53rd Summer School on Astronomy and Astrophysics＠Tokyo University(8/1̶4)

Multi-wavelength observationsof X-ray binaries & cataclysmic
variables

Presenter: Mariko Kimura (Kanazawa University, Japan)1

Todaysʼ topics• Introduction• Transients & accretion disks• Outbursts of X-ray binaries and cataclysmic variables• Unsolved problems• Importance of multi-wavelength observations• Our recent works• Repetitive optical & X-ray variability in the black-hole binary V404 Cyg• Standstill-like phenomenon in the white-dwarf binary SS Cyg• Future observations• Introduction of my career and life3

Introduction

4

Accretion disks in our universe
Time-domain astronomy
Study of transient objects

Binary systems

Easy to observe !

Gamma-ray bursts(GRBs)(c) ESA/ATG medialab

Active galactic
nuclei (AGNs)(c) NASA/ESA(c) NASA/JPL

Accretion disks around compact objects (Black holes, white dwarfs, neutron stars)
Central engine of transient events5

Outbursts in binary systems(Dwarf novae)• Easy to be observed (bright, many objects)• Outbursts = sudden brightening of the disk
• Multi-wavelength emission

X-rays

Optical(Low-mass X-ray binaries)

0

50

100

150

200

250

300

Days
6

(Kimura & Done 2019)

Mechanism of outbursts
Thermal-viscous instability triggered by partial ionization of hydrogen
Thermal equilibrium curveṀacc (Te↵ )Mass accretion
rate

StableṀcritUnstable

Mass accretion(Outbursts)

Hot state(Ionized)

Disk

Unstable(Partiallyionized)Mass accumulation(quiescence)

Cool state(neutral)

StableDisk

Surface density

Disk instability model• limit-cycle instability propagates over the entire disk
• Constant mass transfer rates from the secondary(Hoshi 1979; Meyer & Meyer-Hofmeister 1981; Mineshige & Osaki 1985; Osaki 1996;
Meyer & Meyer-Hofmeister 1983)
7

Unsolved problems
Big data

Discovery of peculiar outbursts that cannot be explained by the simple model

Towards the unified model for rich variety of outbursts
Rebrightening
Optical magnitude

Violent & short-term variability

Time [days]

Low-amplitude & infrequent outbursts

Magnitude

Alternation between standstill andoutbursts

Standstill
BJD—2451000
8

Importance of multi-wavelength observations
X-ray and optical observations were notsimultaneously performed. However, ..
A local phenomenon aﬀects theentire accretion physics.
Variations of temperature
at the outer disk, winds
X-ray irradiation

State transitionat the inner disk

Transfer of mass &angular momentum

Multi-wavelength observations
uncover the entire accretion
physics.

9

Study 1: Repetitive optical &X-ray variability in the blackhole binary V404 Cyg

10What is V404 Cygni ?
• One of black-hole low-mass X-ray(Tanaka & Shibazaki, 1996)

binaries• ~9 solar mass black hole
• Located near the Earth: 7,800 light years• Violent X-ray variations in outburstlog(X-ray flux)log(X-ray flux)↓ Light Curves in the 1989 outbursts(Zycki et al., 1999, MNRAS)

V404 Cyg

Days
Days
11

The 2015 summer outburst in V404 Cyg
Black Hole

X-rays(Kimura et al. 2016)(c) Eiri Ono/Kyoto Univ.

Magnitude (Optical)

OpticalCount Rates (X-rays)↑ Including many amateur observers

days
12

↑ less than 3 minutes after the first detection !

Magnitude (Op0cal)

Violent optical variability with repetitive patterns

Count Rates (X-rays)

13

days

Correlation with X-ray variability
a 100
IC

is
m
e
n
s
t
o
r
e
ot m jr
ch fro
n
Sy

0.1

10−3
10−4

~1 min delay of optical variability against
X-ray variability (Gandhi et al. 2016)

10−5

X−ray
(Swift)

is on

1

0.01

Time [days]

VU

keV2 (Photons cm−2 s−1 keV−1)

keV2 (Photons cm−2 s−1 keV−1)

Optical10

X-ray count rate

Optical magnitude

X-raysb10Blackbodyemission fromthe secondary

radio

1

1

(RATAN−600)

10−8

10−7

10−6

10−5

10−4 10−3
Energy (keV)

0.01

0.1

1

10

Synchrotron emission is not enough ..

