arXiv subjects

M. Ribo

Publications and source records attributed to M. Ribo.

At least 37 records · Page 2Linked to original sources

arXiv2008

Exploring the powering source of the TeV X-ray binary LS 5039

LS 5039 is one of the four TeV emitting X-ray binaries detected up to now. The powering source of its multi-wavelength emission can be accretion in a microquasar scenario or wind interaction in a young non-accreting pulsar scenario. These two scenarios predict different morphologic and peak position changes along the orbital cycle of 3.9 days, which can be tested at milliarcsecond scales using VLBI techniques. Here we present a campaign of 5 GHz VLBA observations conducted in June 2000 (2 runs five days apart). The results show a core component with a constant flux density, and a fast change in the morphology and the position angle of the elongated extended emission, but maintaining a stable flux density. These results are difficult to fit comfortably within a microquasar scenario, whereas they appear to be compatible with the predicted behavior for a non-accreting pulsar.

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arXiv2008

A radio and near-infrared mini-survey of the MGRO J2019+37 complex

MGRO J2019+37 is an unidentified source of very high energy gamma-rays originally reported by the MILAGRO collaboration as the brightest TeV source in the Cygnus region. Despite the poor angular resolution of MILAGRO, this object seems to be most likely an extended source or, alternatively, a superposition of point-like TeV sources. In order to contribute to the understanding of this peculiar object, we have mosaiced it with the Giant Metrewave Radio Telescope (GMRT) in Pune, India, at the 610 MHz frequency covering a field of view of about 6 square degrees down to a typical rms noise of a few tenths of mJy. We also observed the central square degree of this mosaic in the near infrared Ks-band using the 3.5 m telescope and the OMEGA2000 camera at the Calar Alto observatory (Spain). We present here a first account of our observations and results, together with a preliminar cross correlation of the radio and infrared source catalog as well as a correlation with the available X-ray observations of the region.

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arXiv2008

The changing milliarcsecond radio morphology of the gamma-ray binary LS 5039

Context. LS 5039 is one of the few TeV emitting X-ray binaries detected so far. The powering source of its multiwavelength emission can be accretion in a microquasar scenario or wind interaction in a young nonaccreting pulsar scenario. Aims. To present new high-resolution radio images and compare them with the expected behavior in the different scenarios. Methods. We analyze Very Long Baseline Array (VLBA) radio observations that provide morphological and astrometric information at milliarcsecond scales. Results. We detect a changing morphology between two images obtained five days apart. In both runs there is a core component with a constant flux density, and an elongated emission with a position angle (PA) that changes by 12+/-3 degrees between both runs. The source is nearly symmetric in the first run and asymmetric in the second one. The astrometric results are not conclusive. Conclusions. A simple and shockless microquasar scenario cannot easily explain the observed changes in morphology. An interpretation within the young nonaccreting pulsar scenario requires the inclination of the binary system to be very close to the upper limit imposed by the absence of X-ray eclipses.

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arXiv2008

TeV Gamma-Ray Astrophysics

The window of TeV Gamma-Ray Astrophysics was opened less than two decades ago, when the Crab Nebula was detected for the first time. After several years of development, the technique used by imaging atmospheric Cherenkov telescopes like HESS, MAGIC or VERITAS, is now allowing to conduct sensitive observations in the TeV regime. Water Cherenkov instruments like Milagro are also providing the first results after years of integration time. Different types of extragalactic and galactic sources have been detected, showing a variety of interesting phenomena that are boosting theory in very high energy gamma-ray astrophysics. Here I review some of the most interesting results obtained up to now, making special emphasis in the field of X-ray/gamma-ray binaries.

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arXiv2007

Observations of microquasars with the MAGIC telescope

We report on the results from the observations in very high energy band (VHE, E_gamma > 100GeV) of the black hole X-ray binary (BHXB) Cygnus X-1. The observations were performed with the MAGIC telescope, for a total of 40 hours during 26 nights, spanning the period between June and November 2006. We report on the results of the searches for steady and variable gamma-ray signals, including the first experimental evidence for an intense flare, of duration between 1.5 and 24 hours.

