arXiv subjects

Marc Ribó

Publications and source records attributed to Marc Ribó.

17 recordsLinked to original sources

arXiv2025

LST-1 follow-up of the exceptionally bright gamma-ray burst GRB 221009A

On 9 October 2022, the brightest gamma-ray burst (GRB) ever recorded (GRB 221009A) was detected. Its remarkably bright emission, partially due to its close distance to Earth ($z\sim0.15$), makes this GRB a unique event. The outstanding characteristics of GRB 221009A, including the TeV detection by the LHAASO experiment, triggered deep follow-up observations of the source across all wavebands, including very-high-energy gamma rays with the first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory. LST-1 observations started about one day after the onset of the prompt emission, under strong moonlight conditions. This resulted in a hint of a signal with a statistical significance of about 4$σ$. The monitoring of this source continued until the end of November 2022. This constitutes the deepest observation campaign performed on a GRB with LST-1. Here we show the results of this follow-up campaign.

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arXiv2025

LST-1 observations of GRB 221009A: Insights into its late-time VHE afterglow

Gamma-ray bursts (GRBs) originate from explosions at cosmological distances, generating collimated jets. GRB 221009A, exploded on 9 October 2022, has been established as the brightest GRB to date. Its bright and long emission was extensively followed up from radio to gamma rays. LHAASO firmly detected the onset of the afterglow emission at energies up to $\sim$13 TeV within about an hour after the burst, starting just a few minutes after the trigger. While this VHE emission component can be accounted for in a narrow jet scenario, such an interpretation cannot reproduce the broadband emission observed at later times, which exceeds the theoretical expectations. This discrepancy can be settled if more complex models are considered, providing the first strong evidence for a structured jet in a long GRB. Unfortunately, the VHE emission after a few hours is poorly constrained, as sensitive VHE observations by Cherenkov Telescopes were prevented due to strong moonlight conditions. The first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory began observations about one day after the burst under high night sky background conditions. These observations are the first ones performed on GRB 221009A by a Cherenkov telescope, revealing a hint of a signal with a statistical significance of about 4$σ$ during the observations performed at 1.3 days after the burst. The monitoring campaign continued until the end of November 2022, making it the deepest observation campaign performed on a GRB with the LST-1. In this contribution, we will present the analysis results of the LST-1 observation campaign on GRB 221009A in October 2022.

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arXiv2025

Transient Observations with LST-1: Key Results and Future Prospects

The recent detections of the afterglow phase of long gamma-ray bursts (lGRBs) at very high energies (VHE, >100 GeV) mark a significant advance in astrophysics of transient phenomena, offering deeper insights into the acceleration mechanisms, jet structure, and physical processes driving GRB emission. In the multi-messenger landscape, both high-energy neutrino and gravitational wave detections are providing new insights into the physics of extreme cosmic accelerators and highlighting the need for rapid and broadband follow-up observations. The Large-Sized Telescope (LST-1), the first telescope of the LST array, part of the Cherenkov Telescope Array Observatory (CTAO) North site, is particularly well-suited for real-time, rapid follow-up of transients. In this contribution, we present the latest achievements of the transient observational program with LST-1, which is now in advanced commissioning on La Palma, Canary Islands. We outline the observational strategies in place and describe the dynamic handling of events by the transient handler of LST-1 (e.g., its ability to handle poorly localised events, including gravitational waves, GRBs and neutrinos). We present the key results from transient observation campaigns conducted so far, discuss the lessons learned, and outline the promising prospects for the future LST-1+MAGIC combined transient program with fast response, via a Transient Handler.