We can see the violent activity
in the vicinity of the black holeat optical wavelengths !

BH

14

1

Violent variability at low luminosity
101

Eddington luminosity

Lbol (1.35 × 1039 ergs s)

100

10−1

Black: Swift/BAT (15—50 keV)
Red: INTEGRAL/ISGRI(25—60 keV)

10−2

10−3

10−4

0

2

4

6

8

10

BJD−2,457,189

12

Violent optical variability occurred
at 0.001—0.1 LEdd.
15

Violent variability at low luminosity
Optical variability in V404 Cyg

X-ray variations in GRS 1915+105

30 minThermal equilibrium curve

Radiation-pressure instability does notcause this kind of variability in V404 Cyg..

slim disk

Temperature (T)×16(Fender & Belloni 2004)

standard disk
radia0on pressure
dominated
standard disk
gas pressure
dominatedΣmax
ΣminSurface density

Short summary• V404 Cyg entered outbursts in 2015 after 26 yrs dormancy.
• We found large-amplitude & short-term optical variations with repetitive
patterns for the first time.

• The optical variations were well-correlated with the simultaneous X-ray variations.
• X-ray irradiation was dominant in optical emission.
• Violent variability occurred at low luminosity -> Radiation-pressure instability is
unlikely as the mechanism of these variations
Another kind of instability
for long-period systems ?

Object name

Orbital period

V404 Cyg

6.5 d

GRS 1915+105

33.9 dIGR J17091-3624>4d

V4641 Sgr

2.8 d

Kimura et al. (2016, Nature, 54, 529): ``Repetitive patterns in rapid optical variations in the nearby
black-hole binary V404 Cygni’’17Study 2: Standstill-likephenomenon in the whitedwarf binary SS Cyg

18

Classiﬁcation of dwarf novae
Thermal equilibrium curveṀacc (Te↵ )SS Cyg-type stars

Outburst(hot state)Ṁcrit<latexit sha1_base64="oe6WPoGfnNdpXzj4DAXf305PGP0=">AAACinicSyrIySwuMTC4ycjEzMLKxs7BycXNw8vHLyAoFFacX1qUnBqanJ+TXxSRlFicmpOZlxpaklmSkxpRUJSamJuUkxqelO0Mkg8vSy0qzszPCympLEiNzU1Mz8tMy0xOLAEKxQuoxqTkl1T71sZXxxTlKpQU1SrYKKAIJRdlltTGCygb6BmAgQImwxDKUGaAgoB8geUMMQwpDPkMyQylDLkMqQx5DCVAdg5DIkMxEEYzGDIYMBQAxWIZqoFiRUBWJlg+laGWgQuotxSoKhWoIhEomg0k04G8aKhoHpAPMrMYrDsZaEsOEBcBdSowqBpcNVhp8NnghMFqg5cGf3CaVQ02A+SWSiCdBNGbWhDP3yUR/J2grlwgXcKQgdCF180lDGkMFmC3ZgLdXgAWAfkiGaK/rGr652CrINVqNYNFBq+B7l9ocNPgMNAHeWVfkpcGpgbNZuACRoAhenBjMsKM9AzN9AwCTZQdnKBRwcEgzaDEoAEMb3MGBwYPhgCGUKC93QwbGXYx7GbiYTJismSyhihlYoTqEWZAAUwuAEM6mGc=</latexit>

Ṁtr < Ṁcrit
Quiescence(cool state)<latexit sha1_base64="oe6WPoGfnNdpXzj4DAXf305PGP0=">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</latexit>

Ṁtr < Ṁcrit• Thermally unstable disk
• Repeat outbursts(Hoshi 1979; Meyer & Meyer-Hofmeister 1981; Mineshige & Osaki 1985; Osaki 1996;
Meyer & Meyer-Hofmeister 1983)
19

Classiﬁcation of dwarf novae
Thermal equilibrium curve

Nova-like stars (NLs)