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arXiv2007

Exploring the connection between the stellar wind and the non-thermal emission in LS 5039

LS 5039 has been observed with several X-ray instruments so far. The source presents X-ray variability at orbital timescales in flux and photon index. The system harbors an O-type main sequence star with moderate mass loss. At present, the link between the X-rays and the stellar wind is unclear. New XMM-Newton observations have been performed around periastron and apastron passages in September 2005, when the stellar wind activity was apparently higher. April 2005 Chandra observations on LS 5039 are also revisited. Moreover, a compilation of Halpha EW data obtained since 1992, from which the stellar mass loss evolution can be approximately inferred, is carried out. XMM-Newton observations show higher and harder emission around apastron than around periastron. No signatures of thermal emission or a reflection iron line indicating the presence of an accretion disk are found in the spectrum, and the hydrogen column density (NH) is compatible with being the same in both observations and consistent with the interstellar value. 2005 Chandra observations show a hard X-ray spectrum, and possibly high fluxes. The Halpha EW shows yearly variations of a ~ 10%, and does not seem to be correlated with X-ray fluxes obtained at similar phases, unlike it would be expected in the wind accretion scenario. 2005 XMM-Newton and Chandra observations are consistent with 2003 RXTE/PCA results. The constancy of the NH seems to imply that either the X-ray emitter is located at >~ 1012 cm from the compact object, or the real N_H is 3-27 times smaller than the one predicted by a spherical symmetric wind model. We suggest that the multiwavelength non-thermal emission of LS 5039 is related to the radio jets and unlikely produced inside the binary system.

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arXiv2007

Chandra Observations of the Gamma-ray Binary LSI+61303: Extended X-ray Structure?

We present a 50 ks observation of the gamma-ray binary LSI+61303 carried out with the ACIS-I array aboard the Chandra X-ray Observatory. This is the highest resolution X-ray observation of the source conducted so far. Possible evidence of an extended structure at a distance between 5 and 12 arcsec towards the North of LSI+61303 have been found at a significance level of 3.2 sigma. The asymmetry of the extended emission excludes an interpretation in the context of a dust-scattered halo, suggesting an intrinsic nature. On the other hand, while the obtained source flux, of F_{0.3-10 keV}=7.1^{+1.8}_{-1.4} x 10^{-12} ergs/cm^2/s, and hydrogen column density, N_{H}=0.70+/-0.06 x 10^{22} cm^{-2}, are compatible with previous results, the photon index Gamma=1.25+/-0.09 is the hardest ever found. In light of these new results, we briefly discuss the physics behind the X-ray emission, the location of the emitter, and the possible origin of the extended emission ~0.1 pc away from LSI+61303.

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arXiv2007

Kinematics of Black Hole X-ray Binary GRS 1915+105

The space velocity of a stellar black hole encodes the history of its formation and evolution. Here we measure the 3-dimensional motion of the microquasar GRS 1915+105, using a decade of astrometry with the NRAO Very Long Baseline Array, together with the published radial velocity. The velocity in the Galactic Plane deviates from circular rotation by 53-80 +_ 8 km/s, where the range covers any specific distance from 6-12 kpc. Perpendicular to the plane, the velocity is only 10 +_ 4 km/s. The peculiar velocity is minimized at a distance 9-10 kpc, and is then nearly in the radial direction towards the Galactic Center. We discuss mechanisms for the origin of the peculiar velocity, and conclude that it is most likely a consequence of Galactic velocity diffusion on this old binary, rather than the result of a supernova kick during the formation of the 14 Mo black hole. Finally, a brief comparison is made with 4 other BH binaries whose kinematics are well determined.

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arXiv2007

A Multi-wavelength study of 1WGA J1346.5-6255: a new $γ$ Cas analog unrelated to the background supernova remnant G309.2-00.6

