arXiv subjects

I. Negueruela

Publications and source records attributed to I. Negueruela.

At least 109 records · Page 6Linked to original sources

arXiv2010

The X-ray source content of the XMM-Newton Galactic Plane Survey

We report the results of an optical campaign carried out by the XMM-Newton Survey Science Centre with the specific goal of identifying the brightest X-ray sources in the XMM-Newton Galactic Plane Survey of Hands et al. (2004). In addition to photometric and spectroscopic observations obtained at the ESO-VLT and ESO-3.6m, we used cross-correlations with the 2XMMi, USNO-B1.0, 2MASS and GLIMPSE catalogues to progress the identification process. Active coronae account for 16 of the 30 identified X-ray sources. Many of the identified hard X-ray sources are associated with massive stars emitting at intermediate X-ray luminosities of 10^32-34 erg/s. Among these are a very absorbed likely hyper-luminous star with X-ray/optical spectra and luminosities comparable with those of eta Carina, a new X-ray selected WN8 Wolf-Rayet star, a new Be/X-ray star belonging to the growing class of Gamma-Cas analogs and a possible supergiant X-ray binary of the kind discovered recently by INTEGRAL. One of the sources, XGPS-25 has a counterpart which exhibits HeII 4686 and Bowen CIII-NIII emission lines suggesting a quiescent or X-ray shielded Low Mass X-ray Binary, although its properties might also be consistent with a rare kind of cataclysmic variable (CV). We also report the discovery of three new CVs, one of which is a likely magnetic system. The soft (0.4-2.0 keV) band LogN-LogS curve is completely dominated by active stars in the flux range of 1x10^-13 to 1x10^-14 erg/cm2/s. In total, we are able to identify a large fraction of the hard (2-10 keV) X-ray sources in the flux range of 1x10^-12 to 1x10^-13 erg/cm2/s with Galactic objects at a rate consistent with that expected for the Galactic contribution only. (abridged)

astro-ph.HE↗

arXiv2010

INTEGRAL deep observations of the Small Magellanic Cloud

Deep observations of the Small Magellanic Cloud (SMC) and region were carried out in the hard X-ray band by the INTEGRAL observatory in 2008-2009. The field of view of the instrument permitted simultaneous coverage of the entire SMC and the eastern end of the Magellanic Bridge. In total, INTEGRAL detected seven sources in the SMC and five in the Magellanic Bridge; the majority of the sources were previously unknown systems. Several of the new sources were detected undergoing bright X- ray outbursts and all the sources exhibited transient behaviour except the supergiant system SMC X-1. They are all thought to be High Mass X-ray Binary (HMXB) systems in which the compact object is a neutron star.

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arXiv2010

A serendipitous survey for variability amongst the massive stellar population of Westerlund 1

Massive stars exhibit spectroscopic and photometric variability over a wide range of timescales. However the physical mechanisms driving this behaviour remain poorly understood. Westerlund 1 presents an ideal laboratory for studying these processes in a rich, coeval population of post-main sequence stars and we present a pathfinding study aimed at characterising their variability utilising the large body of data that has accumulated since the 1960s. Spectroscopic variability attributable to both wind asphericity and photospheric pulsations was present amongst both the hot and cool stellar populations. Given the limitations imposed by the data, we were unable to determine the physical origin of the wind structure inferred for the OB supergiants, although the inhomogineities in the winds of the Wolf Rayets are likely driven by binary interactions and, conversely, by pulsations in at least one of the cool hypergiants. Photospheric pulsations were found for stars ranging from spectral types as early as O9 I through to the mid F Ia+ yellow hypergiants - with a possible dependence on the luminosity class amongst the OB stars. The spectroscopically variable red supergiants (M2-5 Ia) are also potential pulsators but require further observations to confirm this hypothesis. Determination of the amplitude and periodicity of these pulsations as a function of temperature, luminosity and evolutionary state holds open the prospect of identifying the physical mechanisms driving the instabilities that constrain and define stellar evolution in the upper reaches of the HR diagram, while the presence of highly luminous yellow hypergiants and red supergiants within Wd1 place strong constraints on post-main sequence evolutionary pathways, apparently contradicting current theoretical predictions for >25Msun stars at solar metallicites. [ABRIDGED]

