236 research outputs found
XMM-Newton observations of five INTEGRAL sources located towards the Scutum Arm
Results are presented for XMM-Newton observations of five hard X-ray sources
discovered by INTEGRAL in the direction of the Scutum Arm. Each source received
more than 20 ks of effective exposure time. We provide refined X-ray positions
for all five targets enabling us to pinpoint the most likely counterpart in
optical/infrared archives. Spectral and timing information (much of which are
provided for the first time) allow us to give a firm classification for IGR
J18462-0223 and to offer tentative classifications for the others. For IGR
J18462-0223, we discovered a coherent pulsation period of 997+-1 s which we
attribute to the spin of a neutron star in a highly-obscured (nH = 2e23 /cm2)
high-mass X-ray binary (HMXB). This makes IGR J18462-0223 the seventh
supergiant fast X-ray transient (SFXT) candidate with a confirmed pulsation
period. IGR J18457+0244 is a highly-absorbed (nH = 8e23 /cm2) source in which
the possible detection of an iron line suggests an active galactic nucleus
(AGN) of type Sey-2 situated at z = 0.07(1). A periodic signal at 4.4 ks could
be a quasi-periodic oscillation which would make IGR J18457+0244 one of a
handful of AGN in which such features have been claimed, but a slowly-rotating
neutron star in an HMXB can not be ruled out. IGR J18482+0049 represents a new
obscured HMXB candidate with nH = 4e23 /cm2. We tentatively propose that IGR
J18532+0416 is either an AGN or a pulsar in an HMXB system. The X-ray spectral
properties of IGR J18538-0102 are consistent with the AGN classification that
has been proposed for this source.Comment: 15 pages, 9 figures, 4 tables: accepted for publication in Ap
Broadband Suzaku observations of IGR J16207-5129
An analysis of IGR J16207-5129 is presented based on observations taken with
Suzaku. The data set represents ~80 ks of effective exposure time in a broad
energy range between 0.5 and 60 keV, including unprecedented spectral
sensitivity above 15 keV. The average source spectrum is well described by an
absorbed power law in which we measured a large intrinsic absorption of nH =
16.2(-1.1/+0.9)x10^22 /cm2. This confirms that IGR J16207-5129 belongs to the
class of absorbed HMXBs. We were able to constrain the cutoff energy at
19(-4/+8) keV which argues in favor of a neutron star as the primary. Our
observation includes an epoch in which the source count rate is compatible with
no flux suggesting a possible eclipse. We discuss the nature of this source in
light of these and of other recent results.Comment: 12 pages, 6 figures, accepted for publication in Ap
The nature of the X-ray binary IGR J19294+1816 from INTEGRAL, RXTE, and Swift observations
We report the results of a high-energy multi-instrumental campaign with
INTEGRAL, RXTE, and Swift of the recently discovered INTEGRAL source IGR
J19294+1816. The Swift/XRT data allow us to refine the position of the source
to RA= 19h 29m 55.9s Dec=+18deg 18' 38.4" (+- 3.5"), which in turn permits us
to identify a candidate infrared counterpart. The Swift and RXTE spectra are
well fitted with absorbed power laws with hard (Gamma ~ 1) photon indices.
During the longest Swift observation, we obtained evidence of absorption in
true excess to the Galactic value, which may indicate some intrinsic absorption
in this source. We detected a strong (P=40%) pulsation at 12.43781 (+-0.00003)
s that we interpret as the spin period of a pulsar. All these results, coupled
with the possible 117 day orbital period, point to IGR J19294+1816 being an
HMXB with a Be companion star. However, while the long-term INTEGRAL/IBIS/ISGRI
18--40 keV light curve shows that the source spends most of its time in an
undetectable state, we detect occurrences of short (~2000-3000 s) and intense
flares that are more typical of supergiant fast X-ray transients. We therefore
cannot make firm conclusions on the type of system, and we discuss the possible
implications of IGR J19294+1816 being an SFXT.Comment: 7 pages, 6 figures, accepted for publication in A&
A Suzaku X-ray observation of one orbit of the supergiant fast X-ray transient IGR J16479-4514
We report on a 250 ks long X-ray observation of the supergiant fast X-ray
transient (SFXT) IGR J16479-4514 performed with Suzaku in 2012 February. About
80% of the short orbital period (Porb=3.32 days) was covered as continuously as
possible for the first time. The source light curve displays variability of
more than two orders of magnitude, starting with a very low emission state
lasting the first 46 ks (1E-13 erg/cm2/s, 1-10 keV), consistent with being due
to the X-ray eclipse by the supergiant companion. The transition to the
uneclipsed X-ray emission is energy dependent. Outside the eclipse, the source