1WGA J1346.5-6255 is a ROSAT X-ray source found within the radio lobes of the supernova remnant (SNR) G309.2-00.6. This source also appears to coincide with the bright and early-type star HD 119682, which is in the middle of the galactic open cluster NGC 5281. The radio morphology of the remnant, consisting of two brightened and distorted arcs of emission on opposite sides of the 1WGA J1346.5-6255 source and of a jet-like feature and break in the shell, led to the suggestion that 1WGA J1346.5-6255/G309.2-00.6 is a young analog of the microquasar SS 433 powering the W50 nebula. This motivated us to study this source at X-ray and optical wavelengths. We here present new Chandra observations of 1WGA J1346.5-6255, archival XMM-Newton observations of G309.2-00.6, and optical spectroscopic observations of HD 119682, in order to search for X-ray jets from 1WGA J1346.5-6255, study its association with the SNR, and test for whether HD 119682 represents its optical counterpart. We do not find evidence for jets from 1WGA J1346.5-6255 down to an unabsorbed flux of 2.6E-13 ergs/cm2/s (0.5-7.5 keV), we rule out its association with G309.2-00.6, and we confirm that HD 119682 is its optical counterpart. We derive a distance of 1.2+/-0.3 kpc, which is consistent with the distance estimate to NGC 5281 (1.3+/-0.3 kpc), and much smaller than the distance derived to the SNR G309.2-00.6. We discuss the nature of the source, unveil that HD 119682 is a Be star and suggest it is a new member of the recently proposed group of $γ$-Cas analogs. The Chandra and XMM X-ray lightcurves show variability on timescales of hundreds of seconds, and the presence of a possible period of about 1500 s that could be the rotational period of an accreting neutron star or white dwarf in this $γ$-Cas analog.

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arXiv2006

Simultaneous multiwavelength observations of the Low/Hard State of the X-ray transient source SWIFT J1753.5-0127

We report the results of simultaneous multiwavelength observations of the X-ray transient source SWIFT J1753.5-0127 performed with INTEGRAL, RXTE, NTT, REM and VLA on 2005 August 10-12. The source, which underwent an X-ray outburst since 2005 May 30, was observed during the INTEGRAL Target of Opportunity program dedicated to new X-ray novae located in the Galactic Halo. Broad-band spectra and fast timing variability properties of SWIFT J1753.5-0127 are analyzed together with the optical, near infra-red and radio data. We show that the source was significantly detected up to 600 keV with Comptonization parameters and timing properties typical of the so-called Low/Hard State of black hole candidates. We build a spectral energy distribution and we show that SWIFT J1753.5-0127 does not follow the usual radio/X-ray correlation of X-ray binaries in the Low/Hard State. We give estimates of distance and mass. We conclude that SWIFT J1753.5-0127 belongs to the X-ray nova class and that it is likely a black hole candidate transient source of the Galactic Halo which remained in the Low/Hard State during its main outburst. We discuss our results within the context of Comptonization and jet models.

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arXiv2006

INTEGRAL serendipitous detection of the gamma-ray microquasar LS 5039

LS 5039 is the only X-ray binary persistently detected at TeV energies by the Cherenkov HESS telescope. It is moreover a gamma-ray emitter in the GeV and possibly MeV energy ranges. To understand important aspects of jet physics, like the magnetic field content or particle acceleration, and emission processes, such as synchrotron and inverse Compton (IC), a complete modeling of the multiwavelength data is necessary. LS 5039 has been detected along almost all the electromagnetic spectrum thanks to several radio, infrared, optical and soft X-ray detections. However, hard X-ray detections above 20 keV have been so far elusive and/or doubtful, partly due to source confusion for the poor spatial resolution of hard X-ray instruments. We report here on deep (300 ksec) serendipitous INTEGRAL hard X-ray observations of LS 5039, coupled with simultaneous VLA radio observations. We obtain a 20-40 keV flux of 1.1 +/- 0.3 mCrab (5.9 (+/-1.6) X 10^{-12} erg cm^{-2} s^{-1}), a 40-100 keV upper limit of 1.5 mCrab (9.5 x 10^{-12} erg cm^{-2}s^{-1}), and typical radio flux densities of about 25 mJy at 5GHz. These hard X-ray fluxes are significantly lower than previous estimates obtained with BATSE in the same energy range but, in the lower interval, agree with extrapolation of previous RXTE measurements. The INTEGRAL observations also hint to a break in the spectral behavior at hard X-rays. A more sensitive characterization of the hard X-ray spectrum of LS 5039 from 20 to 100 keV could therefore constrain key aspects of the jet physics, like the relativistic particle spectrum and the magnetic field strength. Future multiwavelength observations would allow to establish whether such hard X-ray synchrotron emission is produced by the same population of relativistic electrons as those presumably producing TeV emission through IC.

