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[en] We present a multi-object optical spectroscopy follow-up study of X-ray sources in a field along the Galactic plane (l = 327.042, b = 2.026) which is part of the Chandra Multi-wavelength Plane survey (ChaMPlane). We obtained spectra for 46 stars, including 15 likely counterparts to X-ray sources, and sources showing an Hα color excess. This has led to the identification of a new cataclysmic variable (CV), CXOPS J154305.5-522709, also named ChaMPlane Bright Source 7 (CBS 7), and we identified eight X-ray sources in the field as active late-type stars. CBS 7 was previously studied in X-rays and showed a hard spectrum and two periods: 1.22 ± 0.08 hr and 2.43 ± 0.26 hr. We present here clear evidence that the source is a CV through the detection of H, He I, and He II emission lines in its optical spectrum. The hard X-ray spectrum and the presence of the He II λ4686 in emission with a large equivalent width suggest a magnetic CV. The near-infrared counterpart is significantly variable, and we found a period consistent with the longest X-ray period at 2.39 ± 0.05 hr but not the shortest X-ray period. If this period is the orbital period, this would place the system in the CV period gap. The possible orbital period suggests a dM4 ± 1 companion star. The distance is then estimated to be ∼1 kpc. The system could be a relatively hard and X-ray luminous polar or an intermediate polar, possibly nearly synchronous.
[en] The mass and radius of the neutron star (NS) in low-mass X-ray binaries can be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass and radius constrains the properties of dense matter in NS cores. A critical ingredient for spectral fits is the composition of the NS atmosphere: hydrogen atmospheres are assumed in most prior work, but helium atmospheres are possible if the donor star is a helium white dwarf. Here we perform spectral fits to XMM-Newton, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13. This NS has the smallest inferred radius from previous spectral fitting. Assuming an atmosphere composed of hydrogen, we find a significantly larger radius, more consistent with those from other quiescent NSs. With a helium atmosphere (an equally acceptable fit), we find even larger values for the radius.
[en] This school, dedicated to young researchers, will clarify our present knowledge of the X-ray sky and give the opportunity to learn about the observatories and tools which are available. The contributions have been organized into 3 issues: -) fundamental physics, -) X-ray and Gamma-ray instruments and analysis techniques, and -) astrophysical objects. This document gathers only the slides of the presentations
[en] The brightest ultra-luminous X-ray source HLX-1 in the galaxy ESO 243-49 provides strong evidence for the existence of intermediate-mass black holes (IMBHs). As the luminosity and thus the mass estimate depend on the association of HLX-1 with ESO 243-49, it is essential to confirm its affiliation. This requires follow-up investigations at wavelengths other than X-rays, which in turn needs an improved source position. To further reinforce the IMBH identification, it is necessary to determine HLX-1's environment to establish whether it could potentially form and nourish a black hole at the observed luminosities. Using the High Resolution Camera on board Chandra, we determine a source position of R.A. = 01h10m28.s3 and decl. = -46004'22.''3. A conservative 95% error of 0.''3 was found following a boresight correction by cross-matching the positions of three X-ray sources in the field with the Two Micron All Sky Survey catalog. Combining all Swift UV/Optical Telescope uvw2 images, we failed to detect a UV source at the Chandra position down to a 3σ limiting magnitude of 20.25 mag. However, there is evidence that the UV emission is elongated in the direction of HLX-1. This is supported by archival data from GALEX and suggests that the far-UV emission is stronger than the near-UV. This could imply that HLX-1 may be situated near the edge of a star-forming region. Using the latest X-ray observations, we deduce the mass accretion rate of a 500 M sun black hole with the observed luminosity and show that this is compatible with such an environment.
[en] In this Letter, we report a spectroscopic confirmation of the association of HLX-1, the brightest ultra-luminous X-ray (ULX) source, with the galaxy ESO 243-49. At the host galaxy distance of 95 Mpc, the maximum observed 0.2-10 keV luminosity is 1.2 x 1042 erg s-1. This luminosity is ∼400 times above the Eddington limit for a 20 M sun black hole and has been interpreted as implying an accreting intermediate-mass black hole with a mass in excess of 500 M sun (assuming that the luminosity is a factor of 10 above the Eddington value). However, a number of other ULX sources have been later identified as background active galaxies or foreground sources. It has recently been claimed that HLX-1 could be a quiescent neutron star X-ray binary at a Galactic distance of only 2.5 kpc, so a definitive association with the host galaxy is crucial in order to confirm the nature of the object. Here, we report the detection of the Hα emission line for the recently identified optical counterpart at a redshift consistent with that of ESO 243-49. This finding definitively places HLX-1 inside ESO 243-49, confirming the extreme maximum luminosity and strengthening the case for it containing an accreting intermediate-mass black hole of more than 500 M sun.
