We present optical identification of counterparts of ULX-4 in spiral galaxy M51. Ultraluminous X-ray sources (ULXs) are variable, non-nuclear point-like X-ray sources with isotropic luminosities (Lx > 1039 erg s−1) above the Eddington limit for a 10 Mʘ black hole. The ULX-4 has observed with Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) in filters F438W (B), F555W (V) and F814W (I), their data are available in archive. Two optical sources have been identified as counterpart candidates within the error radius of 0.2 arcsec and their V magnitudes are ∼23. The possible counterparts of ULX-4 were determined as B type supergiant stars, indicating that this system may be a high-mass X-ray binary.

1.
J.
Poutanen
and
G.
Lipunova
and
S.
Fabrika
 et al,
MNRAS
377
, pp.
1187
1194
(
2007
).
2.
E. J. M.
Colbert
and
R. F.
Mushotzky
,
ApJ
519
,
89
107
. (
1999
).
3.
H.
Feng
and
R.
Soria
,
NewAR
55
,
166
183
(
2011
).
4.
M.
Bachetti
and
F.A.
Harrison
and
D.J.
Walton
 et al,
Nature
514
,
202
204
(
2014
).
5.
G. L.
Israel
and
P.
Esposito
and
G.A. Rodríguez
Castillo
 et al,
MNRAS
462
,
4371
4385
(
2016
).
6.
F.
Fürst
and
D.J.
Walton
and
Harrison
F.A.
 et al,
ApJ
831
, L14,
1
6
(
2016
).
7.
C.
Carpano
and
F.
Haberl
and
C.
Maitra
, et al,
MNRAS
476
,
L45
L49
(
2018
).
8.
R.
Soria
and
M.
Cropper
and
M.
Pakull
, et al,
MNRAS
356
,
12
28
(
2005
).
9.
P. K.
Abolmasov
and
D. A.
Swartz
and
S.
Fabrika
 et al,
ApJ
668
,
124
129
(
2007
).
10.
F.
Griśe
and
P.
Kaaret
and
M. W.
Pakull
, et al,
ApJ
734:23
,
1
14
(
2011
).
11.
H.
Avdan
and
S.
Avdan
and
A.
Akyuz
 et al,
ApJ
828:105
,
1
10
(
2016
).
12.
J. J.
Feldmeier
and
J.
Puls
and
A. W. A.
Pauldrach
,
ApJ
479
,
231
243
(
1997
).
13.
Y.
Terashima
and
A. S.
Wilson
,
ApJ
601
,
735
758
(
2004
).
14.
Y.
Terashima
and
H.
Inoue
and
A. S.
Wilson
,
ApJ
645
,
264
270
(
2006
).
15.
R.
Urguhart
and
R.
Soria
and
H.
Johnston
 et al,
MNRAS
475
,
3561
3576
(
2018
).
16.
D.A
Swartz
and
R.
Soria
and
A. F
Tennant
and
M.
Yukita
,
ApJ
741:49
,
1
10
(
2011
).
17.
G.
de Vaucouleurs
,
A.
de Vaucouleurs
,
H.G.
 Jr. 
Corwin
 et al,
Third Reference Catalog of Bright Galaxies
(
New York
:
Springer
) (
1991
).
18.
D.E.
Osterbrock
and
G.J.
Ferland
,
2006
,
Astrophysics of gaseous nebulae and active galactic nuclei
. ed. by
D.E.
Osterbrock
and
G.J.
Ferland
.
Sausalito, CA
(
University Science Books, Sausalito
, 2006).
19.
A. E.
Dolphin
,
Publications of the Astronomical Society of the Pacific
112:776
,
1397
1410
(
2000
).
20.
E. F.
Schlafly
and
D. P.
Finkbeiner
,
ApJ
737:103
,
1
13
(
2011
).
21.
L.
Tao
and
F.
Hua
and
F.
Grise
 et al,
ApJ
737:81
,
15535
15535
(
2011
).
22.
A.
Bressan
and
P.
Marigo
and
L.
Girardi
 et al,
MNRAS
427
,
127
145
(
2012
).
23.
L.H
Aller
and
I.
Appenzeller
and B. Baschek., et al 
Landolt-Bornstein: Numerical Data and Functional Relationships in Science and Technology
Vol.
2
(
New York
:
Springer
) (
1982
).
24.
J.
Aird
and
K.
Nandra
and
E. S.
Laird
 et al,
MNRAS
401
,
2531
2551
(
2010
).
25.
S.
Avdan
and
A.
Vinokurov
and
S.
Fabrika
, et al,
MNRAS
455
,
L91
L95
(
2016
).
26.
H. P.
Earnshaw
and
T. P.
Roberts
and
R.
Sathyaprakash
,
MNRAS
476
,
4272
4277
(
2018
).
This content is only available via PDF.
You do not currently have access to this content.