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GLAST Science and Opportunities
Seattle AAS Meeting, January 2007
Enhancing GLAST Science Through Complementary Radio Observations
Jim UlvestadPaper 176.02
2Acknowledgments
• Slides from Greg Taylor, Sean Dougherty• Stanford group (Romani, Sowards-Emmerd, Healey)
and others for collaborative VLA programs
3Outline
• Guiding Principles• IDs of New Source Classes• IDs of Individual Sources• Examples: Blazars, Colliding Wind Binaries
4Guiding Principles
• Radio observations should be driven by peer-reviewed science, and by maximizing the combined science outputs of the GLAST mission and the radio telescopes
• Selection of radio telescopes should be governed by those that are uniquely required for the complementary GLAST science
• Radio telescope facilities must balance GLAST science carefully with the rest of their science portfolio
• Bureaucratic headaches and double-jeopardy for proposers and observers should be minimized
• Question: How does one secure GLAST-supporting data (e.g., pulsar timing) that do not represent exciting science from the radio observatory alone?
5IDs of New Source Classes
• EGRET detected approximately 271 individual gamma-ray sources (3EG, Hartman et al. 1999)– Only about 1/3 had high-confidence identifications in 3EG– Many unidentified sources at both low and high galactic
latitudes– Two primary identified classes were blazars and pulsars
• GLAST will detect ~104 individual sources– How can radio observations be used to (help) identify new
classes of sources, such as LLAGNs, supernova remnants, microquasars, etc.?
6Radio Catalogs and New Source Classes
• Correlative studies between gamma-ray error boxes and sources of high/medium/low/absent radio flux density– Large-area radio catalogs at moderately low frequencies of 1-5
GHz (e.g., FIRST, NVSS, SUMSS, Parkes, GB6)• Optical IDs/classifications are incomplete
• Most have poor resolution, and catalogs are not contemporaneous
– Radio surveys of particular classes of sources• Unbiased radio surveys of particular object classes are rare
• Excellent approach may be to use classes of sources identified in SDSS (e.g., SDSS quasars), and look for correlations with the radio fluxes/powers in the individual classes
7IDs of Individual Sources
• Very promising avenue for radio observations AFTER source classes are identified
• Likely correlation of gamma-ray detection/fluence with radio flux density
• Figure of Merit approach developed over last several years has worked very well for blazars (Sowards-Emmerd et al. 2004)
8CRATES Source Distribution
• Flat-spectrum sources, CLASS + VLA + ATCA (Healey et al. 2007)
11,000 flat-spectrum sources, |b|>10 deg., S > 65 mJy
9A Possible VLA Approach to Identifying
Counterparts
• Scaling from NVSS, an all-sky VLA 8.4 GHz survey would require approximately 3,000 * (8.4/1.4)2 = 108,000 hr, or 15-18 years of observing!
• However, one could carry out a targeted survey of 5,000 GLAST source fields at the rate of 1,000 fields per day– Total observing time of 120 hr– Simultaneous 1.4 and 5 GHz observations with 12 antennas
each, for 30 seconds on target, in A configuration of VLA– RMS noise = 0.5 mJy in each band– Resolution ~ 2 arcsec, field of view ~ 9 arcmin
10Hypothetical Targeted VLA Survey
5 GHz
1.4 GHz
11
Gamma-Ray Emission Mechanisms for Blazars
GLAST will detect thousands of gamma-ray blazars that
can only be resolved by VLBI techniques
12Sub-Milliarcsecond Imaging of Blazar Jets
• How do gamma-ray flares relate to changing structures in blazar radio jets? Which comes first?
• What is the origin of the gamma rays? Internal or External Compton?
• There are hints that EGRET blazers are faster (Jorstad et al 2001) and more strongly polarized (Lister & Homan 2005)
• Do we have the observational tools to image jets on the appropriate length scales and time scales?
13Requirements for Imaging Blazar Jets
• High-frequency capability (> 20 GHz) to image jets where they are optically thin
• Full-polarization imaging• Dynamic scheduling for response to gamma-ray
flares at any time of year, and for repeated reliable observations
• Sub-milliarcsecond resolution to detect changes on time scales of days to months
Only the VLBA meets these requirements
14VLBA
• High Sensitivity Array (add VLA, GBT, Effelsberg) may be desirable for LLAGNs, TeV blazars
15Sample Jet Evolution Imaged with VLBA
• Monthly VLBA imaging of radio galaxy 3C 120 at 22 GHz (Gomez et al. 2000)
• What were the gamma rays doing during this period?
• Desire imaging on time scales of weeks or less for z~0.5
16VLBA Imaging Polarimetry Survey (VIPS)
• 1127 sources, S > 85 mJy, 65 > > 20 deg., |b| > 10 deg., at 5 GHz
• First-epoch VLBA observations in 2006– Helmboldt et al. 2007, astro-ph/0611459
• Identifications and redshifts from SDSS, HET, Palomar, Keck, …
• Goals:– Characterize GLAST sources (pre-launch)– Study evolution of radio sources– Probe AGN environments– Find binary supermassive black holes
http://www.phys.unm.edu/~gbtaylor/VIPS
17Which Jets will be Detected by GLAST?
Helmboldt et al. 2007
18
• VLBA observations have enabled an orbit solution
Colliding Wind Binary, WR 140
• Distance – NOT based on stellar parameters! Distance = 1.85 +/- 0.16 kpc
• O supergiant• All important system parms now
defined!!!– Stellar types– Distance– All orbit parameters (including
inclination)
– ALL VERY IMPORTANT to modelling
Dougherty, Pittard et al. 2005, 2006
19
EGRET (100MeV – 20 GeV)
From Benaglia & Romero (2003)
WR140 lies in 3EG J2022+4317 Error Box
• Is WR140 a gamma-ray source?– Are CWBs gamma-
ray sources?
• What should we expect at high energies?
20WR140 Emission at phase 0.8 (from fits to radio data)
Radio ASCA GLAST
21Predicted Luminosities and Fluxes at Phase 0.8
• GLAST 5σ sensitivity at E > 100 MeV for a 2-yr all-sky survey is 1.6 x 10-9 ph s-1 cm-2 (should detect WR140 with GLAST)
• High-energy observations are critical to establishing some model parameters
22Radio Observatories
• NRAO: VLA, VLBA, GBT; eventually EVLA & ALMA– Rapid Response and Large Proposal processes
• Existing surveys (NVSS, FIRST, VIPS, MOJAVE, etc.)• Non-NRAO telescopes
– European VLBI Network (3 sessions/yr, 2-3 weeks)– University Radio Observatories
• History of rapid response science with CARMA
– Arecibo, at frequencies below 10 GHz– Australia Telescope Compact Array, or LBA