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arXiv2024

NIR spectral classification of the companion in the gamma-ray binary HESS J1832-093 as an O6 V star

HESS J1832-093 is a member of the rare class of gamma-ray binaries, as recently confirmed by the detection of orbitally modulated X-ray and gamma-ray emission with a period of ~86 d. The spectral type of the massive companion star has been difficult to retrieve as there is no optical counterpart, but the system is coincident with a near-infrared source. Previous results have shown that the infrared counterpart is consistent with an O or B type star, but a clear classification is still lacking. We observed the counterpart twice, in 2019 and 2021, with the X-Shooter spectrograph operating on the VLT. The obtained spectra classify the counterpart as an O6 V type star. We estimate a distance to the source of $6.7 \pm 0.5$ kpc, although this estimate can be severely affected by the high extinction towards the source. This new O6 V classification for the companion star in HESS J1832-093 provides further support to an apparent grouping around a given spectral type for all discovered gamma-ray binaries that contain an O-type star. This may be due to the interplay between the initial mass function and the wind-momentum-luminosity relation.

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arXiv2019

Following up Transient Sources at Very High Energies with MAGIC

Several classes of sources are known to emit different messengers. Among them, transient sources are a special case, due to their serendipitous occurrence, time variability and duration on different timescales. They are associated with explosive and catastrophic events where very compact objects like neutron stars and black holes are involved. The difficulty of observing such elusive and possibly short-lasting events requires a fast reaction and a well-organized alert network between different experiments. In order to characterize them in the best possible way, instruments with a wide field of view should serve as external triggers for facilities with small sky coverage. MAGIC, as a Cherenkov telescope, belongs to the latter category. The search for transients by MAGIC is possible thanks to an automatic alert system listening to the alerts sent by the Gamma-ray Coordinate Network (GCN). In this contribution we describe the MAGIC alert system, which was designed mainly for the follow-up of Gamma-Ray Bursts in its initial conception. The alert system was recently updated in a multi-messenger context, receiving alerts also from neutrino and GW observatories. Finally we will present the MAGIC program for transient sources and how it was adapted in the current multi-wavelength and multi-messenger panorama.

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arXiv2018

The radio jets of SS 433 at millimetre wavelengths

Context. SS 433 is historically a well-known microquasar in the Galaxy that has been deeply studied during the four decades elapsed since its discovery. However, observations at very high radio frequencies with good angular resolution are still very scarce in the literature. The present paper tries to partially fill this gap using archival data of the source obtained with the Atacama Large Millimeter Array (ALMA). Aims. We aim to study the SS 433 jet properties at radio frequencies corresponding to millimetre wavelengths where the synchrotron emitting particles are expected to lose their energy much faster than at lower frequencies of centimetre wavelengths. Results. A resolved view of the SS 433 radio core and jets is presented. In addition to spectral index and magnetic field measurements, we are able to estimate the age of the oldest visible ejecta still radiating significantly at millimetre wavelengths. By combining our findings with those of previous authors at lower frequencies, we confirm that the energy loss of the radiating electrons is dominated by adiabatic expansion instead of synchrotron radiative losses. In addition, we find suggestive evidence for the previously proposed period of slowed expansion within the first months of the ejecta flow, needed to simultaneously match the radiative lifetime observed in the centimetre domain. Our results argue for the need for future coordinated millimetre and centimetre interferometric observations with good time sampling throughout the SS 433 precessional cycle to better understand energetic processes in stellar relativistic jets.

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arXiv2018

VHE observations of binary systems performed with the MAGIC telescopes

The improvement on the Imaging Air Cherenkov Technique (IACT) led to the discovery of a new type of sources that can emit at very high energies: the gamma-ray binaries. Only six systems are part of this exclusive class. We summarize the latest results from the observations performed with the MAGIC telescopes on different systems as the gamma-ray binary LS I +61$^{\circ}$ 303 and the microquasars SS433, V404 Cygni and Cygnus X-1, which are considered potential VHE gamma-ray emitters. The binary system LS I +61$^{\circ}$ 303 has been observed by MAGIC in a long-term monitoring campaign. We show the newest results of our search for super-orbital variability also in context of contemporaneous optical observations. Besides, we will present the results of the only super-critical accretor known in our galaxy: SS 433. We will introduce the VHE results achieved with MAGIC after 100 hours of observations on the microquasar Cygnus X-1 and report on the microquasar V404 Cyg, which has been observed with MAGIC after it went through a series of exceptional X-ray outbursts in June 2015.

