# Presentation: 366547

## 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

## Outline

- Introduction to radio observations
- New 5 GHz Very Large Array data
- Radio results:
- M31*, and it’s X-ray counterpart ?
- Radio “spectral index” of M31*
- Variability of M31* at 5 GHz
- Summary
-    X-rays by M.Garcia

## 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

## The center of Andromeda

- For reference:    early radio image
- Central extended diffuse emission
- Long baselines (in terms of λ’s)
- Inner r~4’ (1 kpc)
- Pooley 1969

## 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)

## Previous high-resolution cm-wave radio observations

- Still need longer (>200 kλ) 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

## 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

## 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)

## Results: 5 GHz image

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

- 1’

## 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”

- Chandra X-ray image (inner region)

- 1’

## 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 -> find variable X-ray source
- Within ~1-2” of radio

## M31*? variability in X-ray

- 2004Dec06,   2004Dec27,    2005Jan28,    2005Feb21

- Note: All nearby X-ray sources are variable!

- Garcia et al. 2006

## Radio emission of M31*

- M31*, radio point source (VLA, <0.4”, <1.5 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”

## Radio spectral index M31*

- Approximate “spectral index” in “quiescence” seems synchrotron-like : α ≈ -1 ±0.2
- Sgr A* slowly rising with radio frequency (α > 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 year’s 1 GBps 5 GHz EVN/VLBI observations failed
- Seems to have a quiescent and flaring mode; actually just enough S/N in 2 hours...

## Results: M31*, the BH

- Quiescent (?) at ~50 μJy   (20 μJy at 8 GHz)

## Results: M31*, the BH

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

## Results: M31*, the BH

- Next few hours back to average ~50 μJy!

## Results: M31*, the BH

- M31* variable on time scales of hours !

## 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?

## M31* radio variability

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

## M31* radio variability

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

## 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*)

## 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

## M31* radio variability

- Did we miss the giant radio flare......?

- one day

- 3 weeks!

## 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)

## Summary

- Very sensitive 5 GHz VLA image of the central r~1 kpc of Andromeda (2.7 μJy/bm)
- M31* unresolved to <0.4” (<1.5 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

## Summary

- Very sensitive 5 GHz VLA image of the central r~1 kpc of Andromeda (2.7 μJy/bm)
- M31* unresolved to <0.4” (<1.5 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 ..

## Thank you!