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arXiv2010

The eclipsing, double-lined, Of supergiant binary Cyg OB2-B17

Massive, eclipsing, double-lined, spectroscopic binaries are not common but are necessary to understand the evolution of massive stars as they are the only direct way to determine stellar masses. They are also the progenitors of energetic phenomena such as X-ray binaries and gamma-ray bursts. We present a photometric and spectroscopic analysis of the candidate binary system Cyg OB2-B17 to show that it is indeed a massive evolved binary. We utilise V band and white-light photometry to obtain a light curve and period of the system, and spectra at different resolutions to calculate preliminary orbital parameters and spectral classes for the components. Our results suggest that B17 is an eclipsing, double-lined, spectroscopic binary with a period of 4.0217+/-0.0004 days, with two massive evolved components with preliminary classifications of O7 and O9 supergiants. The radial velocity and light curves are consistent with a massive binary containing components with similar luminosities, and in turn with the preliminary spectral types and age of the association.

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arXiv2009

Radio emission from the massive stars in the Galactic Super Star Cluster Westerlund 1

Current mass-loss rate estimates imply that main sequence winds are not sufficient to strip away the H-rich envelope to yield Wolf-Rayet (WR) stars. The rich transitional population of Westerlund 1 (Wd 1) provides an ideal laboratory to observe mass-loss processes throughout the transitional phase of stellar evolution. An analysis of deep radio continuum observations of Wd 1 is presented. We detect 18 cluster members. The radio properties of the sample are diverse, with thermal, non-thermal and composite thermal/non-thermal sources present. Mass-loss rates are ~10^{-5} solar mass/year across all spectral types, insufficient to form WRs during a massive star lifetime, and the stars must undergo a period of enhanced mass loss. The sgB[e] star W9 may provide an example, with a mass-loss rate an order of magnitude higher than the other cluster members, and an extended nebula of density ~3 times the current wind. This structure is reminiscent of luminous blue variables, and one with evidence of two eras of high, possibly eruptive, mass loss. Three OB supergiants are detected, implying unusually dense winds. They also may have composite spectra, suggesting binarity. Spatially resolved nebulae are associated with three of the four RSGs and three of the six YHGs in the cluster, which are due to quiescent mass loss rather than outbursts. For some of the cool star winds, the ionizing source may be a companion star though the cluster radiation density is sufficiently high to provide the necessary ionizing radiation. Five WR stars are detected with composite spectra, interpreted as arising in colliding-wind binaries.

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arXiv2009

Preliminary determinations of the masses of the neutron star and mass donor in the High Mass X-ray Binary system EXO 1722-363

We intended to measure the radial velocity curve of the supergiant companion to the eclipsing high mass X-ray binary pulsar EXO1722-363 and hence determine the stellar masses of the components. We used a set of archival K$_{\rm s}$-band infrared spectra of the counterpart to EXO1722-363 obtained using ISAAC on the VLT, and cross-correlated them in order to measure the radial velocity of the star. The resulting radial velocity curve has a semi-amplitude of $24.5 \pm 5.0$ km s$^{-1}$. When combined with other measured parameters of the system, this yields masses in the range 1.5 $\pm$ 0.4 - 1.6 $\pm$ 0.4 M$_{\odot}$ for the neutron star and 13.6 $\pm$ 1.6 - 15.2 $\pm$ 1.9 M$_{\odot}$ for the B0--1 Ia supergiant companion. These lower and upper limits were obtained under the assumption that the system is viewed edge-on (i = 90$^\circ$) for the lower limit and the supergiant fills its Roche lobe ($β= 1$) for the upper limit respectively. The system inclination is constrained to $i>75^{\circ}$ and the Roche lobe-filling factor of the supergiant is $β>0.9$. Additionally we were able to further constrain our distance determination to be 7.1 $\le$ d $\le$ 7.9 kpc for EXO1722-363. The X-ray luminosity for this distance range is 4.7 $\times$ 10$^{35}$ $\le$ L$_{\rm X}$ $\le$ 9.2 $\times$ 10$^{36}$ erg s$^{-1}$. EXO1722-363 therefore becomes the seventh of the ten known eclipsing X-ray binary pulsars for which a dynamical neutron star mass solution has been determined. Additionally EXO1722-363 is the first such system to have a neutron star mass measurement made utilising near-infrared spectroscopy.

