Exploration of the Time Domain: A New Old Frontier

1. Exploration of the Time Domain: A New Old Frontier S. G. Djorgovski (Caltech) 2nd Zwicky Workshop, Berkeley, May 25, 2005

2. Time Domain Astrophysics Intrinsic Modulation along the LOS: microlensing, ISS, eclipses, variable extinction … • Moving objects:Solar system, Galactic structure, exoplanets • Variability Physical causes of intrinsic variability: • Evolution (structural changes etc.), generally long time scales • Internal processes, e.g., turbulence inside stars • Accretion / collapse, protostars to CVs to GRBs to QSOs • Thermonuclear explosions • Magnetic field reconnections, e.g., stellar flares • Line of sight changes (rotation, jet wiggles…) Variability is known on time scales from ms to 1010 yr Synoptic, panoramic surveys event discovery Rapid follow-up and multi-keys to understanding

3. Intrinsically Variable Phenomena • Things we know about: • Stars: oscillations, noise, activity cycles, atmospheric phenomena (flares, etc.), eclipses, explosions (SNe, GRBs), accretion (CVs, novae), spinning beams (pulsars, SS 433, …) • AGN: accretion power spectrum, beaming phenomena • Things we see, but don’t really understand: • Faint fast transients • Archival OTs • Megaflares on normal stars • Things we expect to see, and maybe we do: • Breakout shocks of Type II SNe • SMBH loss cone accretion events • BH mergers (LIGO, LISA?), QSO formation…? • Things as yet unknown and/or unexpected: • Manifestations of ETCs? (SETF?)

4. Flaring M Dwarfs(a vermin of the synoptic sky surveys?) Lynx OT (Catalina Sky Survey) SDSS Counterpart

5. Megaflares From Normal (?) Stars An example from DPOSS: A normal, main-sequence star which underwent an outburst by a factor of > 300. There is some anecdotal evidence for such megaflares in normal stars (Schaefer). The cause(s), duration, and frequency of these outbursts is currently unknown.

6. Optical Transients in DPOSS DPOSS A possible orphan afterglowdiscovered serendipitously in DPOSS: an 18th mag transient associated with a 24.5 mag galaxy. At zest ~ 1, the observed brightness is ~ 100 times that of a SN at the peak. How many do we expect to see? Depending on the beaming factors, there should be ~ 10 - 100 afterglows down to R ~ 20 mag per sky snapshot. … But it could be something else entirely… Keck

7. Faint, Fast Transients (Tyson et al.) Some flaring M-stars, some extragalactic, …  A heterogeneous population!

8. Accretion Flares From (Otherwise Quiescent) SMBHs: X-Ray Tidal disruption and fallback. Expected rate ~ 10-4 /galaxy/yr Komossa et al. (Rosat) 5 candidate events, amplitude > 102 (quiet state could be Lx ~ 0) Lpeak ~ 1044 erg/s, Etot ~ 1050 erg ~ 10-4 Mc2 Ultra-soft spectra, kT < 0.1 keV

9. Accretion Flares From (Otherwise Quiescent) SMBHs: Visible PALS-1 (Stern et al.) Possible gravitationally magnified U-band dropout (z ~ 3.3?) behind Abell 267

10. Accretion Flares From (Otherwise Quiescent) SMBHs: Visible Variable sources in the centers of apparently normal galaxies at z ~ few tenths Low-L AGN? L ~ 10-2 Lhost Totani et al., SUBARU

11. Accretion Flares From Our Galaxy’s Own Central Black Hole? A. Ghez et al.

12. A Systematic Search for Transients and Highly Variable Objects Using DPOSS Plate Overlaps B. Granett, A. Mahabal, S.G. Djorgovski, and the DPOSS Team ~ 1.5 O overlaps between adjacent plates  ~ 40% of the total survey area Baselines from days to ~ 10 yrs, typical ~ 2-4 yrs Typical limiting mags r ~ 20, using 3 bandpasses (JFN  gri) Effective Area Coverage in a “Snapshot” Survey: If texp < tburst and baseline ∆t >> tburst , then Effective Area = Useful Area  Npasses  Nfilters For DPOSS: ~ 15,000 deg2 0.4 2  3 ~ 0.9 Sky

13. DPOSS Plate Overlap Survey: High-Amplitude (non-OT) Variables Spectroscopic Source Identifications: 35% QSOs (1/2 radio loud) 18% CVs 18% M dwarfs 6% Earlier type stars 23% Unidentified (likely BL Lacs?)