spends most of the time at a level of (6-7)x10^-12 erg/cm2/s punctuated by two
structured faint flares with a duration of about 10 and 15 ks. Remarkably, the
first faint flare occurs at a similar orbital phase of the bright flares
previously observed in the system. This indicates the presence of a
phase-locked large scale structure in the supergiant wind, driving a higher
accretion rate onto the compact object. The scattered component visible during
the X-ray eclipse allowed us to directly probe the wind density at the orbital
separation, resulting in rho=7E-14 g/cm3. Assuming a spherical geometry for the
supergiant wind, the derived wind density translates into a ratio
Mdot_w/v_terminal = 7E-17 solar masses/km which, assuming terminal velocities
in a large range 500-3000 km/s, implies an accretion luminosity two orders of
magnitude higher than that observed. As a consequence, a mechanism is at work
reducing the mass accretion rate. Different possibilities are discussed.Comment: Accepted for publication in MNRAS. 10 pages, 5 figure
Investigating the Optical Counterpart Candidates of Four INTEGRAL Sources localized with Chandra
We report on the optical spectroscopic follow up observations of the
candidate counterparts to four INTEGRAL sources: IGR J04069+5042, IGR
J06552-1146, IGR J21188+4901 and IGR J22014+6034. The candidate counterparts
were determined with Chandra, and the optical observations were performed with
1.5-m RTT-150 telescope (T\"{U}B\.{I}TAK National Observatory, Antalya, Turkey)
and 2.4-m Hiltner Telescope (MDM Observatory, Kitt Peak, Arizona). Our
spectroscopic results show that one of the two candidates of IGR J04069+5042
and the one observed for IGR J06552-1146 could be active late-type stars in RS
CVn systems. However, according to the likelihood analysis based on Chandra and
INTEGRAL, two optically weaker sources in the INTEGRAL error circle of IGR
J06552-1146 have higher probabilities to be the actual counterpart. The
candidate counterparts of IGR J21188+4901 are classified as an active M-type
star and a late-type star. Among the optical spectra of four candidates of IGR
J22014+6034, two show H\alpha emission lines, one is a late-type star and the
other is a M type. The likelihood analysis favors a candidate with no
distinguishing features in the optical spectrum. Two of the candidates
classified as M type dwarfs are similar to some IGR candidates claimed to be
symbiotic stars. However, some of the prominent features of symbiotic systems
are missing in our spectra, and their NIR colors are not consistent with those
expected for giants. We consider the IR colors of all IGR candidates claimed to
be symbiotic systems and find that low resolution optical spectrum may not be
enough for conclusive identification.Comment: 24 pages, 12 figures; accepted for publication in Ap
Chandra Observations of Eight Sources Discovered by INTEGRAL
We report on 0.3-10 keV observations with the Chandra X-ray Observatory of
eight hard X-ray sources discovered within 8 degrees of the Galactic plane by
the INTEGRAL satellite. The short (5 ks) Chandra observations of the IGR source
fields have yielded very likely identifications of X-ray counterparts for three
of the IGR sources: IGR J14091-6108, IGR J18088-2741, and IGR J18381-0924. The
first two have very hard spectra in the Chandra band that can be described by a
power-law with photon indices of Gamma = 0.6+/-0.4 and -0.7(+0.4)(-0.3),
respectively (90% confidence errors are given), and both have a unique near-IR
counterpart consistent with the Chandra position. IGR J14091-6108 also displays
a strong iron line and a relatively low X-ray luminosity, and we argue that the
most likely source type is a Cataclysmic Variable (CV), although we do not
completely rule out the possibility of a High Mass X-ray Binary. IGR
J18088-2741 has an optical counterpart with a previously measured 6.84 hr
periodicity, which may be the binary orbital period. We also detect five cycles
of a possible 800-950 s period in the Chandra light curve, which may be the
compact object spin period. We suggest that IGR J18088-2741 is also most likely
a CV. For IGR J18381-0924, the spectrum is intrinsically softer with Gamma =
1.5(+0.5)(-0.4), and it is moderately absorbed, nH = (4+/-1)e22 cm-2. There are
two near-IR sources consistent with the Chandra position, and they are both
classified as galaxies, making it likely that IGR J18381-0924 is an Active
Galactic Nucleus (AGN). For the other five IGR sources, we provide lists of
nearby Chandra sources, which may be used along with further observations to
identify the correct counterparts, and we discuss the implications of the low
inferred Chandra count rates for these five sources.Comment: Accepted by ApJ, 14 page
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