Ṁacc (Te↵ )<latexit sha1_base64="oe6WPoGfnNdpXzj4DAXf305PGP0=">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</latexit>

Ṁtr < ṀcritṀcrit<latexit sha1_base64="Nfvsrnxki2U5K1S1iCEyvR4Pww8=">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</latexit>

Ṁtr > Ṁcrit• Thermally stable disk
• Constant luminosity(Hoshi 1979; Meyer & Meyer-Hofmeister 1981; Mineshige & Osaki 1985; Osaki 1996;
Meyer & Meyer-Hofmeister 1983)20

Classiﬁcation of dwarf novae
Thermal equilibrium curve<latexit sha1_base64="oe6WPoGfnNdpXzj4DAXf305PGP0=">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</latexit>ṀcritṀtr <
ṀcritMagnitudeṀacc (Te↵ )Z Cam-type stars

Standstill
BJD—2451000

Ṁtr ⇠ Ṁcrit• Intermediate type between NLs and SS Cyg
stars• Fluctuations of mass transfer rates ?
The standstill is not explained by the simple disk instability model withconstant mass transfer rates.
The unified model for outbursts is not completed ..
(Hoshi 1979; Meyer & Meyer-Hofmeister 1981; Mineshige & Osaki 1985; Osaki 1996;
Meyer & Meyer-Hofmeister 1983)
21

SS Cyg: the brightest dwarf nova
• Monitored at optical

wavelengths for >100 years• Monitored at X-rays for >20
years• Repeat dwarf-nova outbursts
with intervals of ~1 month• Recognized as the prototypeof SS Cyg-type dwarf novae(Historical light curve of AAVSO)

22

The 2021 anomalous event & its precursor• SS Cyg is no longer the prototype dwarf nova ..

Standstill-like phenomenon ?

• First event in the long history of observations (the 2021 anomalous event)
What causes standstill in dwarf novae ?

23

The 2021 anomalous event & its precursor
Precursor: gradual brightening at optical & X-ray wavelengths
OpticalAnomalous event

X-raysOur campaigns Optical: Amateur groups (VSNET, AAVSO), Tomo-e Gozen at
Kiso Observatory
X-rays: NICER, NuSTAR, MAXI, Swift24

Possible scenarios
How did the increase in the optical & X-ray flux in quiescence occur ?
1. Increase of mass-transfer rates from the secondary star
No positive evidence
2. Strong X-ray irradiation of the disk and/or the secondary star
3. The accretion rate in the disk increases for some reasons25

Strong X-ray irradiation ?
Reflection

Background• Practically, the X-ray spectra of DNe have been
fitted by multi-temperature plasma models.

• We have to consider the reflection by the WD
and/or the disk.

Our model

Accretion disk

WD
X-ray emitting
regionTo express Fe fluorescence line

• Tbabs * (reflect * cevmkl + gaussian)Bremsstrahlung emissivity(Done & Osborne 1997; Ishida et al. 2009; Nakaniwa et al. 2019)26

Strong X-ray irradiation ?
• Green: NICER (0.5–8 keV), Purple: NuSTAR (3.0–50 keV), Black: modelPrecursor• T_max ~ 25–30 keV• solid angle ~ 0.2—0.3• Flux increases by ~2—5 times• Lx ~ 1033 erg / s

Anomalous event

The X-ray irradiation of the disk andthe secondary star is ineﬃcient27

Geometry of X-ray emitting plasma
Normal quiescence

Precursor

Anomalous
event

Compact
Vertically extended• The solid angle of the reflector

Higher temperature → larger pressure→ larger scale height• T_max ~ 20 keV• The solid angle of the reflector

was >1

was 0.2—0.3(Ishida et al. 2009)• T_max ~ 25–30 keV
28

Possible scenarios
How did the increase in the optical & X-ray flux in quiescence occur ?
1. Increase of mass-transfer rates from the secondary star
No positive evidence
2. Strong X-ray irradiation of the disk and/or the secondary starWeak
3. The accretion rate in the disk increases for some reasons