14. Examples of DPOSS Transients

15. DPOSS Pilot Project Conclusions: • Faint, variable sky has a very rich phenomenology • Spectroscopic follow-up will be a key bottleneck for any synoptic sky surveys • Most high-amplitude variable sources down to ~ 20 mag are QSOs (Blazars, OVVs…), CVs, and flaring late-type dwarfs, with some early-type stars • Asteroids may be a significant contaminant in a search for transients • We find many more transients (~ 103/Sky) than expected from current models for orphan afterglows. • Most of them are probably QSOs, CVs, flaring stars, and distant SNe; some may well be afterglows; and some may be new types of phenomena

16. The Palomar-Quest Digital Sky Survey • Using a 112-CCD camera on the P48 • 50% P&S, 50% DS (the PQ survey) • A Caltech-JPL-Yale-… collaboration • Data rate ~ 1 TB/month; ~100 TB total; ~ 10 TB of DS already in hand • DS: ~ 500 deg2 / night in up to 4 filters, down to ~ 21 mag per pass • DS: multiple passes over ~ 15,000 deg2, time baselines minutes to years (decades) • DS: UBRI and rizz filters; PS: Rwide • VO connections and standards built in • Exploration of the time domain is one of the key science goals

17. PQ Search for Low-z Supernovae C. Baltay, R. Ellis, A. Gal-Yam, S.R. Kulkarni, and the LBL SNF (Using the image subtraction technique) • Calibration of the SN Ia Hubble diagram • New standard candles from SN II • Endpoints of massive star evolution

18. Optical Transients and Asteroids (Exploratory work; A. Mahabal, with P. Kollipara, a Caltech undergrad)

19. Palomar-Quest Real-Time Transients Discovery System Data Flow P48 Image Archive Off-site Archives LBL SNF Other? CIT Data Broker Yale NCSA Master Archive Variables Archive CIT Next-Day Pipeline CIT Fast Pipeline Comparison Engine: Vars./Trans. Detector Website Known Variables Checker Source Classification Engine Alert Decision Engine JPL NEAT Archive Broadcast Alert Asteroid Separator Engine NVO / Multi- Off-site Archives

20. Some Challenges Ahead • Automated, reliable, adaptive data cleaning • High volume data generators  lots of glitches • Cutting-edge systems  poor stability • High completeness / Low contamination • Integrate event discovery and (multi-) follow-up • Must work with “solar system people” - moving objects are the major contaminant for extra-solar-system variables and transients (and vice versa?) • Automated, reliable event classification and alert decisions (need Machine Learning methods) • Sparse data from the event originator; folding in heterogeneous external data; VO connections; etc.

21. Dumkopfs! I already said it all back in 1935…

23. PQ Survey Sky Coverage • Range -25°<  < +30°, excluding the Galactic plane • Ultimately cover ~ 14,000 - 15,000 deg2 • Rate ~ 500 deg2/night in 4 bands • As of Jan’05, covered ~ 13,000 deg2 in UBRI, of which ~ 11,000 deg2 at least twice, and ~ 4,700 deg2 at least 4 times; and ~ 14,100 deg2 in rizz, of which ~ 11,600 deg2 at least twice, and ~ 4,200 deg2 at least 4 times

24. Overview • Discoveries are often made through a systematic exploration of observable parameter space, e.g., the Time Domain • A poorly explored portion of the observable parameter space at any  (also , GW) and at many time scales • A range of exciting astrophysical phenomena • Possibility of fundamental new discoveries • Some things that go bang in the night • Known, expected, unknown/unexpected • A pilot project using DPOSS plate overlaps • A rich phenomenology of the time-variable sky • Palomar-Quest survey: status and plans