The Changing Universe: Catching Truths in Fleeting Events

In the supposedly unchanging eternal sky, ancient Chinese observers were drawn to unusual appearances: “guest stars” were portents of earthly events to court astrologers. Today, cosmic explosions and variability announce ever-stranger physical phenomena to modern astronomers, while improved observational tools illuminate intriguing new aspects of known systems. Supernovae, variable stars, gamma-ray bursts, and gravitational lenses are just some of the creatures of the transient Universe that LSST will reveal.

LSST will make fundamental contributions to characterizing populations and mapping Galactic structure as it identifies large quantities of known variable types. Astronomers will be able to study huge numbers of multiple star systems to reveal what types of stars frequent these systems. LSST will also see the dimming of stellar light due to the transit of orbiting planets to greater distances and lesser brightness than any existing instrument. LSST’s all-sky coverage, consistent long-term monitoring, and flexible criteria for event identification will allow researchers to extend observations from other programs and probe a large unexplored region of parameter space to discover new types of transients.

What goes boom in the Universe?

LSST will capture explosions in our Local Universe and those at cosmological distances — different types of objects dominate observable explosions in these two areas of space. Our Local Universe is defined as the area of space less than or equal to 200 megaparsecs (a megaparsec is also known as 1021, a sextillion kilometers) and objects sought in the magnitude “gap” between brightest novae and subluminous supernovae are best found locally. These observations will tell us about events such as the collapse of massive stars and the coalescence of neutron stars. LSST will also complement the search for gravitational waves, ultra high energy cosmic rays, TeV photons, and astrophysical neutrinos in the Local Universe. Identifying and characterizing the many transients of the Local Universe will rely on distinguishing those interesting events from known kinds of variable objects in both the foreground and background sky.

Great rewards will come from LSST for transients in the distant Universe as well, especially for events with decay times of less than one day. Gamma-ray bursts (GRBs) are the most relativistic explosions in the known Universe. They signal the birth of rapidly spinning stellar black holes resulting from the death of massive stars and can release more energy in 10 seconds than what the Sun will emit in its entire 10 billion year lifetime. Long GRBs typically last 2-100 seconds and short GRBs last less than 2 seconds but the more complicated picture that LSST will explore includes hydrid GRBs. GRB energy is released in jets so scientists estimate that for every 1 GRB observed here on Earth, there are about 500 GRBs that we don’t see. LSST will advance the detection of orphan GRB afterglows from unseen GRBs to understand the stellar deaths that produce them. Of course, even stranger objects such as peculiar transients, very fast transients, and exotic “unknown unknowns” will challenge and excite researchers.

What about geometric variables?

LSST will not only monitor stars and explosions, but will also see variations due to the geometry of certain systems, such as planetary transits and microlensing events. LSST will dramatically increase the number of known hot Jupiter systems and expand the range of observation to greater distances, potentially observing ~20,000 transiting hot Jupiters. The LSST data will lead to gains in understanding planetary migration theories and the effects of intense stellar irradiation of planet atmospheres.

Gravitational lensing is simply a foreground object deflecting and enhancing light from a distant object. Other LSST Science Book chapters deal with weak and strong lensing, but microlensing also exists due to the relative motion of the observer, lens, and source. These events will teach us about dark matter, planets, distant stellar populations and our solar neighborhood.

What can we count on?

LSST has three major objectives for research on variable stars: produce very large samples of already known types, discover theoretically predicted types, and discover new variable types. LSST will identify variables across all types of stars. Pulsating variable stars make up the majority of periodic variables and LSST data will illuminate the birth of systems, the death rattles of stars, and their tumultuous lives in between.

How to handle all these data?

LSST will produce 100 times more data than the current generation transient searches, delivering observations on tens of thousands of transients every night. Filtering time-critical information from the torrent of data - 30 terabytes each night - will be challenging. Recognizing interesting phenomenon for follow-up studies will require sophisticated data management and clever follow-up.

Because a transient is an object that hasn’t been seen before, the LSST team has to figure out just how to classify and handle the data. The team is laying out the following steps: 1) a quick initial classification, 2) short listing the nominees — deciding which possible follow-up resources or observations will result in a classification, 3) obtaining the follow-up and reclassification with LSST or other facilities and 4) adding new data and repeating the classification steps as needed. With the details of this system in place, LSST will lead the way to unexpected advances in our knowledge of transients and variables, finding out what mechanisms produce the changeable objects in the Universe.

Article written by Anna H. Spitz and Lucianne M. Walkowicz