29Bright inner disk during quiescence
Anomalous event
Precursor

Hot

Normalquiescence

Anomalous
event &
precursor

Cool<latexit sha1_base64="I3jnU6bFdJKi175/DD2eZJwiEo0=">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</latexit><latexit sha1_base64="jSBUZzGQGZuM0AqGV0oQPQf68vQ=">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</latexit>Ṁin / ↵cool ⌃TGM ṀinLX ⇠2rin

We can explain the increase of both of optical & X-ray flux.
Temporal enhancement of viscosity in the cool state ?
30

Eﬀect of enhanced viscosity• Enhancement of viscosity in the cool state
Smaller limit cycle

Increase of the flux level in quiescence
Frequent small and inside-out outbursts

Thermal equilibrium curveṀacc (Te↵ )Ṁcrit↑ Optical light curve in the precursor

Bigger α

The transition wave does not easily propagate
over the disk.
31

Nature of the 2021 anomalous event• If the viscosity in the cool state is extremely enhanced ..
Light curves just oscillate and show no clear outbursts

Standstill-like
The 2021 anomalous
phenomenon
event ?

Hot
Cool

A series of very small outbursts(Mineshige & Osaki 1985)

Kimura et al. (2021, PASJ, 73, 1262): ``On the nature of the anomalous event in 2021 in the dwarf nova
SS Cygni and its multi-wavelength transition’’32

High-speed observations (optical & X-ray)
Simultaneous observations with
Tomo-e & NICER

Tomo-e Gozen (2019〜)• 400—700 nm• Wide-field video survey
by CMOS cameras

NICER (2018〜)

Suitable for observationsof transient events↑ Provided by Nishino-kun at Tokyo Univ.

• 0.3—10.0 keV• High time resolution

33

Highly-correlated optical & X-ray variations

Past• Weak correlation (UV & X-ray)• X-ray delay (~100—200 sec)

In 2020• Strong correlation• Optical delay (~1 sec)

34

Enhanced X-ray irradiation ?
X-ray emitting plasma expanded

X-ray irradiation was enhanced

Optical delay against X-rays

Nishino, Kimura, Sako et al. (2022, PASJ, 73, L17):``Detection of highly correlated optical and X-ray
variations in SS Cygni with Tomo-e Gozen and NICER’’

X-ray emitting plasma(c) Kiso observatory

35

Short summary• SS Cyg showed an anomalous event like standstill in early 2021.
• We performed X-ray and optical monitoring of this event and its forerunner.
• Our analyses suggest that X-ray irradiation was weak, and that the viscosity inthe disk may be enhanced during quiescence.
→ applicable for Z Cam-type standstill ?

• The 2021 anomalous event is a group of very small outbursts ?
• The X-ray emitting region was hotter and expanded.
→ X-ray irradiation became stronger.
→ Strong correlation between optical & X-ray variations

36

Future observations

37

Future observations by upcoming telescopes
X-rays

OpticalNICER＠ISS (2018~)

Tomo-e Gozen（2019~）

Multi-wavelength & high-speed
photometry, X-ray detailed
spectroscopy

Time [sec]

XRISM (2023~)
TriCCS＠Seimei（2021~）

Plasma geometry under thestrong gravitational field38

Future observations of SS Cyg• Coordinated observations by XRISM, NuSTAR, and Tomo-e Gozen of SS Cyg (priority A
target during the XRISM PV phase)

XRISM (2023~)

Tomo-e Gozen (2019~)• Return to the normal state ?
• Consistency between X-ray spectra and high-speed multiwavelength observations• Density & temperature profiles of X-ray plasma
NuSTAR

39

Future observations (KOYOH・HiZ-GUNDAM)
KOYOH (micro satellite)
KOYOH

HiZ-GUNDAM
Wide-field X-ray
monitorGamma-raydetector

Wide-field X-ray monitor and optical/IR telescopes
Discovery of many kinds of transients (GRBs
synchronized with GW, outbursts of CVs and
XBs, magnetar flares, ..)Discovery of high-redshift GRBs andinvestigate early universe

Multi-wavelength follow-up observations
Optical

X-ray

Approach to important problems byusing small telescopes !
Radio
40

Thank you for your attention.

44