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arXiv2006

Multi-wavelength properties of the high-energy bright Seyfert 1 galaxy IGR J18027-1455

A new sample of hard X-ray sources in the Galactic Plane is being revealed by the regular observations performed by the INTEGRAL satellite. The full characterization of these sources is mandatory to understand the hard X-ray sky. Here we report new multifrequency radio, infrared and optical observations of the source IGR J18027-1455, as well as a multi-wavelength study from radio to hard X-rays. The radio counterpart of IGR J18027-1455 is not resolved at any observing frequency. The radio flux density is well fitted by a simple power law with a spectral index alpha=-0.75+/-0.02. This value is typical of optically thin non-thermal synchrotron emission originated in a jet. The NIR and optical spectra show redshifted emission lines with z=0.034, and a broad Halpha line profile with FWHM ~3400 km/s. This suggests an Active Galactic Nucleus (AGN) of type 1 as the optical counterpart of IGR J18027-1455. We confirm the Seyfert 1 nature of the source, which is intrinsically bright at high energies both in absolute terms and when scaled to a normalized 6 cm luminosity. Finally, comparing its X-ray luminosity with isotropic indicators, we find that the source is Compton thin and AGN dominated. This indicates that INTEGRAL might have just seen the tip of the iceberg, and several tens of such sources should be unveiled during the course of its lifetime.

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arXiv2006

Wind accretion in the massive X-ray binary 4U 2206+54: abnormally slow wind and a moderately eccentric orbit

Massive X-ray binaries are usually classified depending on the properties of the donor star in classical, supergiant and Be X-ray binaries. The massive X-ray binary 4U 2206+54 does not fit in any of these groups, and deserves a detailed study to understand how the transfer of matter and the accretion on to the compact object take place. To this end we study an IUE spectrum of the donor and obtain a wind terminal velocity (v_inf) of ~350 km/s, which is abnormally slow for its spectral type. We also analyse here more than 9 years of available RXTE/ASM data. We study the long-term X-ray variability of the source and find it to be similar to that observed in the wind-fed supergiant system Vela X-1, reinforcing the idea that 4U 2206+54 is also a wind-fed system. We find a quasi-period decreasing from ~270 to ~130 d, noticed in previous works but never studied in detail. We discuss possible scenarios and conclude that long-term quasi-periodic variations in the mass-loss rate of the primary are probably driving such variability in the measured X-ray flux. We obtain an improved orbital period of 9.5591 d with maximum X-ray flux at MJD 51856.6. Our study of the orbital X-ray variability in the context of wind accretion suggests a moderate eccentricity around 0.15. Moreover, the low value of v_inf solves the long-standing problem of the relatively high X-ray luminosity for the unevolved nature of the donor, BD +53 2790, which is probably an O9.5 V star. We note that changes in v_inf and/or the mass-loss rate of the primary alone cannot explain the diferent patterns displayed by the orbital X-ray variability. We finally emphasize that 4U 2206+54, together with LS 5039, could be part of a new population of wind-fed HMXBs with main sequence donors, the natural progenitors of supergiant X-ray binaries. (Abridged)

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arXiv2005

A possible black hole in the gamma-ray microquasar LS 5039

The population of high energy and very high energy gamma-ray sources, detected with EGRET and the new generation of ground-based Cherenkov telescopes, conforms a reduced but physically important sample. Most of these sources are extragalactic (e.g., blazars), while among the galactic ones there are pulsars and SN remnants. The microquasar LS 5039, previously proposed to be associated with an EGRET source by Paredes et al. (2000), has recently been detected at TeV energies, confirming that microquasars should be regarded as a class of high energy gamma-ray sources. To model and understand how the energetic photons are produced and escape from LS 5039 it is crucial to unveil the nature of the compact object, which remains unknown. Here we present new intermediate-dispersion spectroscopy of this source which, combined with values reported in the literature, provides an orbital period of 3.90603+/-0.00017 d, a mass function f(M)=0.0053+/-0.0009 M_sun, and an eccentricity e=0.35+/-0.04. Atmosphere model fitting to the spectrum of the optical companion, together with our new distance estimate of d=2.5+/-0.1 kpc, yields R_opt=9.3+0.7-0.6 R_sun, log (L_opt/L_sun)=5.26+/-0.06, and M_opt=22.9+3.4-2.9 M_sun. These, combined with our dynamical solution and the assumption of pseudo-synchronization, yield an inclination i=24.9+/-2.8 degree and a compact object mass M_X=3.7+1.3-1.0 M_sun. This is above neutron star masses for most of the standard equations of state and, therefore, we propose that the compact object in LS 5039 is a black hole. We finally discuss about the implications of our orbital solution and new parameters of the binary system on the CNO products, the accretion/ejection energetic balance, the SN explosion scenario, and the behaviour of the TeV emission with the new orbital period.