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arXiv2009

Near-infrared survey of High Mass X-ray Binary candidates

We combine infrared spectra in the I, J, H and K bands together with JHK photometry to characterize the spectral type, luminosity class and distance to the infrared counterparts to five INTEGRAL sources. For SAX J18186-1703 and IGR J18483-0311, we present the first intermediate-resolution spectroscopy reported. We find that four systems harbour early-type B supergiants. All of them are heavily obscured, with E(B-V) ranging between 3 and 5, implying visual extinctions of ~ 9 to 15 magnitudes. We refine the published classifications of IGR J18027-2016 and IGR J19140+0951 by constraining their luminosity class. In the first case, we confirm the supergiant nature and rule out class III. In the second case, we propose a slightly higher luminosity class (Ia instead of Iab) and give an improved value of the distance based on new optical photometry. Owing to their infrared and X-ray characteristics, IGR J18027-2016 and IGR J19140+0951, emerge as Supergiant X-ray binaries with X-ray luminosities of the order of Lx ~ [1-2] x 10^{36} erg/s, while SAX J1818.6-1703 and IGR J18483-0311, turn out to be Supergiant Fast X-ray Transients at 2 and 3 kpc, respectively. Finally, XTE J1901+014 emerges as a puzzling source: its X-ray behaviour is strongly reminiscent of the SFXTs but a supergiant nature is firmly ruled out for the counterpart. We discuss several alternative scenarios to explain its behaviour.

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arXiv2009

Spectroscopic monitoring of the luminous blue variable Westerlund1-243 from 2002 to 2009

The massive post-Main Sequence star W243 in the galactic starburst cluster Westerlund 1 has undergone a spectral transformation from a B2Ia supergiant devoid of emission features in 1981 to an A-type supergiant with a rich emission-line spectrum by 2002/03. We used VLT/UVES and VLT/FLAMES to obtain high-resolution spectra on six epochs in 2003/04 (UVES) and ten epochs in 2008/09 (FLAMES). These spectra are used alongside other VLT/FLAMES and NTT/EMMI spectra to follow the evolution of W243 from 2002 to 2009. W243 displays a complex, time-varying spectrum with emission lines of Hydrogen, Helium and Lyman-pumped metals, forbidden lines of Nitrogen and Iron, and a large number of absorption lines from neutral and singly-ionized metals. Many lines are complex emission/absorption blends. The LBV has a current temperature of ~8500K (spectral type A3Ia+), and displays signs of photospheric pulsations and weak episodic mass loss. Nitrogen is highly overabundant, with Carbon and Oxygen depleted, indicative of surface CNO-processed material and considerable previous mass-loss, although current time-averaged mass-loss rates are low. The emission-line spectrum forms at large radii, when material lost by the LBV in a previous mass-loss event is ionized by an unseen hot companion. Monitoring of the near-infrared spectrum suggests that the star has not changed significantly since it finished evolving to the cool state, close to the Humphreys-Davidson limit, in early 2003. [ABRIDGED]

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arXiv2009

A VLT/FLAMES survey for massive binaries in Westerlund 1: I. first observations of luminous evolved stars

