arXiv subjects

Josep Martí

Publications and source records attributed to Josep Martí.

At least 19 recordsLinked to original sources

arXiv2026

A Galactic microblazar as a potential accelerator of ultra-high-energy particles

Context. Persistent jets from X-ray binaries which are aligned very close to the line of sight could be considered to be Galactic equivalents of blazars, or 'microblazars'. They are also expected to power gamma-ray sources. Aims. We intend to assess a serious candidate apparently fulfilling many of the requirements to be considered a genuine member of this class: IRAS 18293-0941. Methods. An intense multi-wavelength observational and theoretical study has been carried out on our proposed candidate source. Results. With photometric and spectroscopic properties typical of a binary star, this system exhibits clear collimated and one-sided radio emission matching the effects of relativistic motion along a reduced ejection angle. