits of computational resource and time. We could sample from a wide variety of transient or variable objects that could be employed for a science use case. Here we describe five use cases that exemplify the potential capabilities of the antares system, especially in regard to the focus of the prototype being the search for the rarest of the rare. 2 Electromagnetic Counterparts to Gravitational Wave Sources One of the most exciting results in modern physics is the direct detection of gravitational waves (GWs) by interferometric systems. The Advanced Laser Interferometer Gravitational- Wave Observatory (aligo; Aasi et al., 2015) has announced two significant detections of GW sources (Abbott et al., 2016c,a), each of which was the subject of intensive searches for an electromagnetic (EM) counterpart, (e.g., Abbott et al., 2016b; Cowperthwaite et al., 2016). These follow-up campaigns conducted wide-field imaging surveys of the likely regions of the sky given the localizations as determined by aligo. These localizations are fairly broad, as illustrated in Figure 1. For GW150914, the total area of the 90% credible region for the GW source was over 600 square degrees on the sky. Only a small fraction of that credible region was imaged to look for the EM counterpart (Abbott et al., 2016b). With upgrades and additional LIGO facilities, the localizations should become more feasible to search in the near future, with 50% of events having localizations 20 square degrees (Abbott et al., 2016d). Moreover, the Large Synoptic Survey Telescope (LSST, Ivezić et al., 2008; Kantor, 2014; Kahn, 2016) field-of-view is 10 square degrees, enabling deep, wide-field coverage at a rate unprecedented today. Even so, with tens of square degrees to search, and each LSST field of 10 square degrees having several thousand alerts, there is still a problem in identifying the actual EM counterpart. The first major step in the antares architecture is the association of an alert with known astrophysical sources at the same position on the sky, as well as previous alerts, if any. For an aligo alert, the expectation is that there will not be a previously detected source, but that there will be a host galaxy. Information known about the host (distance, type, etc.) 1

2 Figure 1: The 90% credible region for the localization of GW The colors denote different algorithms used to determine the credible region. The total area is greater than 600 square degrees while follow-up images on the ground generally have a field-of-view of less than one square degree. Figure from Abbott et al. (2016b). will be annotated in the alert. Such galaxy catalogs do not exist with the completeness necessary at this point, but LSST itself will eventually provide such a catalog. 2

3 There is an extensive literature predicting the optical characteristics of a GW source such as a merger of two neutron stars, two black holes, or a black hole/neutron star system, (e.g., Narayan et al., 1992; Nakar, 2007; Metzger & Berger, 2012; Berger, 2014; Fong et al., 2015; Metzger, 2016). Most predictions indicate a short-lived ( days) relatively red optical transient. The distinguishing features will include this color information as well as constraints inferred from the host galaxy. Visualization and evaluation of the features in relation to each other is necessary to determine which features are most useful for singling out the EM counterpart. Part of this distinction is eliminating known objects with some overlap in the feature space of interest. Note that this presumes that LSST will engage in multi-filter observations during a campaign to identify an EM counterpart, rather than the single-filter cadence that will make up the bulk of the survey. In addition, the predicted rapid time-scale of the EM counterpart highlights the necessity to distinguish it early so that the maximum amount of follow-up observations will be possible. There may not be enough information from early, limited observations, so a list of likely alerts may be produced, rather than a single, sure thing. Tools to assess the probability of an object s categorization, especially when combining inferences from several techniques, will be needed to provide realistic rankings for alerts. In addition, feedback from subsequent observations will be useful, although that is beyond the scale of the prototype antares system. Through a robust, dynamic, and efficient system, one can find the truly rare object such as an aligo source. 3 Tidal Disruption Events The study of stars disrupted by tidal forces as they pass near a super-massive black hole has grown tremendously in the past decade, with several likely events directly observed (Gezari, 2012; Komossa, 2015). These tidal disruption events (TDEs) can shed light on the nature of black holes, including accretion mechanisms and jet formation (e.g., Metzger et al., 2012). With the number of TDEs detected so far only in the tens of objects, there are still many mysteries yet to be solved. For example, there is an apparent preference for these events in post-starburst galaxies (French et al., 2016), but it is not clear why that is the case. This use case demonstrates another element of the antares architecture, namely multiwavelength association. It is likely that all galaxies contain a massive black hole at their center, but they are only detectable by their effect on the environment around them. If there is material actively falling into the black hole, there is direct evidence in the form of radiation emitted from that material as it interacts with the black hole, directly or via an accretion disk. This emission can span the electromagnetic spectrum, but they are prodigious producers of x-rays compared to other astronomical sources. This accretion of material is 3

4 Figure 2: Artist s rendition of a tidal disruption event. Figure from Komossa (2015). not continuous, leading to variation in the emission from such active nuclei, so such objects (active galactic nuclei AGN) would appear as alerts in the LSST data stream. If a star moves too close to a massive black hole, the tidal forces can essentially rip it apart, illustrated in Figure 2. These TDEs will also appear as alerts in the LSST data stream with characteristics quite similar to AGN. The antares architecture takes each alert and, if there is more than one possible association at the location of the alert, produces replicas that associate the alert with all of the potential AstroObjects at that position. Often, these associations are unrelated, as in the case of a foreground star and a distant galaxy where the alignment is coincidental. There are cases, however, where the interaction of association, especially across the electromagnetic spectrum, provides valuable information that can aid in the categorization of an alert. These are called combos in the antares architecture. Active galaxies are more common than TDEs, so most alerts that occur at the center of a galaxy are likely to be AGN (and there are other, less common possibilities). To find the TDEs, antares must eliminate the AGN. The use of the multiwavelength association allows for this distinction. If there is a known source of x-rays (or other high-energy emission), then it is likely to be an AGN, and thus removed from consideration as a TDE. Even association with a radio source increases the likelihood that the object is an AGN. By using the combo approach, antares can combine information to provide a categorization. In addition, broad-band photometry and multiwavelength data can distinguish post-starbust 4

5 galaxies (A. Zabludoff, priv. comm.), so ancillary information about potential host galaxies can provide even more of a distinction for likely TDEs. 4 Supernovae on Demand The explosive disruption of a star, either the thermonuclear explosion of a white dwarf or the core-collapse of a massive star, is a spectacular event. They are highly energetic and thus bright and visible over cosmological distances. Supernovae help us to understand the end states of stellar evolution (e.g., Langer, 2012), physics in extreme environments (e.g., Sukhbold & Woosley, 2016), the chemical evolution of the Universe (e.g., McWilliam, 1997), and some can be used as standardizeable candles to map the Universe on cosmological scales (e.g., Riess et al., 1998; Perlmutter et al., 1999). One thing they are not, though, is rare. Today, they are discovered by the hundreds every year (see the Transient Name Server site 1 ), while LSST is likely to produce thousands per year (Rau et al., 2007). Given the bulk of the science that is done with supernovae, the key is not the discovery of a single, unique object, but rather the detailed follow up of large samples. The resources to conduct observational follow up of astronomical objects are limited, especially when it comes to transient objects that are only available for a short period. In addition, many resources are scheduled in ways not necessarily favorable to time-domain astronomy. The so-called classical method of scheduling telescopes assigns nights to astronomers and they need to have targets available when their time is scheduled. When a time-domain survey such as LSST can provide relatively common objects every night, this issue is resolved, as long as the targets of interest can be identified. This could include selecting thermonuclear supernovae as part of a study to characterize their diversity in order to better constrain cosmological parameters or selecting only the very youngest supernovae where early-time photometry and spectroscopy has proven extraordinarily valuable in understanding the progenitors of the explosions (e.g., Bloom et al., 2012; Khazov et al., 2016). The way to fill this need with antares is to alter the set of filters in use by the system so that, in addition to rare objects, objects that are not rare, but desired that night can be identified. This would involve changing the filtering algorithms during the day so that the nightly run can respond to specific requests. (This is an action of the conductor described in the Architecture document.) The ability of an individual astronomer or team to request such a change would have to rely on rankings of an allocation committee, just as with any other observational program. This capability is not implemented in the prototype, but the architecture is designed to accommodate it. 1 AT=yes&classified sne=1&num page=500 5

6 5 Superflare Stars While many late-type stars exhibit flaring (e.g., Benz & Güdel, 2010; Hawley et al., 2014, and references therein), the Kepler spacecraft revealed a class of flares with energies greater than ergs (approximately 100 times more energetic than flares observed on the Sun, Candelaresi et al., 2014). These superflare events are rare and may have implications for dynamo activity in late-type stars, and thus our understanding of stellar structure and evolution in solar-type stars. In addition, the frequency of such superflares has implications for systems with exoplanets and the possibility of habitability, including whether our own G-type star is capable of producing a superflare (e.g., Lundkvist et al., 2016; Karoff et al., 2016). See also the discussion in Chapter 5 of Najita et al. (2016). This type of object would trigger the VPDF stage early in the antares system as a particularly unusual brightening of a source. In addition, the progenitor star would have been observed before the flare as part of the regular LSST survey. The photometric information from the prior observations would put strong constraints on the type of star, and thus enable rapid identification of the alert as a flare on a G- or K-type star. 6 The Unknown Astronomy continues to have a strong discovery-based element. Despite theoretical consideration of Einstein s cosmological constant (Carroll et al., 1992), no one predicted the revolution that occurred in 1998 when observations of supernovae revealed that the Universe was not just expanding, but accelerating (Riess et al., 1998; Perlmutter et al., 1999). The nature of the so-called dark energy that drives this acceleration and makes up two-thirds of the Universe is still a mystery. It was first found through careful analysis of an unexpected result from time-domain astronomy. The even more prodigious explosions that generate super-luminous supernovae were only identified within the last 15 years (Gal-Yam, 2012). Figure 3 illustrates one method of characterizing the parameter space of transient events via luminosity (or energy) and time scale. While classical novae, core-collapse supernovae, and thermonuclear supernovae have been known and characterized for decades, the other objects identified in the figure have only been recognized for about a decade. These include the super-luminous supernovae mentioned before, as well as luminous red novae (Martini et al., 1999; Kasliwal et al., 2011), calcium-rich transients (Kasliwal et al., 2012; Foley, 2015), and.ia supernovae (so-called because they are a tenth of the energy and time scale of a Type Ia supernova, Kasliwal et al., 2010; Perets et al., 2010). Not all regions of this particular parameter space may be physically possible, but there are clearly still empty regions that have yet to be fully explored. The prototype antares 6

7 REFERENCES Figure 3: The discovery space of time-variable explosive astrophysical objects. The parameters in this diagram are luminosity (M V, or inverse logarithmic brightness) and the time scale for the change in flux. Only the gray areas have been explored in any detail. There are large regions of parameter space still unknown and this is just explosive transients. Figure from Kasliwal (2012). system is designed to winnow out from alert streams objects that are known and common. Rare objects that have been seen are flagged, as described above for EM counterparts to gravitational wave sources or superflare stars. Objects for which there is no comparable match in known catalogs or feature spaces are also identified as rare, giving us a way to identify the unknown when we see it. References Aasi, J. et al Characterization of the LIGO detectors during their sixth science run. Classical and Quantum Gravity, 32, , astro-ph/