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arXiv2018

The geometric distance and binary orbit of PSR B1259-63

The pulsar/massive star binary system PSR B1259-63 / LS 2883 is one of the best-studied gamma-ray binaries, a class of systems whose bright gamma-ray flaring can provide important insights into high-energy physics. Using the Australian Long Baseline Array we have conducted very long baseline interferometric observations of PSR B1259-63 over 4.4 years, fully sampling the 3.4-year orbital period. From our measured parallax of $0.38\pm0.05$ mas we use a Bayesian approach to infer a distance of $2.6^{+0.4}_{-0.3}$ kpc. We find that the binary orbit is viewed at an angle of $154\pm3$ degrees to the line of sight, implying that the pulsar moves clockwise around its orbit as viewed on the sky. Taking our findings together with previous results from pulsar timing observations, all seven orbital elements for the system are now fully determined. We use our measurement of the inclination angle to constrain the mass of the stellar companion to lie in the range 15-31$M_{\odot}$. Our measured distance and proper motion are consistent with the system having originated in the Cen OB1 association and receiving a modest natal kick, causing it to have moved $\sim$8 pc from its birthplace over the past $\sim3\times10^5$ years. The orientation of the orbit on the plane of the sky matches the direction of motion of the X-ray synchrotron-emitting knot observed by the Chandra X-ray Observatory to be moving away from the system.

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arXiv2017

MAGIC VHE Gamma-Ray Observations Of Binary Systems

There are several types of Galactic sources that can potentially accelerate charged particles up to GeV and TeV energies. We present here the results of our observations of the source class of gamma-ray binaries and the subclass of binary systems known as novae with the MAGIC telescopes. Up to now novae were only detected in the GeV range. This emission can be interpreted in terms of an inverse Compton process of electrons accelerated in a shock. In this case it is expected that protons in the same conditions can be accelerated to much higher energies. Consequently they may produce a second component in the gamma-ray spectrum at TeV energies. The focus here lies on the four sources: nova V339 Del, SS433, LS I +61 303 and V404 Cygni. The binary system LS I +61 303 was observed in a long-term monitoring campaign for 8 years. We show the newest results on our search for superorbital variability, also in context with contemporaneous optical observations. Furthermore, we present the observations of the only super-critical accretion system known in our galaxy: SS433. Finally, the results of the follow-up observations of the microquasar V404 Cygni during a series of outbursts in the X-ray band and the ones of the nova V339 Del will be discussed in these proceedings.

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arXiv2016

Peculiar objects towards 3FGL J0133.3+5930: an eclipsing Be star and an active galactic nucleus

Aims. We aim to contribute to the identification of unassociated gamma-ray sources in the galactic plane in order to enlarge the currently known population of gamma-ray binaries and related systems, such as radio emitting X-ray binaries and microquasars. These objects are currently regarded as excellent test beds for the understanding of high energy phenomena in stellar systems. Methods. Potential targets of study are selected based on cross-identification of the 3rd Fermi Large Area Telescope catalogue with historical catalogues of luminous stars often found as optical counterparts in known cases. Follow-up observations and analysis of multi-wavelength archival data are later used to seek further proofs of association beyond the simple positional agreement. Results. Current results enable us to present here the case of the Fermi source 3FGL J0133.3+5930 where two peculiar objects have been discovered inside its region of uncertainty. One of them is the star TYC 3683-985-1 (LS I +59 79) whose eclipsing binary nature is reported in this work. The other one is the X-ray source SWIFT J0132.9+5932, that we found to be a likely low-power Active Galactic Nucleus at z = 0:1143 \pm 0:0002. If this second object is of blazar type, it could easily account for the observed gamma-ray photon flux. However, this is not confirmed at present, thus rendering still open the star system TYC 3683-985-1 as an alternative counterpart candidate to the Fermi source.

