arXiv subjects

G. Rauw

Publications and source records attributed to G. Rauw.

At least 109 records · Page 6Linked to original sources

arXiv2006

On the nature of the hard X-ray source IGR J2018+4043

We found a very likely counterpart to the recently discovered hard X-ray source IGR J2018+4043 in the multi-wavelength observations of the source field. The source, originally discovered in the 20-40 keV band, is now confidently detected also in the 40-80 keV band, with a flux of (1.4 +/- 0.4) x 10(-11) erg cm(-2) s(-1). A 5 ks Swift observation of the IGR J2018+4043 field revealed a hard point-like source with the observed 0.5-10 keV flux of 3.4(+0.7)(-0.8) x 10(-12) erg cm(-2) s(-1) (90% confidence level) at alpha = 20h18m38.55s, delta = +40d41m00.4s (with a 4.2" uncertainty). The combined Swift-INTEGRAL spectrum can be described by an absorbed power-law model with photon index gamma = 1.3 +/- 0.2 and N_H = 6.1(+3.2)(-2.2) x 10(22) cm(-2). In archival optical and infrared data we found a slightly extended and highly absorbed object at the Swift source position. There is also an extended VLA 1.4 GHz source peaked at a beam-width distance from the optical and X-ray positions. The observed morphology and multiwavelength spectra of IGR J2018+4043 are consistent with those expected for an obscured accreting object, i.e. an AGN or a Galactic X-ray binary. The identification suggests possible connection of IGR J2018+4043 to the bright gamma-ray source GEV J2020+4023 (3EG J2020+4017) detected by COS B and CGRO EGRET in the gamma-Cygni SNR field.

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arXiv2006

The first orbital solution for the massive colliding-wind binary HD93162 (=WR25)

Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD93162 (=WR25), this object has been predicted to be a colliding-wind binary system. However, radial-velocity variations that would prove the suspected binary nature have yet to be found. We spectroscopically monitored this object to investigate its possible variability to address this discordance. We compiled the largest available radial-velocity data set for this star to look for variations that might be due to binary motion. We derived radial velocities from spectroscopic data acquired mainly between 1994 and 2006, and searched these radial velocities for periodicities using different numerical methods. For the first time, periodic radial-velocity variations are detected. Our analysis definitively shows that the Wolf-Rayet star WR25 is an eccentric binary system with a probable period of about 208 days.

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arXiv2006

GCIRS16SW: a massive eclipsing binary in the Galactic Center

We report on the spectroscopic monitoring of GCIRS16SW, an Ofpe/WN9 star and LBV candidate in the central parsec of the Galaxy. SINFONI observations show strong daily spectroscopic changes in the K band. Radial velocities are derived from the HeI 2.112 um line complex and vary regularly with a period of 19.45 days, indicating that the star is most likely an eclipsing binary. Under various assumptions, we are able to derive a mass of ~ 50 Msun for each component.

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arXiv2006

An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population

In this second paper, we pursue the analysis of the 180 ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30% can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their X-ray luminosities scale with their bolometric luminosities as $\log L_\mathrm{X} - \log L_\mathrm{bol}=-6.912\pm0.153$. Such a scaling law holds in the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in the hard band. While the two colliding wind binaries in our sample clearly deviate from this scheme, the remaining O-type objects show a very limited dispersion (40% or 20% according to whether `cool' dwarfs are included or not), much smaller than that obtained from previous studies. At our detection threshold and within our sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. The intrinsic X-ray emission of non-peculiar O-type stars seems thus constant for a given star and the level of its X-ray emission is accurately related to the its luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25% in the sub-type range B0-B4 and remains mostly uniform throughout the different sub-populations while it drops significantly at later sub-types. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the detected emission being associated with a physical PMS companion >... [see paper for the complete abstract]

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arXiv2006

The atypical emission-line star Hen3-209

We analyse observations, spanning 15 years, dedicated to the extreme emission-line object Hen3-209. Our photometric data indicate that the luminosity of the star undergoes marked variations with a peak-to-peak amplitude of 0.65mag. These variations are recurrent, with a period of 16.093+-0.005d. The spectrum of Hen3-209 is peculiar with many different lines (HI, HeI, FeII,...) showing P Cygni profiles. The line profiles are apparently changing in harmony with the photometry. The spectrum also contains [OIII] lines that display a saddle profile topped by three peaks, with a maximum separation of about 600km/s. Hen3-209 is most likely an evolved luminous object suffering from mass ejection events and maybe belonging to a binary system.

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arXiv2006

Early-type stars in the young open cluster IC1805. II. The probably single stars HD15570 and HD15629, and the massive binary/triple system HD15558

Aims: We address the issue of the multiplicity of the three brightest early-type stars of the young open cluster IC1805, namely HD15570, HD15629 and HD15558. Methods: For the three stars, we measured the radial velocity by fitting Gaussian curves to line profiles in the optical domain. In the case of the massive binary HD15558, we also used a spectral disentangling method to separate the spectra of the primary and of the secondary in order to derive the radial velocities of the two components. These measurements were used to compute orbital solutions for HD15558. Results: For HD15570 and HD15629, the radial velocities do not present any significant trend attributable to a binary motion on time scales of a few days, nor from one year to the next. In the case of HD15558 we obtained an improved SB1 orbital solution with a period of about 442 days, and we report for the first time on the detection of the spectral signature of its secondary star. We derive spectral types O5.5III(f) and O7V for the primary and the secondary of HD15558. We tentatively compute a first SB2 orbital solution although the radial velocities from the secondary star should be considered with caution. The mass ratio is rather high, i.e. about 3, and leads to very extreme minimum masses, in particular for the primary object. Minimum masses of the order of 150 \pm 50 and 50 \pm 15 M_\odot are found respectively for the primary and the secondary. Conclusions: We propose that HD15558 could be a triple system. This scenario could help to reconcile the very large minimum mass derived for the primary object with its spectral type. In addition, considering new and previously published results, we find that the binary frequency among O-stars in IC1805 has a lower limit of 20%, and that previously published values (80%) are probably overestimated.

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arXiv2006

An XMM-Newton view of the young open cluster NGC 6231 -- I. The catalogue

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC 6231 is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 6E-15 erg/s/cm^2 but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 3 arcsec at maximum. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin^2 and a core radius close to 3.1arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects -- corresponding to the early-type stars of the cluster -- most of the sources are relatively faint (~5E-15 erg/s/cm^2) with an energy distribution peaked around 1.0-2.0 keV.

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arXiv2005

INTEGRAL detection of hard X-rays from NGC 6334: Nonthermal emission from colliding winds or an AGN?

We report the detection of hard X-ray emission from the field of the star-forming region NGC 6334 with the the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The JEM-X monitor and ISGRI imager aboard INTEGRAL and Chandra ACIS imager were used to construct 3-80 keV images and spectra of NGC 6334. The 3-10 keV and 10-35 keV images made with JEM-X show a complex structure of extended emission from NGC 6334. The ISGRI source detected in the energy ranges 20-40 keV and 40-80 keV coincides with the NGC 6334 ridge. The 20-60 keV flux from the source is (1.8+-0.37)*10(-11) erg cm(-2) s(-1). Spectral analysis of the source revealed a hard power-law component with a photon index about 1. The observed X-ray fluxes are in agreement with extrapolations of X-ray imaging observations of NGC 6334 by Chandra ACIS and ASCA GIS. The X-ray data are consistent with two very different physical models. A probable scenario is emission from a heavily absorbed, compact and hard Chandra source that is associated with the AGN candidate radio source NGC 6334B. Another possible model is the extended Chandra source of nonthermal emission from NGC 6334 that can also account for the hard X-ray emission observed by INTEGRAL. The origin of the emission in this scenario is due to electron acceleration in energetic outflows from massive early type stars. The possibility of emission from a young supernova remnant, as suggested by earlier infrared observations of NGC 6334, is constrained by the non-detection of 44Ti lines.

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arXiv2005

An exceptional X-ray view of the young open cluster NGC 6231: what XMM-Newton has taught us

Considered as the core of the Sco OB1 association, the young open cluster NGC 6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite targeted the cluster for a nominal duration of about 180 ks. Thanks to the detector sensitivity, the EPIC cameras provided an unprecedented X-ray view of NGC 6231, revealing about 600 point-like sources. In this contribution, we review the main results that have been obtained thanks to this unprecedented data set. Concerning the O-type stars, we present the latest developments related to the so-called 'canonical' Lx-Lbol relation. The dispersion around this relation might actually be much smaller than previously thought. In our data set, the sole mechanism that yields a significant deviation from this scheme is wind interaction. It is also the sole mechanism that induces a significant variation of the early-type star X-ray flux. In a second part of this contribution, we probe the properties of the optically faint X-ray sources. Most of them are believed to be low mass pre-main sequence stars. Their analysis provides direct insight into the star formation history of the cluster.

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arXiv2005

CPD-41deg7742: an unusual wind interaction

We summarize the results of a multiwavelength observing campaign on the massive eclipsing binary CPD-41deg7742, another remarkable object in the young open cluster NGC 6231. Our campaign relies on high resolution echelle spectroscopy, narrow-band optical photometry, and XMM-Newton X-ray observations. Combined with the spectroscopic analysis, the light curve analysis provides a direct measurement of the masses and sizes of the system components. However, the most outstanding results come from the XMM-Newton observations. Our 180 ks campaign towards NGC 6231, and CPD-41deg7742, provides an unprecedented phase coverage of such a close early-type binary. The EPIC-MOS light curves almost fully cover the 2.4 day period of the system and the brightness of the object is sufficient to yield a time resolution as tight as 1 ks. The X-ray flux presents clear variations along the orbit, that we interpret as the signature of an unusual wind interaction. We indeed expect that, in this O+B system, the dominant primary wind crashes into the secondary surface, leading to a wind-photosphere interaction. As a strong support to our interpretation, we provide a geometrical model that associates an extra X-ray emission to the secondary inner surface. Though quite simple, the present model matches the main features of the X-ray light curve.

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arXiv2005

The massive binary CPD-41degr7742 II. Optical light curve and X-ray observations

In the first paper of this series, we presented a detailed high-resolution spectroscopic study of CPD - 41degr7742, deriving for the first time an orbital solution for both components of the system. In this second paper, we focus on the analysis of the optical light curve and on recent XMM-Newton X-ray observations. In the optical, the system presents two eclipses, yielding an inclination i ~ 77degr. Combining the constraints from the photometry with the results of our previous work, we derive the absolute parameters of the system. We confirm that the two components of CPD - 41degr7742 are main sequence stars (O9 V + B1-1.5 V) with masses (M_1 ~ 18 Msol and M_2 ~ 10 Msol) and respective radii (R_1 ~ 7.5 Rsol and R_2 ~ 5.4 Rsol) close to the typical values expected for such stars. We also report an unprecedented set of X-ray observations that almost uniformly cover the 2.44-day orbital cycle. The X-ray emission from CPD - 41degr7742 is well described by a two-temperature thermal plasma model with energies close to 0.6 and 1.0 keV, thus slightly harder than typical early-type emission. The X-ray light curve shows clear signs of variability. The emission level is higher when the primary is in front of the secondary. During the high emission state, the system shows a drop of its X-ray emission that almost exactly matches the optical eclipse. We interpret the main features of the X-ray light curve as the signature of a wind-photosphere interaction, in which the overwhelming primary O9 star wind crashes into the secondary surface. Alternatively the light curve could result from a wind-wind interaction zone located near the secondary star surface. As a support to our interpretation, we provide a phenomenological geometric model that qualitatively reproduces the observed modulations of the X-ray emission.

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arXiv2005

Are WC9 Wolf-Rayet stars in colliding-wind binaries?

