Multiwavelength Approach

1
The Multiwavelength Approach to Unidentified
Gamma-Ray Sources David J. Thompson NASA Goddard
Space Flight Center djt_at_egret.gsfc.nasa.gov
2
Outline

• MOTIVATION Why gamma-ray sources should be
  multiwavelength objects.
• X-RAYS Counterpart searches from the top
  down.
• RADIO Counterpart searches from the
  bottom up.
• PULSATION/ROTATION Periodic variability.
• FLARING Other forms of variability.
• POPULATIONS Collective properties of sources.
• THEORY The glue connecting all these
  observations.
• CONCLUSIONS

3

• MOTIVATION
• Why High-Energy Gamma-Ray Sources Should Be
  Multiwavelength Objects

4
Gamma-ray Sources Inherently Multiwavelength

• In the 10 MeV range and above, gamma radiation is
  non-thermal
• ? produced by interactions of energetic
  particles
• Synchrotron radiation Particles in magnetic
  fields
• Compton scattering Particles and lower-energy
  photons
• Bremsstrahlung Particles in fields of atomic
  nuclei
• ?o decay Decay of particles from
  inelastic collisions of particles

5
Effect of Energetic Particles - 1

• Nature rarely produces monoenergetic particle
  beams. A broad range of particle energies leads
  to a broad range of photon energies.

(Mori, 1997)
?o decay, monoenergetic in the rest frame,
produces a gamma-ray spectrum over orders of
magnitude when cosmic ray interactions are
involved.
Cosmic ray particle spectrum
6
Effect of Energetic Particles - 2

• Charged particles rarely interact by only one
  process.
• Different processes radiate in different energy
  bands.

Synchrotron emission radio to optical
Inverse Compton emission X-ray/gamma
ray
Multiwavelength spectrum of PKS 0537-441, showing
the Synchrotron and Compton components (Pian et
al., 2002)
7
Effect of Energetic Particles - 3

• High-energy particles, as they lose energy, can
  radiate in lower-energy bands.
• Particles with enough energy to produce gamma
  rays can also produce X-rays, optical, radio,
  etc.
• The reverse is not true thermal emission and
  emission from low-energy particles can radiate
  predominantly in narrow bands.

High-energy cutoffs in some black hole candidates
(Grove et al. 1998)
Gamma Ray Burst afterglows
8
EGRET All Sky Map (gt1 GeV)
3C279 (blazar)
3EG J18355918 (Isolated Neutron Star?)
Vela (radio pulsar)
Geminga (radio-quiet pulsar)
3EG J00107309 (CTA 1 SNR?)
Crab (radio pulsar SNR)
Orion Cloud (Cosmic ray interactions with ISM)
3EG J02416103 (LSI 61o303 Binary System?)
3EG J20204017 (? Cygni SNR?)
LMC (Cosmic ray interactions with ISM)
3EG J1746-2851 (Galactic Center?)
3EG J1837-0423 (unidentified
transient)
PKS 0208-512 (blazar)
9

• Third EGRET Catalog

Over half the total are unidentified
10

• X-RAYS
• Counterpart searches from the highest energies
  down

11
The Top-Down Multiwavelength Approach

• Concept at some level, gamma-ray sources will
  have X-ray counterparts.
• IF the X-ray counterpart can be found, the better
  X-ray position information allows deep
  searches at longer wavelengths.
• The approach using an X-ray image of a
  gamma-ray source error box, eliminate most of the
  X-ray sources from consideration based on their
  X-ray, optical, and radio properties. Look for a
  nonthermal source with a plausible way to produce
  gamma rays.
• The classic example is Geminga. Bignami,
  Caraveo, Lamb, and Halpern started this search in
  1983. The final result appeared in 1992 with the
  detection of pulsations from this isolated
  neutron star.

12
3EG J18355918 A New Geminga?

• Parallel effort by two groups, headed by
  Mirabal/Halpern and Reimer/Carramiñana used the
  same approach and reached the same conclusion for
  3EG J18355918

Take deep optical images to try to identify all
the X-ray sources. Most turn out to be stars or
QSOs, unlikely gamma-ray sources. One candidate
has no obvious optical counterpart RX
J1836.25925.
Use radio search to look for possible radio
pulsar. None found.
Start with deep ROSAT image (soft X-rays)
Construct MW spectrum. It resembles that of
Geminga, a spin-powered pulsar. No pulsations
have yet been found for 3EG J18355918.
Use Chandra to obtain X-ray spectrum of the
candidate two components, one thermal, one power
law.
13
Other Top-Down Efforts

• Although 3EG J18355918 is one clear example,
  other similar searches have been done and are
  continuing

Caraveo et al. XMM Newton observations,
analysis in progress.
Mukherjee et al. possible blazar in the Cygnus
region.
Roberts, Romani, Kawai ASCA searches, evidence
of pulsar wind nebulae. (Hard X-rays)
14

• RADIO
• Counterpart searches from the lowest energies up

15
One Bottom-up Multiwavelength Approach

• Concept the largest class of identified
  gamma-ray sources is blazars, all of which have
  radio emission.
• IF a flat-spectrum radio source with strong,
  compact emission at 5 GHz or above is found in a
  gamma-ray source error box, it becomes a blazar
  candidate.
• The approach use radio catalogs to search for
  flat-spectrum radio sources. If a candidate is
  found, follow up with other observations to
  locate other blazar characteristics such as
  polarization and time variability.
• The EGRET team used this approach in compiling
  the EGRET catalogs. Mattox et al. quantified the
  method based on proximity and radio intensity.
  Sowards-Emmerd, Romani, and Michelson have
  expanded the number of known blazars with this
  approach.