Multiwavelength observations of the young massive cluster Westerlund 1 have revealed evidence for a large number of OB supergiant and Wolf-Rayet binaries. However, in most cases these findings are based on secondary binary characteristics such as hard X-ray emission and/or non-thermal radio spectra and hence provide little information on binary properties such as mass ratio and orbital period. To overcome this shortcoming we have initiated a long temporal baseline, VLT/FLAMES+GIRAFFE multi-epoch radial velocity survey that will provide the first direct constraints on these parameters. This study presents first results from twenty of the most luminous supergiant stars in Wd1. Statistically significant radial velocity changes are detected in ~60% of targets. W43a is identified as a short-period binary, while W234 and the newly-identified cluster member W3003 are probable binaries and W2a is a strong binary candidate. The cool hypergiants W243 and W265 display photospheric pulsations, while a number of early-mid B supergiants display radial velocity changes that we cannot distinguish between orbital or photospheric motion in our initial short-baseline survey. When combined with existing observations, we find 30% of our sample to be binary (6/20) while additional candidate binaries support a binary fraction amongst Wd1 supergiants in excess of ~40% [ABRIDGED]

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arXiv2009

Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415

We report near-infrared observations of the mass donors of the eclipsing high-mass X-ray binary (HMXB) systems EXO 1722-363 and OAO 1657-415 in order to derive their accurate spectral classifications. We determined that EXO 1722-363 was of spectral type B0 - B1 Ia, positioned at a distance 8.0 +2.5/-2.0 kpc with a progenitor mass in the range 30 - 40 M_Sun. Luminosity calculations imply that L_X ~ 10^35 - 10^37 erg s^-1 for this distance range. We conclude that EXO 1722-363 shares many of the properties associated with other X-ray binary B-type supergiant donors. We found that OAO 1657-415 correlates closely with the spectra of a class of transitional objects, the Ofpe/WNL, an intermediate evolutionary stage between massive O type stars leaving the main sequence and evolving into Wolf-Rayets. Due to the wide range range in Luminosity displayed by Ofpe/WNL stars, (log L/L_Sun ~ 5.3 - 6.2) distance determinations are problematic. For OAO 1657-415 we report a distance of 4.4 <= d <= 12 kpc, implying an X-ray luminosity of 1.5 x 10^36 <= L_X <= 10^37 erg s^-1. We have used our new classification of OAO 1657-415 to explain the physical processes responsible for its unique position within the Corbet diagram. Ofpe/WNL stars demonstrate a high rate of mass-loss through a dense stellar wind combined with a low terminal velocity. This combination of wind properties leads to a high accretion rate and transfer of angular momentum to the neutron star in this system. We believe this in turn leads to a smaller instantaneous equilibrium spin period with respect to normal OB supergiants.

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arXiv2009

A third red supergiant rich cluster in the Scutum-Crux Arm

We aim to characterise the properties of a third massive, red supergiant dominated galactic cluster. To accomplish this we utilised a combination of near/mid-IR photometry and spectroscopy to identify and classify the properties of cluster members, and statistical arguments to determine the mass of the cluster. We found a total of 16 strong candidates for cluster membership, for which formal classification of a subset yields spectral types from K3-M4 Ia and luminosities between log(L/L_sun)~4.5-4.8 for an adopted distance of 6+/-1 kpc. For an age in the range of 16-20 Myr, the implied mass is 2-4x10^4 M_sun, making it one of the most massive young clusters in the Galaxy. This discovery supports the hypothesis that a significant burst of star formation occurred at the base of Scutum-Crux arm between 10-20 Myr ago, yielding a stellar complex comprising at least ~10^5M_sun of stars (noting that since the cluster identification criteria rely on the presence of RSGs, we suspect that the true stellar yield will be significantly higher). We highlight the apparent absence of X-ray binaries within the star formation complex and finally, given the physical association of at least two pulsars with this region, discuss the implications of this finding for stellar evolution and the production and properties of neutron stars.

astro-ph.GA↗

arXiv2008

Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 0.001-1000 s. A cyclotron line at ~30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Our new long and uninterrupted RXTE observations allow us to search for long (~1 hr) pulsations for the first time. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 x 10^{35} erg s^{-1} with a ratio Fmax/Fmin ~ 5. This ratio implies an eccentricity of ~0.4, somewhat higher than previously suggested. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit.