We present results from a spectroscopic search for massive companions to dust-making Galactic WC9 stars as a step to testing the paradigm that dust formation in these systems requires colliding winds to produce over densities. We find evidence for OB companions to the WC9 stars WR 59 and WR 65, but not WR 121 or WR 117. We identify lines of N III-V and possibly N II in the spectrum of WR 88, one of the few Galactic WC9 stars which do not make circumstellar dust, and suggest that WR 88 is a transitional WN-WC9 object and less evolved than the other WC9 stars. On the other hand, the possible identification of a strong emission line at 4176A in the spectrum of WR 117 with Ne I suggests that this star is more evolved than other WC9 stars studied.

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arXiv2005

An XMM-Newton observation of the multiple system HD167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC6604

We discuss the results of two XMM-Newton observations of the open cluster NGC6604 obtained in April and September 2002. We concentrate mainly on the multiple system HD167971 (O5-8V + O5-8V + (O8I)). The soft part of the EPIC spectrum of this system is thermal with typical temperatures of about 2 10^6 to 9 10^6 K. The nature (thermal vs non-thermal) of the hard part of the spectrum is not unambiguously revealed by our data. If the emission is thermal, the high temperature of the plasma (~ 2.3 10^7 to 4.6 10^7 K) would be typical of what should be expected from a wind-wind interaction zone within a long period binary system. This emission could arise from an interaction between the combined winds of the O5-8V + O5-8V close binary system and that of the more distant O8I companion. Assuming instead that the hard part of the spectrum is non-thermal, the photon index would be rather steep (~3). Moreover, a marginal variability between our two XMM-Newton pointings could be attributed to an eclipse of the O5-8V + O5-8V system. The overall X-ray luminosity points to a significant X-ray luminosity excess of about a factor 4 possibly due to colliding winds. Considering HD167971 along with several recent X-ray and radio observations, we propose that the simultaneous observation of non-thermal radiation in the X-ray (below 10.0 keV) and radio domains appears rather unlikely. Our investigation of our XMM-Newton data of NGC6604 reveals a rather sparse distribution of X-ray emitters. Including the two bright non-thermal radio emitters HD168112 and HD167971, we present a list of 31 X-ray sources... (see paper for full abstract)

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arXiv2005

Non-thermal radio emission from O-type stars. I. HD 168112

We present a radio lightcurve of the O5.5 III(f+) star HD 168112, based on archive data from the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). The fluxes show considerable variability and a negative spectral index, thereby confirming that HD 168112 is a non-thermal radio emitter. The non-thermal radio emission is believed to be due to synchrotron radiation from relativistic electrons that have been Fermi accelerated in shocks. For HD 168112, it is not known whether these shocks are due to a wind-wind collision in a binary system or to the intrinsic instability of the stellar wind driving mechanism. Assuming HD 168112 to be a single star, our synchrotron model shows that the velocity jump of the shocks should be very high, or there should be a very large number of shocks in the wind. Neither of these is compatible with time-dependent hydrodynamical calculations of O star winds. If, on the other hand, we assume that HD 168112 is a binary, the high velocity jump is easily explained by ascribing it to the wind-wind collision. By further assuming the star to be an eccentric binary, we can explain the observed radio variability by the colliding-wind region moving in and out of the region where free-free absorption is important. The radio data presented here show that the binary has a period between one and two years. By combining the radio data with X-ray data, we find that the most likely period is about 1.4 yr.

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arXiv2005

Optical spectroscopy of X-Mega targets in the Carina Nebula - V. The spectroscopic binary HD93161A and its visual companion HD93161B

We present the analysis of an extensive set of high resolution spectroscopic observations of HD93161, a visual binary with a separation of 2 arcsec. HD93161 A is a spectroscopic binary, with both components clearly detected throughout the orbit. The primary star is most probably of spectral type O8V, while the secondary is likely an O9V. We obtain the first orbital solution for this system, characterized by a period of 8.566\pm0.004 days. The minimum masses of the primary and secondary stars are 22.2\pm0.6 M_sol and 17.0\pm0.4 M_sol respectively. These values are quite large, suggesting a high inclination of the orbit. The second object, HD93161 B, displays an O6.5V((f)) spectral type and is thus slightly hotter than its neighbour. This star is at first sight single but presents radial velocity variations. We finally study HD93161 in the X-ray domain. No significant variability is detected. The X-ray spectrum is well described by a 2T model with kT_1~0.3keV and kT_2~0.7keV. The X-ray luminosity is rather moderate, without any large emission excess imputable to a wind interaction.

