A broad-band study of the 2015 outburst of EXO 1745-248 with INTEGRAL and XMM-Newton
Low Mass X-ray Binaries (LMXBs), binary systems containing a compact object,
are among the brightest and most extreme systems in the Universe. In these
systems a neutron star (1.4-2 M⊙) or black hole
(5-15 M⊙) accretes matter transferred by a low-mass (less
than 1 M⊙) companion star. This matter in-spirals toward the
compact object usually forming an accretion disk in which a large amount of
potential energy is dissipated reaching temperatures of tens to hundreds of
millions of degrees Kelvin and making LMXBs powerful sources in the soft and
hard X-ray band. The low magnetic field of the compact objects allows the disk
to extend to small radii, experiencing strong gravity and reaching high
velocities, thus making these systems ideal laboratories to study the behavior
of the accretion flow in the relativistic regime.
With the aid of the ESA missions XMM-Newton and INTEGRAL, a transient neutron
star LMXB, EXO 1745-248, hosted in the Globular Cluster Terzan 5, has been
studied during an X-ray outburst. The high-quality broad-band spectra provided
by INTEGRAL have helped to constrain the continuum, dominated by a
high-temperature (40 keV) thermal Comptonization, allowing the high energy
resolution, spectroscopic instruments onboard XMM-Newton to unveil a wealth of
narrow and broad emission lines superimposed to the continuum.
Features at energies compatible with K-α transitions of ionized Sulfur,
Argon, Calcium, and Iron were detected, with a broadness compatible with
Doppler broadening in the inner part of an accretion disk truncated at about
40 km from the neutron star center. Strikingly, at least one narrow emission
line ascribed to neutral or mildly ionized Iron is needed to model the
prominent emission complex detected between 5.5 and 7.5 keV. The different
ionization states and broadness suggest an origin in a region located farther
from the neutron star than where the other emission lines are produced.
In the figure the light curve of the 2015 outburst displayed by EXO 1745-248
as observed by IBIS/ISGRI and JEM-X on board INTEGRAL is shown. For completeness,
the light curve obtained from Swift/XRT (and published previously by
Tetarenko, 2016) is also shown. The hard-to-soft spectral state transition of
EXO 1745-248 around 57131 MJD is marked with a dashed vertical line in the plots.
Around this date, the count-rate of the source in the IBIS/ISGRI decreases
significantly, while it continues to increase in JEM-X. The times of the
XMM-Newton observation are also marked by red dashed vertical lines.
Broad-band spectra of the source during the outburst are also shown together
with the best fit model (upper panel), and residuals in units of sigma with
respect to the best fit model (bottom panel). The spectra from different
instruments have been fitted simultaneously. These are XMM-Newton/RGS1 (red),
XMM-Newton/RGS2 (green), XMM-Newton/EPIC-pn (black), INTEGRAL/JEMX1 (blue),
INTEGRAL/JEMX2 (cyan), and INTEGRAL/ISGRI (magenta).
This study has been led by the University of Palermo (Italy) and the INAF -
Astronomical Observatory of Rome (Italy), has been partially performed at the
Institut de Ciéncies de l'Espai (IEEC-CSIC) in Barcelona (Spain), in
collaboration with the ISDC - Data Centre for Astrophysics in Versoix
(Switzerland), the University of Cagliari (Italy), and other European
institutions.
Reference:
"XMM-Newton and INTEGRAL view of the hard state of EXO 1745-248 during its 2015 outburst",
M. Matranga, A. Papitto, T. Di Salvo, E. Bozzo, D. F. Torres, R. Iaria, L. Burderi,
N. Rea, D. de Martino, C. Sanchez-Fernandez, A. F. Gambino, C. Ferrigno, L. Stella, 2017, A&A, 603, A39
