Radio and Xray variability of M31, the supermassive black hole in Andromeda

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Radio (and X-ray) variability of M31, the
super-massive black hole in Andromeda

• Loránt Sjouwerman
• National Radio Astronomy Observatory
• Michael Garcia (Smithsonian), Albert Kong, John
  Dickel,
• Michael Garrett, Kelley Hess, Benjamin Williams,
  Kelsey Johnson

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Outline

• Introduction to radio observations
• New 5 GHz Very Large Array data
• Radio results
• M31, and its X-ray counterpart ?
• Radio spectral index of M31
• Variability of M31 at 5 GHz
• Summary
• X-rays by M.Garcia

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The center of Andromeda

• Normal galaxy nuclear black hole
• Nearest, apart from our own Sgr A
• Individual, local to M31 objects/topics
• SN1885A S And, 16 from center
• Compare structure, etc., to MWG
• Major constituent of Local Group
• Dynamics, evolution, interaction

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The center of Andromeda

• For reference early radio image
• Central extended diffuse emission
• Long baselines (in terms of ?s)
• Inner r4 (1 kpc)
• Pooley 1969

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Previous high-resolution cm-wave radio
observations

• Arc-second resolution instruments resolve out
  most of the central clump in some individual
  sources
• Hjellming Smarr 1982 VLA, 5 GHz
• (van der Kruit 1972, WSRT 1.4 GHz)
• (Braun 1990, VLA 1.4 GHz)

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Previous high-resolution cm-wave radio
observations

• Still need longer (gt200 k?) baselines to resolve
  clump to detect central sources
• e.g. VLA in A or B array at a few GHz for
  SN1885A/S And
• Resolution sub-arcsec
• e.g. 0.4 is 1.5 pc at 780 kpc, i.e. includes
  CND
• VLBI best to resolve SMBH emission structure
  (mas)
• may need highest sensitivity

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Previous high-resolution cm-wave radio
observations

• No radio remnant SN1885A / S And
• Spencer Burke 1973 Crane et al. 1992
• However
• Detection of variable
• nuclear radio source
• (20-50 µJy at 8 GHz)
• Crane et al. 1992, 1993a,b
• Radio dubbed M31
• (cf. Sgr A, also M81, ..)
• Melia 1993 (after Brown 1982)
• Radio flux 1/4 Sgr A at
  8 GHz (but 30-60x more
  massive)
• Crane et al. 1992, Bender et al. 2005

• Crane et al. 1992, 1993a,b
• plus additional VLA archive data

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New 5 GHz VLA data

• Meant to complement 8 GHz VLA archive data from
  Crane et al.
• Sjouwerman Dickel 2000
• M31, other M31 sources, spectral indices, ..
• Total 100 hours VLA (3 projects)
• 2002-2005 VLA A, BnA, B-array
• Latest simultaneous with Chandra X-ray
• Current best image rms 2.7 µJy/bm
• Resolution 0.4, 1.5 pc (cf. Chandra)

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Results 5 GHz image

• Extended and resolved sources
• Point sources
• Variable, or steep-spectrum sources
• Either in M31 or background AGN
• Polarized filaments?

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Radio/X-ray registration

• However not much overlap with X-ray
• Garcia et al. 2000
• M31 in X-ray?
• Two obvious candidates (?)
• X-ray Sgr A ?
• Chandra astrometry 1

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Chandra X-ray image (inner region)
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Radio/X-ray registration

• No direct alignment
• between R/X frames
• Only a couple radio/X-ray
• sources far from the field
• centers
• No statistical source in
• X-ray source positions
• No statistical/systematic offset
• Radio variable -gt find variable X-ray source
• Within 1-2 of radio

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M31? variability in X-ray

• 2004Dec06, 2004Dec27, 2005Jan28,
  2005Feb21

Garcia et al. 2006
Note All nearby X-ray sources are variable!
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Radio emission of M31

• M31, radio point source (VLA, lt0.4, lt1.5 pc)
• VLA A/B-array archive data
• 8 GHz varies from 20-50 µJy (1990-1995)
• 5 GHz varies from 50-100 µJy (2002-2005)
• 1.4 GHz varies from 100-140 µJy (1986/1988)
• No simultaneous radio data on variable source..
• Hard to define an instantaneous spectral index
• Faint hard to measure 2 bands near-simultaneous
• Assume low state is quiescent state

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Radio spectral index M31

• Approximate spectral index in quiescence
  seems synchrotron-like a -1 0.2
• Sgr A slowly rising with radio frequency (a gt 0)
• Non-thermal component?, spectral break?
• Note, resolution 1.5 pc
• VLBI size and structure unknown
• e.g. is there a small jet? talk on NGC4395 by J
  Wrobel
• last years 1 GBps 5 GHz EVN/VLBI observations
  failed
• Seems to have a quiescent and flaring mode
  actually just enough S/N in 2 hours

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Results M31, the BH

• Quiescent (?) at 50 µJy (20 µJy at 8 GHz)

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Results M31, the BH

• Start of next day not detected! (16 hours)

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Results M31, the BH

• Next few hours back to average 50 µJy!

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Results M31, the BH

• M31 variable on time scales of hours !

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M31 radio variability

• Possible reasons for variability
• Calibration, weather, etc.?
• Should be apparent on other sources too
• Interstellar/intergalactic scintillation?
• Should be seen on other sources too
• Chance alignment of M31 with another unrelated
  variable radio source?
• Within 0.4 (1.5 pc) e.g. both P1,P2 BHs?

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M31 radio variability

• Possible reasons for variability
• Eclips?
• Non-detection

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M31 radio variability

• Possible reasons for variability
• Eclips?
• Non-detection
• Non-detection only seen once so far

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M31 radio variability

• Possible reasons for variability
• Eclips?
• Non-detection
• Non-detection only seen once so far
• Intrinsic?
• Flares (high state), seen occasionally (Sgr A)

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M31 radio variability

• Possible reasons for variability
• Eclips?
• Non-detection
• Non-detection only seen once so far
• Intrinsic?
• Flares (high state), seen occasionally (Sgr A)
• Flare flux above quiescent flux

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M31 radio variability

• Did we miss the giant radio flare?

one day
3 weeks!
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M31 radio variability

• Possible reasons for variability
• Eclips?
• Non-detection
• Non-detection only seen once so far
• Intrinsic?
• Flares (high state), seen occasionally (Sgr A)
• Flare flux above quiescent flux
• Quiescent intrinsic, or nearby other source?
• Related to X-ray variability? (recent Chandra)

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Summary

• Very sensitive 5 GHz VLA image of the central r1
  kpc of Andromeda (2.7 µJy/bm)
• M31 unresolved to lt0.4 (lt1.5 pc)
• VLBI, milli-arcsec structure not yet known
• hope to try again
• Synchrotron-like spectrum in quiescence
• M31 varies on time scales of hours
• maybe a bit too weak for current instruments

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Summary

• Very sensitive 5 GHz VLA image of the central r1
  kpc of Andromeda (2.7 µJy/bm)
• M31 unresolved to lt0.4 (lt1.5 pc)
• VLBI, milli-arcsec structure not yet known
• hope to try again
• Synchrotron-like spectrum in quiescence
• M31 varies on time scales of hours
• maybe a bit too weak for current instruments
• Michael Garcia will now take over ..

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Thank you!