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arXiv2004

NGC 6231: X-ray properties of the early-type star population

Based on a deep XMM-Newton observation of the young open cluster NGC 6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are associated with early-type stars. We investigate their $L_\mathrm{X} / L_\mathrm{bol}$ relationship and we confirm the clear dichotomy between O- and B-type stars. The cut-off line between the two behaviours occurs at $L_\mathrm{bol} \sim 10^{38}$ erg s$^{-1}$ as previously proposed by \citet{BSC97}. The distinction between single and binary stars is not clear cut, except for the colliding wind system HD 152248. The X-ray detected B-stars in NGC 6231 appear to be more luminous than predicted from the \citeauthor{BSC97} relation. Though this suggests a bimodal distribution of the B-star X-ray emission, we caution however that these results might be biased by detection limits. Finally we investigate the X-ray variability of the detected sources and we find that about 40% of the X-ray emitters in the field of view present consistent signs of variability in the EPIC instruments. This fraction is much larger than previously thought. About one third of the early-type stars population, either singles or binaries, further shows variability. These variations do not seem to be exclusively related to binarity and could thus not be systematically interpreted in the framework of a wind-wind collision phenomenon.

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arXiv2004

Hard X-ray Emission Clumps in the gamma-Cygni Supernova Remnant: an INTEGRAL-ISGRI View

Spatially resolved images of the galactic supernova remnant G78.2+2.1 (gamma-Cygni) in hard X-ray energy bands from 25 keV to 120 keV are obtained with the IBIS-ISGRI imager aboard the International Gamma-Ray Astrophysics Laboratory INTEGRAL. The images are dominated by localized clumps of about ten arcmin in size. The flux of the most prominent North-Western (NW) clump is (1.7 +/- 0.4) 10^{-11} erg/cm^2/s in the 25-40 keV band. The observed X-ray fluxes are in agreement with extrapolations of soft X-ray imaging observations of gamma-Cygni by ASCA GIS and spatially unresolved RXTE PCA data. The positions of the hard X-ray clumps correlate with bright patches of optical line emission, possibly indicating the presence of radiative shock waves in a shocked cloud. The observed spatial structure and spectra are consistent with model predictions of hard X-ray emission from nonthermal electrons accelerated by a radiative shock in a supernova interacting with an interstellar cloud, but the powerful stellar wind of the O9V star HD 193322 is a plausible candidate for the NW source as well.

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arXiv2004

Line profile variability in the spectra of Oef stars: II. HD192281, HD14442 and HD14434

We present the very first analysis of the spectroscopic variability of the three rapidly rotating Oef stars HD192281 (O5(ef)), HD14442 (O5.5ef) and HD14434 (O6.5(ef)). Radial velocities of the HeII lambda 4541 line reveal no evidence of binarity on time scales of a few days, or from one year to the next, for any of the targets. The HeII lambda 4686 double-peaked emission and, to some extent, the Hbeta absorption line display significant profile variability in the spectra of all three stars. Data gathered during different observing runs spread over six years reveal a rather stable time scale for HD192281 and HD14442, whereas the variability pattern changes significantly from one year to the other. The case of HD14434 is less clear as no obvious time scale emerges from our analysis. In a temptative way to interpret this variability, stellar rotation remains a possible clock for HD192281 and HD14442. However, currently available models addressing stellar rotation fail to explain some crucial aspects of the observed variability behaviour, which appear to be even more complex in the case of HD14434.

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