astro-ph↗

arXiv2008

Optical properties of High-Mass X-ray Binaries (HMXB) in the Small Magellanic Cloud

The SMC represents an exciting opportunity to observe the direct results of tidal interactions on star birth. One of the best indicators of recent star birth activity is the presence of signicant numbers of High-Mass X-ray Binaries (HMXBs) - and the SMC has them in abundance! We present results from nearly 10 years of monitoring these systems plus a wealth of other ground-based optical data. Together they permit us to build a picture of a galaxy with a mass of only a few percent of the Milky Way but with a more extensive HMXB population. However, as often happens, new discoveries lead to some challenging puzzles - where are the other X-ray binaries (eg black hole systems) in the SMC? And why do virtually all the SMC HMXBs have Be starcompanions? The evidence arising from these extensive optical observations for this apparently unusual stellar evolution are discussed.

astro-ph↗

arXiv2008

Very peculiar wind from BD+53 2790, the optical counterpart to 4U 2206+54

BD+53 2790, an O9.5Vp star, is the optical counterpart to the HMXRB 4U 2206+54. This system was classified initially as a BeX, but observational evidence soon stressed the need to revise this classification. The permanent asymmetry in the H-alpha line profiles (in contrast with the cyclic variations shown by Be stars), the variations in the profile of this line in time scales of hours (while time scales from weeks to months are expected in Be stars), and the lack of correlation between IR observables and H-alpha line parameters, strongly suggest that, while BD+53 2790 contains a circunstellar disc, it is not like the one present in Be stars. Furthermore, there is evidence of overabundance of He in BD+53 2790. Together with the presence of an anomalous wind, found through UV spectroscopy, the possibility to link this star with the group of He rich stars is open. We will discuss the work done with IUE data from BD+53 2790 and the unexpected finding of a slow and dense wind, very rare for an O9.5V star.

astro-ph↗

arXiv2008

INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302

In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds.Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1-200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity.

astro-ph↗

arXiv2008

Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud

The spectral distributions of Be/X-ray binaries in the Large Magellanic Cloud and Galaxy have been shown to differ significantly from the distribution of isolated Be stars in the Galaxy. Population synthesis models can explain this difference in spectral distributions through substantial angular momentum loss from the binary system. In this work we explore the spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud (SMC) using high signal-to-noise spectroscopy of a sample of 37 optical counterparts to known X-ray pulsars. Our results show that the spectral distribution of Be/X-ray binaries in the SMC is consistent with that of the Galaxy, despite the lower metallicity environment of the SMC. This may indicate that, although the metallicity of the SMC is conducive to the formation of a large number of HMXBs, the spectral distribution of these systems is likely to be most strongly influenced by angular momentum losses during binary evolution, which are not particularly dependent on the local metallicity.

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arXiv2007

The XMM-Newton Serendipitous Survey V. Optical identification of the XMM-Newton Medium sensitivity Survey (XMS)

We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. We report on the optical identification of the XMS samples, complete to 85-95%. At the intermediate flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

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arXiv2007

INTEGRAL observations of the Small Magellanic Cloud

The first INTEGRAL observations of the Small Magellanic Cloud (carried out in 2003) are reported in which two sources are clearly detected. The first source, SMC X-1, shows a hard X-ray eclipse and measurements of its pulse period indicate a continuation of the long-term spin-up now covering ~30 years. The second source is likely to be a high mass X-ray binary, and shows a potential periodicity of 6.8s in the IBIS lightcurve. An exact X-ray or optical counterpart cannot be designated, but a number of proposed counterparts are discussed. One of these possible counterparts shows a strong coherent optical modulation at ~2.7d, which, together with the measured hard X-ray pulse period, would lead to this INTEGRAL source being classified as the fourth known high mass Roche lobe overflow system.

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