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arXiv2015

Simulations of an inhomogeneous stellar wind interacting with a pulsar wind in a binary system

Binary systems containing a massive star and a non-accreting pulsar present strong interaction between the stellar and the pulsar winds. The properties of this interaction, which largely determine the non-thermal radiation in these systems, strongly depend on the structure of the stellar wind, which can be clumpy or strongly anisotropic, as in Be stars. We study numerically the influence of inhomogeneities in the stellar wind on the structure of the two-wind interaction region. We carried out for the first time axisymmetric, relativistic hydrodynamical simulations, with Lorentz factors of ~6 and accounting for the impact of instabilities, to study the impact in the two-wind interaction structure of an over-dense region of the stellar wind. We also followed the evolution of this over-dense region or clump as it faces the impact of the pulsar wind. For typical system parameters, and adopting a stellar wind inhomogeneity with a density contrast >~10, clumps with radii of a few percent of the binary size can significantly perturb the two-wind interaction region, pushing the two-wind interface to <~40% of the initial distance to the pulsar. After it is shocked, the inhomogeneity quickly expands and is disrupted when it reaches the smallest distance to the pulsar. It eventually fragments, being advected away from the binary system. The whole interaction region is quite unstable, and the shocked pulsar wind can strongly change under small perturbations. We confirm the sensitive nature of the two-wind interaction structure to perturbations, in particular when the stellar wind is inhomogeneous. For realistic over-dense regions of the stellar wind, the interaction region may shrink by a factor of a few, with the shocked flow presenting a complex spatial and temporal pattern. This can lead to strong variations in the non-thermal radiation.

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arXiv2013

Periodic radio morphology of gamma-ray binaries

Gamma-ray binaries allow us to study physical processes such as particle acceleration up to very-high energies and gamma-ray emission and absorption with changing geometrical configurations on a periodic basis. These sources produce outflows of radio-emitting particles whose structure can be imaged with Very Long Baseline Interferometry (VLBI). We have studied the changing morphology of the gamma-ray binaries LS I +61 303 and LS 5039, and we have discovered the extended emission of PSR B1259-63 and HESS J0632+057. Based on these results, we have established the basic properties and behaviour of the radio emission of gamma-ray binaries on AU scales, and we have contributed to find characteristics that are common to all of them. Here we present the most relevant properties of each source and the general properties of gamma-ray binaries, and we describe the implications on the nature of these binary systems.

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arXiv2012

First LOFAR Observations of Gamma-Ray Binaries

A few binary systems display High Energy (100 MeV - 100 GeV) and/or Very High Energy (> 100 GeV) gamma-ray emission. These systems also display non-thermal radio emission that can be resolved with long-baseline radio interferometers, revealing the presence of outflows. It is expected that at very low frequencies the synchrotron radio emission covers larger angular scales than has been reported up to now. Here we present preliminary results of the first deep radio observations of the gamma-ray binary LS I +61 303 with LOFAR, which is sensitive to extended structures on arcsecond to arcminute scales.

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arXiv2012

Optical photometric monitoring of gamma-ray binaries

Four gamma-ray binaries, namely PSR B1259-63, HESS J0632+057, HD 215227 and LS I +61 303, contain compact objects orbiting around massive Be stars. The nature of the compact object is only known in the case of PSR B1259-63, but the other systems could also contain young non-accreting pulsars with relativistic winds. Around periastron passage the compact objects should produce significant changes in the structure of the Be discs due to gravitational forces and eventually by ram pressure from the putative pulsar wind. Indeed, variability in the Halpha emission line has been detected in all these systems, and periodic variability in the optical photometry has been detected in two of them. However, there is lack of a systematic monitoring with accurate photometry, which could be used to constrain the shape of the disc during the periastron passage. This information is important to build accurate physical models to explain the broadband spectral energy distribution of these sources. Here we present an ongoing program to monitor the optical photometry of gamma-ray binaries and we show preliminary results for the case of HD 215227.

