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ASAS-‐SN: Big Science with Small Telescopes
Thomas W.-‐S. Holoien (Ohio State) and the ASAS-‐SN Team
ASAS-‐SN is supported by NSF grant AST-‐1515927, the Center for Cosmology and AstroParLcle Physics (CCAPP) at OSU, the Mt. Cuba Astronomical FoundaLon, and the Robert MarLn Ayers Sciences Fund.
TW-‐SH is supported by the DOE ComputaLonal Science Graduate Fellowship, grant number DE-‐FG02-‐97ER25308. Contact: [email protected]‐state.edu
Presented at the 2016 DOE CSGF Annual Program Review in Arlington, VA, July 2016
IntroducLon
About ASAS-‐SN
Even in the modern era, only human eyes can scan the enLre opLcal sky for the violent, variable, and transient events that shape our universe. The "All-‐Sky Automated Survey for Supernovae" (ASAS-‐SN or "Assassin") is changing this by monitoring the extra-‐galacLc sky to a limiLng magnitude of 17 every 2-‐3 days using mulLple telescopes, hosted by Las Cumbres Observatory Global Telescope Network, in the northern and southern hemispheres. Our automated pipeline schedules observaLons, processes data, and scans images for new potenLal discoveries without human interacLon, requiring sophisLcated algorithms to avoid false posiLve detecLons. The primary goal of ASAS-‐SN is to discover bright, nearby supernovae, and in three years of operaLon we have discovered over 300 supernovae, including 179 in 2015 alone, accounLng for more than 60% of supernovae brighter than 17th magnitude. ASAS-‐SN is finding nearby supernovae that would not be found by any other professional or amateur survey, and the nearby nature of ASAS-‐SN discoveries allows for detailed follow-‐up across a wide wavelength coverage. Here we present updated details on the ASAS-‐SN survey and its data-‐processing pipeline as well as analyses of the supernova discoveries made during its first three years of operaLon.
Discovery StaLsLcs The primary science goal of ASAS-‐SN is a complete and unbiased census of bright, nearby supernovae, a task that has never
been acempted by any previous project. Our early results with supernovae have been extremely encouraging, but ASAS-‐SN has also discovered many other types of bright transients. In just over two years of operaLon, ASAS-‐SN has found:
• 650+ new cataclysmic variable (CV) stars, • 40+ M-‐dwarf flares with ΔV≥4, including the largest-‐magnitude flare ever discovered, • 18 blazar flares, • 3 Ldal disrupLon events (TDEs), and • 355 Supernovae (265 Type Ia, 74 Type II, 11 Type Ib/Ic, 1 SLSN-‐I, 4 Untyped)
Figures 4 and 5 below compare ASAS-‐SN supernova discoveries to those of other professional and amateur surveys since the ASAS-‐SN “Cassius” unit came online in May 2014. ASAS-‐SN is discovers more than half of all bright supernovae, and the ASAS-‐SN sample appears to be less biased in terms of host galaxy magnitude and distance from the host galaxy nucleus than other searches. ASAS-‐SN also finds most of the remaining bright supernovae, but is not the first to report them.
Figure 4: Histogram showing the number of bright (V ≤ 17.0) supernovae discovered per month in 2014 and 2015. Color indicates whether they were discovered by ASAS-‐SN (red), discovered by others but recovered in ASAS-‐SN data (green), or neither discovered nor recovered by ASAS-‐SN (blue). ASAS-‐SN has consistently discovered more than half of bright supernovae since Cassius came online in May of 2014.
Figure 5: The distribuLon of bright SNe as a funcLon of absolute host galaxy magnitude and the offset of the SN from the center of the host, color-‐coded by whether they were discovered by ASAS-‐SN (red), other professional surveys (blue), or amateur astronomers (black). The horizontal and verLcal lines indicate the corresponding median values. The median offset for ASAS-‐SN supernovae is less than half that of either of the other two samples (4.9 arcseconds vs. 12.2 or 15.0).
Notable Discoveries Below we highlight some of the most notable discoveries made by ASAS-‐SN in the last year.
ASASSN-‐15oi (Holoien et al. 2016) This is the third Ldal disrupLon event discovered by ASAS-‐SN, and the third-‐nearest TDE candidate ever found. It was more luminous and faded more rapidly than most TDEs, but its spectra are a good match to helium-‐rich TDEs in literature. With 3 TDEs compared to 265 type Ia supernova discoveries, ASAS-‐SN is finding TDEs at a significantly higher rate than other surveys.
Figure 6: MulL-‐band light curves (lep) and spectral Lme sequence (right) of ASASSN-‐15oi. The strong blue conLnuum and broad helium emission features are characterisLc of a Ldal disrupLon event, though it fades more rapidly than other ASAS-‐SN TDEs.
References & Websites Works cited: • Dong et al. 2016, Science, 351, 257 • Godoy-‐Rivera et al. 2016, arXiv: 1605.00645 • Holoien et al. 2016, arXiv: 1602.01088 For more informaLon, please see our websites:
ASAS-‐SN Homepage ASAS-‐SN Supernovae TW-‐SH Homepage
ASAS-‐SN Data Pipeline With roughly 60 GB of raw data taken per night and thousands of possible transient sources in every image, the ASAS-‐SN pipeline requires automaLon and sophisLcated source detecLon algorithms to run with as licle human interacLon as possible. The basics of the ASAS-‐SN data processing pipeline and image subtracLon are summarized in the Figures below:
ASASSN-‐15lh (Dong et al. 2015, Godoy-‐Rivera et al. 2016) This is the most luminous supernova ever discovered, and it exhibited a second peak in its UV light curve roughly 150 days aper explosion.
Figure 7: UV and opLcal light curves of ASASSN-‐15lh. The unusual UV re-‐brightening is not associated with any spectral evoluLon.
Figure 1: Sky map showing the number of Lmes each ASAS-‐SN field was observed between July 16, 2015 and July 15, 2016. Every field was observed over 40 Lmes, and most fields were observed over 100 Lmes, during this year-‐long span.
ASAS-‐SN currently consists of two units, one in each hemisphere, hosted by the Las Cumbres Observatory Global Telescope Network (LCOGT). Our northern hemisphere unit, “Brutus,” is deployed at the LCOGT Haleakala staLon, and our Southern hemisphere unit, “Cassius,” is deployed at the LCOGT Cerro Tololo staLon. Both units are composed of four 14-‐cm telescopes on a common mount, with each camera having a roughly 20 square degree field of view. Together, these telescopes allow us to survey approximately 20,000 square degrees each night down to roughly 17th magnitude in V-‐band, giving us full sky coverage every 2-‐3 nights, weather permisng. UlLmately, we hope to add an addiLonal unit in each hemisphere, allowing nightly full-‐sky observaLon. A map of our recent sky coverage is shown below.
Figure 3: DemonstraLon of image subtracLon with recent discovery ASASSN-‐16fp. The new observaLon (top-‐right) is subtracted from the reference image (top lep), leaving a clean detecLon in the subtracted image (bocom). The red circle highlights the supernova.
Reference Image New ObservaLon
SubtracLon Figure 2: Flowchart outlining the ASAS-‐SN pipeline. ObservaLons are scheduled to maximize the discovery rate of transients, and data are downloaded reduced throughout the night in real-‐Lme. PotenLal transient sources are flagged for human scanning, and confirmed discoveries are publicly announced through Astronomer’s Telegrams and the ASAS-‐SN transients website.