16
Example 3EG J04332908
Assembling the Puzzle (Foreman et al.) From
catalogs Flat-spectrum radio source, 475 mJy at
5 GHz IR, optical, X-ray source From
observations Radio variability Gamma-ray
variability Featureless optical spectrum Probable
optical and X-ray variability Spectral energy
distribution is bimodal like other
blazars Conclusion probably a BL Lac
17
Other Bottom-up Methods
Blazars are not the only objects where radio is a
useful tracer.
Combi and Romero, Torres et al. Radio maps used
to search for SNR associations with gamma-ray
sources.
Digel et al. Extended gamma-ray source in Orion,
compared to model based on HI and CO radio maps
Paredes et al. found a microquasar in the error
box of 3EG J1824-1514.
18

• PERIODIC VARIABILITY
• Rotational and Orbital

19
Periodicity as a Multiwavelength Tool

• Concept periodic variability at multiple
  wavelengths is a definitive identifier.
• Pulsars (rotating neutron stars) are the
  prototype of this approach.
• The approach search out a periodic signal at
  one wavelength, then fold data from other
  wavelength bands at the expected period.
• Thus far, all gamma-ray pulsars were first found
  at other wavelengths, but GLAST will have the
  capability for independent period searches.
• In addition to identification, periodic sources
  allow exploration of physical processes.

20
The Seven Highest-Confidence Gamma-ray Pulsars
Once identified, the important next step is to
learn what sources tell us about the Universe.
Multiwavelength light curves of pulsars provide
information about the physics and geometry of the
emission regions near the neutron stars.
21
Pulsars Multiwavelength Spectra
Very different from blazar spectra. Multiple
emission components are visible.
22
Orbital Motion from Binary Systems
The HESS team has recently reported TeV emission
from the binary system of PSR B1259-63 and a Be
star, with the emission seen near periastron.
23

• FLARING
• Other types of variability

24
Variability as a Multiwavelength Tool

• Concept transient or long-term variability
  helps to identify sources and to understand their
  physics.
• Variability studies have shown that pulsars have
  low variability (except for their pulsations),
  while blazars typically show more variability.
• The approach use degree of variability as a
  guide to source identification, then examine
  variability correlations at different wavelengths
  as a diagnostic of emission processes.
• Transients, including gamma-ray bursts, require
  fast response in order to obtain multiwavelength
  data.

25
Blazar Identification Example 3EG J2006-2321
First Clue Gamma-ray variability
Radio sources in the error box
One flat-spectrum radio source, 260 mJy at 5 GHz
one marginally-flat source, 49 mJy other sources
are much weaker
Optical observations The 49 mJy source is a
normal galaxy The 260 mJy source has an optical
counterpart with a redshift z0.83
Variable optical polarization is seen. Only an
X-ray upper limit found.
Spectral energy distribution is bimodal like
other blazars Conclusion a flat spectrum radio
quasar (FSRQ)
Wallace et al.
26

MW Needs for Blazar Exploration

• Blazars are characterized by both short-term and
  long-term variability at essentially all
  wavelengths. The relationship between changes at
  different wavelengths is a powerful tool for
  studying the jets in these sources.
• Two general aspects to exploration of blazar
  variability
• Monitoring to know what the long-term behavior is
  and to identify short-term flares.
• Campaigns for intensive study, particular during
  flares.

3C279 Gamma rays X-rays UV Optical IR Radio
27

• POPULATION STUDIES
• Collective properties of sources

28
Population Studies as a Multiwavelength Tool

• Concept even if individual source
  identifications are not possible, statistical
  analysis may show a pattern of correlation with a
  given type of object.
• Because gamma-ray source error boxes are large,
  positional agreement with a candidate object is
  rarely a strong argument by itself for physical
  association.
• The approach compare a gamma-ray source catalog
  (or subset of a catalog) with known classes of
  objects, using some statistical measure of
  correlation.
• If an association can be found, then it is
  valuable to study the implications of this source
  class.

29
Population Studies Low Galactic Latitude Sources
Examples Kaaret and Cottam correlation with OB
associations Yadigaroglu and Romani OB
associations, pulsars, SNR Romero et al. SNR,
OB associations
30
Population Studies Away from the Galactic Plane
Examples Grenier, Gehrels et al. Steady
sources a new population, distinct from the
sources along the Galactic Plane. Associated with
the Gould Belt nearby, low luminosity objects,
possibly pulsars or microquasars. No candidate
objects yet identified at other wavelengths.
31

• THEORY AND MODELING
• Glue that connects the various observations

32

Importance of Theory and Modeling

• Theoretical work provides explanation and
  understanding of what has already been seen.
• Moves us from What is there? to How do they
  work?
• Theory gives observations more predictive power.
• For known sources, theory predicts what should be
  seen (or not seen) at wavelengths not yet
  explored.
• For the future, theory predicts what new sources
  should be expected.

33

Explanation of Observations Example
Synchrotron Radiation
Curvature Radiation
Model for Crab Pulsar Spectrum Cheng, Ruderman,
Zhang
34
Theory - Predicting the Unobserved
Spectrum of 3EG J1714-3857/SNR RXJ1713-3946.
With the limited multiwavelength coverage, no
simple model explains the source (Reimer and
Pohl).
35
Theory - Predicting Future Detections
Predictions of gamma-ray pulsar fluxes for known
radio pulsars by various models.
36

CONCLUSIONS

• The value of multiwavelength studies for both
  identification and exploration of gamma-ray
  sources has been clearly established.
• Despite substantial efforts, these methods have
  not solved all the mysteries of the unidentified
  EGRET sources.
• The multiwavelength approach will be well
  prepared for the new generation of satellite and
  ground-based gamma-ray telescopes INTEGRAL,
  Swift, AGILE, GLAST, MILAGRO, HESS, MAGIC,
  CANGAROO-3, VERITAS.