Exploration of the Time Domain:

A New Old Frontier

S. G. Djorgovski(Caltech)

2nd Zwicky Workshop,

Berkeley, May 25, 2005

Time Domain Astrophysics• 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 discoveryRapid follow-up and multi- keys to understanding

IntrinsicModulation along the LOS: microlensing, ISS, eclipses, variable extinction …

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?)

Flaring M Dwarfs(a vermin of the synoptic sky surveys?)

Lynx OT (Catalina Sky Survey) SDSS Counterpart

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.

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…

Optical Transients in DPOSS DPOSS

Keck

Faint, Fast Transients (Tyson et al.)

Some flaring M-stars, some extragalactic, … A heterogeneous population!

Accretion Flares From (Otherwise Quiescent) SMBHs: X-Ray

Komossa et al. (Rosat)

5 candidate events, amplitude > 102 (quiet state could be Lx ~ 0)

Lpeak ~ 1044 erg/s,Etot ~ 1050 erg ~ 10-4 Mc2

Ultra-soft spectra, kT < 0.1 keV

Tidal disruption and fallback. Expected rate ~ 10-4 /galaxy/yr

Accretion Flares From (Otherwise Quiescent) SMBHs: Visible

PALS-1(Stern et al.)

Possible gravitationally magnified U-band dropout (z ~ 3.3?) behind Abell 267

Variable sources in the centers of apparently normal galaxies at z ~ few tenths

Low-L AGN? L ~ 10-2 Lhost

Totani et al., SUBARU

Accretion Flares From (Otherwise Quiescent) SMBHs: Visible

Accretion Flares From Our Galaxy’s Own Central Black Hole?

A. Ghez et al.

A Systematic Search for Transients and Highly Variable Objects Using DPOSS Plate Overlaps

~ 1.5 O overlaps between adjacent plates ~ 40% of the total survey area

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

B. Granett, A. Mahabal, S.G. Djorgovski,and the DPOSS Team

Baselines from days to ~ 10 yrs, typical ~ 2-4 yrsTypical limiting mags r ~ 20, using 3 bandpasses (JFN gri)

DPOSS Plate Overlap Survey: High-Amplitude (non-OT) Variables

SpectroscopicSource Identifications:35% QSOs (1/2 radio loud)

18% CVs18% M dwarfs 6% Earlier type stars23% Unidentified (likely BL Lacs?)

Examples of DPOSS Transients

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

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

PQ Search for Low-z Supernovae

• Calibration of the SN Ia Hubble diagram• New standard candles from SN II• Endpoints of massive star evolution

C. Baltay, R. Ellis, A. Gal-Yam, S.R. Kulkarni, and the LBL SNF

(Using the image subtraction technique)

Optical Transients and Asteroids

(Exploratory work; A. Mahabal, with P. Kollipara, a Caltech undergrad)

CITData

Broker

JPLNEAT

Archive

VariablesArchive

MasterArchive

ImageArchive

Yale

AlertDecisionEngine

SourceClassification

Engine

KnownVariablesChecker

AsteroidSeparator

Engine

CITFast

Pipeline

CIT Next-Day Pipeline

NCSA

Other?LBL SNF

Website

P48

BroadcastAlert

Off-site Archives

NVO / Multi-Off-site Archives

Palomar-Quest Real-Time TransientsDiscovery System Data Flow

ComparisonEngine:

Vars./Trans.Detector

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.

Dumkopfs!I alreadysaid it all

back in 1935…

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

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