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arXiv2005

Orbital X-Ray Variability of the Microquasar LS 5039

The properties of the orbit and the donor star in the high mass X-ray binary microquasar LS 5039 indicate that accretion processes should mainly occur via a radiatively driven wind. In such a scenario, significant X-ray variability would be expected due to the eccentricity of the orbit. The source has been observed at X-rays by several missions, although with a poor coverage that prevents to reach any conclusion about orbital variability. Therefore, we conducted RossiXTE observations of the microquasar system LS 5039 covering a full orbital period of 4 days. Individual observations are well fitted with an absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line emission that is probably a diffuse background feature. In addition, we have taken into account that the continuum is also affected by significant diffuse background contamination. Our results show moderate power-law flux variations on timescales of days, as well as the presence of miniflares on shorter timescales. The new orbital ephemeris of the system recently obtained by Casares et al. have allowed us to show, for the first time, that an increase of emission is seen close to the periastron passage, as expected in an accretion scenario. Moreover, the detected orbital variability is a factor of ~4 smaller than the one expected by using a simple wind accretion model, and we suggest that an accretion disk around the compact object could be responsible for this discrepancy. On the other hand, significant changes in the photon index are also observed clearly anti-correlated with the flux variations. We interpret the overall X-ray spectral characteristics of LS 5039 in the context of X-ray radiation produced by inverse Compton and/or synchrotron processes in the jet of this microquasar.

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arXiv2005

Further evidence for the presence of a neutron star in 4U 2206+54. INTEGRAL and VLA observations

The majority of High Mass X-ray Binaries (HMXBs) behave as X-ray pulsars, revealing that they contain a magnetised neutron star. Among the four HMXBs not showing pulsations, and that do not show the characteristics of accreting black holes, there is the unusual HMXB 4U 2206+54. Here we present contemporaneous high-energy and radio observations of this system conducted with INTEGRAL and the VLA in order to unveil its nature. The high-energy spectra show clear indications of the presence of an absorption feature at ~32 keV. This is the third high-energy observatory which reveals marginal evidence of this feature, giving strong support to the existence of a cyclotron resonance scattering feature, which implies a magnetic field of 3.6 x 10^12 G. On the other hand, the source is not detected at centimetre radio wavelengths with a 3 sigma upper limit of 0.039 mJy. The expected radio emission for an accreting black hole in the low/hard state, inferred from X-ray flux measurements, would be at least 60 times greater than the measured upper limit. Both results firmly indicate that, in spite of the absence of pulsations, 4U 2206+54 hosts a magnetic accreting neutron star, the first one not to be observed as an X-ray pulsar.

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arXiv2004

The asymmetric compact jet of GRS 1915+105

We present multiepoch VLBA observations of the compact jet of GRS 1915+105 conducted at 15.0 and 8.4 GHz during a {\it plateau} state of the source in 2003 March-April. These observations show that the compact jet is clearly asymmetric. Assuming an intrinsically symmetric continuous jet flow, using Doppler boosting arguments and an angle to the line of sight of $θ=70\degr$, we obtain values for the velocity of the flow in the range 0.3--0.5$c$. These values are much higher than in previous observations of such compact jet, although much lower than the highly relativistic values found during individual ejection events. These preliminary results are compatible with current ideas on the jet flow velocity for black holes in the low/hard state.

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arXiv2004

Optical polarimetric observations of the microquasar LS 5039

We present the first optical polarimetric observations of the runaway microquasar LS 5039. Our results reveal the presence of a large amount (~5%) of polarized emission towards this binary system. By combining polarimetric and spectroscopic observations of some stars in the field together with available statistical information on the galactic interstellar polarization of the region, we have estimated and subtracted the contribution of the interstellar polarization in this direction. As a result, we obtain an intrinsic polarization of ~3% for the object, much higher than what would be expected from jet emission in the optical domain. We suggest that the polarized light originates by electron Thomson scattering in the stellar envelope of the companion star. This allows us to constrain the size of the region where the polarized emission originates, as well as estimating the scattering electronic density and the wind velocity at such distance.

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