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Discovery of powerful
millisecond flares
from Cyg X-1
Marek Gierliński
University of Durham, England
Andrzej Zdziarski
N. Copernicus Astronomical Center, Warsaw, Poland
Cyg X-1 variability
from years to milliseconds
Cyg X-1: a tame source
• Persistent source
• Spectral states: hard and
soft
• Variability nothing like as
dramatic as in transients
• Cyg X-1 is a well-behaved
black hole
Powerful millisecond flares from Cyg X-1
PCA+HEXTE
28 October 2003
4
Seven years of Cyg X-1
ASM
BATSE
2001/2002 soft state
(400 days)
Powerful millisecond flares from Cyg X-1
28 October 2003
5
Spectral transitions – days to weeks
• Typical transition between
states takes about a week
• There are faster events
with timescales of a day
Cui, Feng & Ertmer 2002
Powerful millisecond flares from Cyg X-1
28 October 2003
6
Kilosecond outbursts
20-300 keV
20-300 keV
Stern, Beloborodov & Poutanen 2001
Powerful millisecond flares from Cyg X-1
28 October 2003
7
Power spectrum of Cyg X-1
Not much variability
above 100 Hz
Revnivtsev, Gilfanov & Churazov 2000
Powerful millisecond flares from Cyg X-1
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8
Other black holes too
Sunyaev & Revnivtsev 2000
Powerful millisecond flares from Cyg X-1
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9
Cyg X-1 high-frequency PDS
Revnivtsev, Gilfanov & Churazov 2000
Powerful millisecond flares from Cyg X-1
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10
Very short events
from Cyg X-1
X-ray archaeology: 1973
Rothschild et al. 1974
Powerful millisecond flares from Cyg X-1
28 October 2003
12
Shots
• Flares or ‘shots’ are
weak, each of them 2–3
times brighter than the
continuum
• Superposition of
hundreds of ‘shots’
• Timescales of ~10 and
~100 ms (sum of two
exponentials)
Feng, Li & Chen 1999
Powerful millisecond flares from Cyg X-1
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13
So, you don’t expect sudden
dramatic events from Cyg X-1…
…until this happened
• Enormous flare on 31
July 2002 (during
extended soft state)
• PCA count rate increased
by factor 20 in 100 ms
Powerful millisecond flares from Cyg X-1
20
28 October 2003
PCA 2–60 keV
(4 PCUs)
15
Is it real?
• An external event not
from Cyg X-1?
• Perhaps an energetic
particle hit the PCA
detector?
• Or is it a solar flare?
• Or is it a gamma-ray
burst?
Powerful millisecond flares from Cyg X-1
27 August 1998:
SGR 1900+14 flare
recorded by PCA
looking somewhere else
28 October 2003
16
Multiple detection
• The flare was detected
by four units of PCA
(PCU1 was switched off)
and one HEXTE cluster
Powerful millisecond flares from Cyg X-1
28 October 2003
17
Even BATSE can see it!
PCA
16-12-1996
BATSE
Powerful millisecond flares from Cyg X-1
28 October 2003
18
It really comes from Cyg X-1
• HEXTE (15–150 keV)
contains two detector
clusters switching
between the source and
background 1.5 deg
away
• When cluster B detected
the flare, cluster A
looked at background
and did not see anything
• The flare comes from
Cyg X-1
Powerful millisecond flares from Cyg X-1
source
background
28 October 2003
19
Flare properties
More flares
• We’ve scanned all the
PCA data from Cyg X-1
• Found 13 strong flares
with cpeak- c > 10
• 12 flares in the hard
state, 1 in the soft state
16-12-1996
14-02-1988
31-07-2002
Powerful millisecond flares from Cyg X-1
28 October 2003
21
Flare profile (soft state)
exp(-|t/|)
  20 ms
  0.7
PCA
20HEXTE
Powerful millisecond flares from Cyg X-1
28 October 2003
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Very short precursor
2 ms ~ 40GM/c3
Increase 10
in 2 ms
Powerful millisecond flares from Cyg X-1
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Flare anatomy (soft state)
Flux (3–30 keV)
30 flux increase
Lpeak ~ 1.4 1038 erg/s
 (2–15 keV)
Spectral hardening  ~ 1.8
 (6–60 keV)
Spectral hardening  ~ 1.3
Powerful millisecond flares from Cyg X-1
28 October 2003
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Flare anatomy (hard state)
10 flux increase
Lpeak ~ 0.9 1038 erg/s
Flux (3–30 keV)
Spectral softening  ~ 0.3
 (2–15 keV)
 (6–60 keV)
Powerful millisecond flares from Cyg X-1
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Flare spectrum (soft state)
Flare spectrum
SAX/OSSE
soft state
Model luminosity
Lbol ~ 0.3LEdd
PCA/HEXTE
continuum
around flare
Powerful millisecond flares from Cyg X-1
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Flare spectrum (hard state)
Flare spectrum
Model luminosity
Lbol ~ 0.25LEdd
PCA/HEXTE
continuum
around flare
SAX/OSSE
hard state
Powerful millisecond flares from Cyg X-1
28 October 2003
27
Not only Cyg X-1
XTE J1118+480
Powerful millisecond flares from Cyg X-1
28 October 2003
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Not only Cyg X-1
GX 339–4
Powerful millisecond flares from Cyg X-1
28 October 2003
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Rapid flare in Sgr A*
Shortest timescale ~30GM/c3
Baganoff et al. 2001
Powerful millisecond flares from Cyg X-1
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Conclusions
• We have discovered several strong millisecond
X-ray flares from Cyg X-1 and other black
holes
• Shortest observed timescales correspond to
~40GM/c3 or half of the Keplerian period on
the marginally stable orbit
• The flares must be due to a sudden release of
accretion energy in the inner region of the disc
• Magnetic flares? e.g. Beloborodov 1999; Poutanen &
Fabian 1999; Machida & Matsumoto 2003
• Challenge for theorists!
Powerful millisecond flares from Cyg X-1
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Magnetic flares in plunging region
Machida & Matsumoto 2003
Powerful millisecond flares from Cyg X-1
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