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arXiv2011

Search for VHE signals from microquarsars with MAGIC

Microquasars are accreting binary systems displaying relativistic radio jets where very high energy gamma rays might be produced via Inverse Compton scattering. The detection of the microquasar Cygnus X-3 above 100 MeV by both Fermi/LAT and AGILE satellites, together with the short flares from Cygnus X-1 reported by AGILE, confirmed that this class of astrophysical objects are interesting candidates for very high energy gamma-ray observations. The stand-alone imaging atmospheric Cherenkov MAGIC telescope, and later on the recently inaugurated stereoscopic system, made a significant effort to search for signals from microquasars. This paper reviews all MAGIC results of Cygnus X-3, Cygnus X-1, Scorpius X-1, and SS 433 observations. The stand-alone MAGIC telescope observed Cygnus X-3 for almost 60 hrs from March 2006 until August 2009 in many different X-ray spectral states where a very high energy emission is predicted and also simultaneously with a flux enhancement at high energies detected by Fermi/LAT. No significant signal was found in any of the observed conditions. The MAGIC stereoscopic system pointed at the Z-type low-mass X-ray binary Scorpius X-1 in May 2010 for ~8 hrs. Simultaneous soft X-ray measurements allowed to define the X-ray spectral state of the source which could emit very-high-energy photons in the horizontal branch. MAGIC did not detect the source and put some constraints on the maximum TeV luminosity to jet power ratio. Further observations of Cygnus X-1 were also carried out with the new and twice as sensitive stereoscopic system in Autumn 2009 for a total amount of 30 hrs. Finally, we also report on SS 433 observations obtained during two periods of minimum absorption processes from the accretion disk.

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arXiv2011

Gamma Ray Signal from the Pulsar Wind in the Binary Pulsar system PSR B1259-63/LS2883

Binary pulsar systems emit potentially detectable components of gamma ray emission due to Comptonization of the optical radiation of the companion star by relativistic electrons of the pulsar wind, both before and after termination of the wind. The recent optical observations of binary pulsar system PSR B1259-63/LS 2883 revealed radiation properties of the companion star which differ significantly from previous measurements. In this paper we study the implications of these observations for the interaction rate of the unshocked pulsar wind with the stellar photons and the related consequences for fluxes of high energy (HE) and very high energy (VHE) gamma rays. We show that the signal should be strong enough to be detected with Fermi close to the periastron passage, unless the pulsar wind is strongly anisotropic or the Lorentz factor of the wind is smaller than $10^3$ or larger that $10^5$. The higher luminosity of the optical star also has two important implications: (i) attenuation of gamma rays due to photon-photon pair production, and (ii) Compton drag of the unshocked wind. While the first effect has an impact on the lightcurve of VHE gamma rays, the second effect may significantly decrease the energy available for particle acceleration after termination of the wind.

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arXiv2011

Astrophysical parameters of LS2883 and implications for the PSR B1259-63 gamma-ray binary

Only a few binary systems with compact objects display TeV emission. The physical properties of the companion stars represent basic input to understand the physical mechanisms behind the particle acceleration, emission, and absorption processes in these so-called gamma-ray binaries. Here we present high-resolution and high signal-to-noise optical spectra of LS2883, the Be star forming a gamma-ray binary with the young non-accreting pulsar PSR B1259-63, showing it to rotate faster and be significantly earlier and more luminous than previously thought. Analysis of the interstellar lines suggest that the system is located at the same distance as (and thus is likely a member of) CenOB1. Taking the distance to the association, d=2.3kpc, and a color excess of E(B-V)=0.85 for LS2883, results in Mv=-4.4. Because of fast rotation, LS2883 is oblate (R_eq=9.7R_sun and R_pole=8.1R_sun) and presents a temperature gradient (T_eq=27500K, log g_eq=3.7; T_pole=34000K, log g_pole=4.1). If the star did not rotate, it would have parameters corresponding to a late O-type star. We estimate its luminosity at log(L*/L_sun)=4.79, and its mass at M=30M_sun. The mass function then implies an inclination of the binary system i_orb=23deg,slightly smaller than previous estimates. We discuss the implications of these new astrophysical parameters of LS2883 for the production of high energy and very high energy gamma rays in the PSR B1259-63/LS2883 gamma-ray binary system. In particular, the stellar properties are very important for prediction of the line-like bulk Comptonization component from the unshocked ultra-relativistic pulsar wind.

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