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Draft

version

February

1,

2008

Preprint

typeset

using

L

A

T

E

X

style

emulateapj

v.

11/26/04

XMM-NEWTON

DISCOVERY

OF

217

S

PULSATIONS

IN

THE

BRIGHTEST

PERSISTENT

SUPERSOFT

X-RAY

SOURCE

IN

M31

Sergey

P.

Trudolyubov

1

and

William

C.

Priedhorsky

2

Draft

version

February

1,

2008

ABSTRACT

We

report

on

the

discovery

of

a

periodic

modulation

in

the

bright

supersoft

X-ray

source

XMMU

J004252.5+411540

detected

in

the

2000-2004

XMM-Newton

observations

of

M31.

The

source

exhibits

X-ray

pulsations

with

a

period

P

∼

217.7

s

and

a

quasi-sinusoidal

pulse

shape

and

pulsed

fraction

∼

7-11%.

We

did

not

detect

statistically

significant

changes

in

the

pulsation

period

on

the

time

scale

of

4

years.

The

X-ray

spectra

of

XMMU

J004252.5+411540

are

extremely

soft

and

can

be

approximated

with

an

absorbed

blackbody

of

temperature

62-77

eV

and

a

weak

power

law

tail

of

photon

index

Γ

∼

1.7-3.1

in

the

0.2-3.0

keV

energy

band.

The

X-ray

properties

of

the

source

and

the

absence

of

an

optical/UV

counterpart

brighter

than

19

mag

suggest

that

it

belongs

to

M31.

The

estimated

bolometric

luminosity

of

the

source

varies

between

∼

2

×

10

38

and

∼

8

×

10

38

ergs

s

−

1

at

760

kpc,

depending

on

the

choice

of

spectral

model.

The

X-ray

pulsations

and

supersoft

spectrum

of

XMMU

J004252.5+411540

imply

that

it

is

almost

certainly

an

accreting

white

dwarf,

steadily

burning

hydrogen-rich

material

on

its

surface.

We

interpret

X-ray

pulsations

as

a

signature

of

the

strong

magnetic

field

of

the

rotating

white

dwarf.

Assuming

that

the

X-ray

source

is

powered

by

disk

accretion,

we

estimate

its

surface

field

strength

to

be

in

the

range

4

×

10

5

G

<

B

0

<

8

×

10

6

G.

XMMU

J004252.5+411540

is

the

second

supersoft

X-ray

source

in

M31

showing

coherent

pulsations,

after

the

transient

supersoft

source

XMMU

J004319.4+411758

with

865.5

s

pulsation

period.

Subject

headings:

galaxies:

individual

(M31)

—

novae,

cataclysmic

variables

—

X-rays:

binaries

—

X-rays:

stars

1.

INTRODUCTION

Luminous

supersoft

X-ray

sources

(SSSs)

(Kahabka

&

van

den

Heuvel

2006,

and

references

therein)

were

first

discovered

in

the

Magellanic

Clouds

with

the

Einstein

observatory

and

later

were

established

as

a

major

new

source

class

based

on

the

results

of

ROSAT

observations.

SSSs

have

very

soft

spectra

typically

described

by

black-

body

models

with

temperatures

of

∼

20

−

80

eV

with

no

strong

hard

component,

and

luminosities

of

∼

10

35

−

10

38

ergs

s

−

1

.

Although

supersoft

X-ray

sources

are

not

a

ho-

mogeneous

class,

the

observed

properties

of

the

major-

ity

of

SSSs

are

consistent

with

those

of

accreting

white

dwarfs

(WD)

in

binary

systems

that

are

steadily

or

cycli-

cally

burning

hydrogen-rich

matter

(van

den

Heuvel

et

al.

1992).

The

required

accretion

rates

in

these

systems

can

be

as

high

as

10

−

7

M

⊙

year

−

1

.

Another

subclass

of

SSSs

are

single

highly

evolved

stars

on

their

way

to

WD

phase.

In

addition,

a

number

of

more

luminous

(

L

X

∼

10

38

−

10

40

ergs

s

−

1

)

X-ray

sources

also

classified

as

SSS

have

been

recently

discovered

in

nearby

galaxies,

with

some

of

them

proposed

as

intermediate

mass

black

hole

(IMBH)

candidates

(Fabbiano

2006,

and

references

therein).

The

nearby,

giant

spiral

M31

presents

an

excellent

opportunity

to

study

various

X-ray

source

populations.

Earlier

observations

of

M31

with

the

ROSAT

satellite

re-

vealed

a

significant

population

of

supersoft

X-ray

sources

(Supper

et

al.

1997,

2001;

Kahabka

1999;

Greiner

2000).

The

advent

of

a

new

generation

of

X-ray

telescopes

(

Chandra

and

XMM-Newton

)

has

allowed

us

to

study

1

Institute

of

Geophysics

and

Planetary

Physics,

University

of

California,

Riverside,

CA

92521

2

Los

Alamos

National

Laboratory,

Los

Alamos,

NM

87545

SSSs

in

a

much

greater

detail

(Osborne

et

al.

2001;

Trudolyubov

et

al.

2001,

2005;

DiStefano

et

al.

2004;

Greiner

et

al.

2004;

Orio

2006).

Most

of

the

SSS

detected

in

M31

appear

to

be

transient/recurrent

or

highly

vari-

able

in

X-rays.

The

observations

with

XMM-Newton

and

Chandra

are

starting

to

provide

valuable

information

on

the

short-term

variability

of

M31

SSSs:

timing

studies

of

M31

X-ray

sources

have

led

to

the

discovery

of

865

s

pul-

sations

in

the

transient

SSS

XMMU

J004319.4+411758

(Osborne

et

al.

2001),

and

significant

short-term

vari-

ations

in

other

SSSs

(Orio

2006;

Trudolyubov,

Pried-

horsky

&

C´

o

rdova

2007).

The

X-ray

source

XMMU

J004252.5+411540

was

dis-

covered

in

the

M31

field

by

the

Einstein

observatory

(source

#69

in

Trinchieri

&

Fabbiano

(1991))

and

de-

tected

in

subsequent

observations

with

ROSAT

(source

#58

in

Primini,

Forman

&

Jones

(1993)),

Chandra

(source

r2-12

in

Kong

et

al.

(2002))

and

XMM-Newton

(source

#352

in

Pietsch

et

al.

(2005)).

The

Chandra

and

XMM-Newton

spectroscopy

of

the

source

revealed

a

thermal

spectrum

with

effective

temperature

of

∼

60

−

70

eV

(DiStefano

et

al.

2004;

Orio

2006;

Trudolyubov,

Priedhorsky

&

C´

o

rdova

2007),

placing

it

in

the

super-

soft

source

class.

Assuming

the

distance

of

760

kpc

(van

den

Bergh

2000),

the

estimated

unabsorbed

luminosity

of

J004252.5+411540

in

the

0.3-1.5

keV

energy

band

can

be

as

high

as

∼

5

×

10

38

ergs

s

−

1

,

making

it

the

brightest

persistent

supersoft

source

in

M31.

In

this

paper,

we

report

on

the

discovery

of

the

coherent

217.7

s

pulsations

in

the

flux

of

XMMU

J004252.5+411540,

using

the

archival

data

of

XMM-

Newton

observations.

We

also

study

X-ray

spectral

prop-

erties

of

the

source,

search

for

its

optical/UV

counter-

parts

and

discuss

its

nature.