[en] Dwarf novae (DNe) in globular clusters (GCs) seem to be rare with only 13 detections in the 157 known Galactic GCs. We report the identification of a new DN in M13, the 14th DN identified in a GC to date. Using the 2 m Faulkes Telescope North, we conducted a search for stars in M13 that show variability over a year (2005-2006) on timescales of days and months. This led to the detection of one DN showing several outbursts. A Chandra X-ray source is coincident with this DN and shows both a spectrum and variability consistent with that expected from a DN, thus supporting the identification. We searched for a counterpart in Hubble Space Telescope Advanced Camera for Surveys/Wide Field Camera archived images and found at least 11 candidates, of which we could characterize only the 7 brightest, including one with a 3σ Hα excess and a faint blue star. The detection of one DN when more could have been expected likely indicates that our knowledge of the global Galactic population of cataclysmic variables is too limited. The proportion of DNe may be lower than found in catalogs, or they may have a much smaller mean duty cycle (∼1%) as proposed by some population synthesis models and recent observations in the field.
[en] We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large Telescope, V-, and R-band) observations of the intermediate-mass black hole candidate HLX-1 before and during the 2012 outburst. We show that the V-band magnitudes vary with time, thus proving that a portion of the observed emission originates in the accretion disk. Using the first quiescent optical observations of HLX-1, we show that the stellar population surrounding HLX-1 is fainter than V ∼ 25.1 and R ∼ 24.2. We show that the optical emission may increase before the X-ray emission consistent with the scenario proposed by Lasota et al. in which the regular outbursts could be related to the passage at periastron of a star circling the intermediate-mass black hole in an eccentric orbit, which triggers mass transfer into a quasi-permanent accretion disk around the black hole. Further, if there is indeed a delay in the X-ray emission we estimate the mass-transfer delivery radius to be ∼1011 cm
[en] We aim at characterizing the structure of the gas and dust around the high-mass X-ray binary GX 301-2, a highly obscured X-ray binary hosting a hypergiant (HG) star and a neutron star, in order to better constrain its evolution. We used Herschel PACS to observe GX 301-2 in the far infrared and completed the spectral energy distribution of the source using published data or catalogs from the optical to the radio range (0.4 to 4 × 104 μm). GX 301-2 is detected for the first time at 70 and 100 μm. We fitted different models of circumstellar (CS) environments to the data. All tested models are statistically acceptable, and consistent with an HG star at ∼3 kpc. We found that the addition of a free-free emission component from the strong stellar wind is required and could dominate the far-infrared flux. Through comparisons with similar systems and discussion on the estimated model parameters, we favor a disk-like CS environment of ∼8 AU that would enshroud the binary system. The temperature goes down to ∼200 K at the edge of the disk, allowing for dust formation. This disk is probably a rimmed viscous disk with an inner rim at the temperature of the dust sublimation temperature (∼1500 K). The similarities between the HG GX 301-2, B[e] supergiants, and the highly obscured X-ray binaries (particularly IGR J16318-4848) are strengthened. GX 301-2 might represent a transition stage in the evolution of massive stars in binary systems, connecting supergiant B[e] systems to luminous blue variables
[en] The hyperluminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the best intermediate-mass black hole (BH) candidate, displays spectral transitions similar to those observed in Galactic BH binaries, but with a luminosity 100-1000 times higher. We investigated the X-ray properties of this unique source by fitting multi-epoch data collected by Swift, XMM-Newton, and Chandra with a disk model computing spectra for a wide range of sub- and super-Eddington accretion rates assuming a non-spinning BH and a face-on disk (i = 0°). Under these assumptions we find that the BH in HLX-1 is in the intermediate-mass range (∼2 × 104 M☉) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is η = 0.11 ± 0.03. We also show that the source does follow the LX ∝ T4 relation for our mass estimate. At the outburst peaks, the source radiates near the Eddington limit. The accretion rate then stays constant around 4 × 10–4 M☉ yr–1 for several days and then decreases exponentially. Such 'plateaus' in the accretion rate could be evidence that enhanced mass-transfer rate is the driving outburst mechanism in HLX-1. We also report on the new outburst observed in 2011 August by the Swift X-Ray Telescope. The time of this new outburst further strengthens the ∼1 year recurrence timescale.
[en] We present Hubble Space Telescope and simultaneous Swift X-ray Telescope observations of the strongest candidate intermediate-mass black hole (IMBH) ESO 243-49 HLX-1. Fitting the spectral energy distribution from X-ray to near-infrared wavelengths showed that the broadband spectrum is not consistent with simple and irradiated disk models, but is well described by a model comprised of an irradiated accretion disk plus a ∼106 M☉ stellar population. The age of the population cannot be uniquely constrained, with both young and old stellar populations allowed. However, the old solution requires excessive disk reprocessing and an extremely small disk, so we favor the young solution (∼13 Myr). In addition, the presence of dust lanes and the lack of any nuclear activity from X-ray observations of the host galaxy suggest that a gas-rich minor merger may have taken place less than ∼200 Myr ago. Such a merger event would explain the presence of the IMBH and the young stellar population.