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9 REFERENCES Gezari, S. 2012, in European Physical Journal Web of Conferences, Vol. 39, European Physical Journal Web of Conferences, Hawley, S. L., Davenport, J. R. A., Kowalski, A. F., Wisniewski, J. P., Hebb, L., Deitrick, R., & Hilton, E. J Kepler Flares. I. Active and Inactive M Dwarfs. ApJ, 797, 121, astro-ph/ Ivezić, Ž., et al LSST: from Science Drivers to Reference Design and Anticipated Data Products. ArXiv e-prints, arxiv: Kahn, S. 2016, in Proc. SPIE, Vol. 9906, Ground-Based and Airborne Telescopes IV, ed. H. J. Hall, R. Gilmozzi, & H. K. Marshall, in press Kantor, J. 2014, in The Third Hot-wiring the Transient Universe Workshop, ed. P. R. Wozniak, M. J. Graham, A. A. Mahabal, & R. Seaman, Karoff, C. et al Observational evidence for enhanced magnetic activity of superflare stars. Nature Communications, 7, Kasliwal, M. M Systematically Bridging the Gap Between Novae and Supernovae. PASA, 29, 482 Kasliwal, M. M. et al PTF 10fqs: A Luminous Red Nova in the Spiral Galaxy Messier 99. ApJ, 730, 134, astro-ph/ Calcium-rich Gap Transients in the Remote Outskirts of Galaxies. ApJ, 755, 161, astro-ph/ Rapidly Decaying Supernova 2010X: A Candidate.Ia Explosion. ApJL, 723, L98, astro-ph/ Khazov, D. et al Flash Spectroscopy: Emission Lines from the Ionized Circumstellar Material around <10-day-old Type II Supernovae. ApJ, 818, 3, astro-ph/ Komossa, S Tidal disruption of stars by supermassive black holes: Status of observations. Journal of High Energy Astrophysics, 7, 148, astro-ph/ Langer, N Presupernova Evolution of Massive Single and Binary Stars. ARA&A, 50, 107, astro-ph/ Lundkvist, M. S. et al Hot super-earths stripped by their host stars. Nature Communications, 7, 11201, astro-ph/

10 REFERENCES Martini, P., Wagner, R. M., Tomaney, A., Rich, R. M., della Valle, M., & Hauschildt, P. H Nova Sagittarii (V4332 Sagittarii): The Discovery and Evolution of an Unusual Luminous Red Variable Star. AJ, 118, 1034, astro-ph/ McWilliam, A Abundance Ratios and Galactic Chemical Evolution. ARAA, 35, 503 Metzger, B. D The Kilonova Handbook. ArXiv e-prints, astro-ph/ Metzger, B. D., & Berger, E What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger? ApJ, 746, 48, astro-ph/ Metzger, B. D., Giannios, D., & Mimica, P Afterglow model for the radio emission from the jetted tidal disruption candidate Swift J MNRAS, 420, 3528, astroph/ Najita, J. et al Maximizing Science in the Era of LSST: A Community-Based Study of Needed US Capabilities. ArXiv e-prints, astro-ph/ Nakar, E Short-hard gamma-ray bursts. PhR, 442, 166, astro-ph/ Narayan, R., Paczynski, B., & Piran, T Gamma-ray bursts as the death throes of massive binary stars. ApJL, 395, L83, astro-ph/ Perets, H. B. et al A faint type of supernova from a white dwarf with a helium-rich companion. Nature, 465, 322, astro-ph/ Perlmutter, S. et al Measurements of Omega and Lambda from 42 High-Redshift Supernovae. ApJ, 517, 565, arxiv:astro-ph/ Rau, A. et al The Incidence of Dwarf Novae in Large Area Transient Searches. ApJ, 664, 474, arxiv:astro-ph/ Riess, A. G. et al Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. AJ, 116, 1009, arxiv:astro-ph/ Sukhbold, T., & Woosley, S. E The Most Luminous Supernovae. ApJL, 820, L38, astro-ph/

LISA: Probing the Universe with Gravitational Waves. Tom Prince Caltech/JPL. Laser Interferometer Space Antenna LISA

LISA: Probing the Universe with Gravitational Waves. Tom Prince Caltech/JPL. Laser Interferometer Space Antenna LISA : Probing the Universe with Gravitational Waves Tom Caltech/JPL Laser Interferometer Space Antenna http://lisa.nasa.gov Gravitational Wave Astronomy is Being Born LIGO, VIRGO, GEO, TAMA 4000m, 3000m, 2000m,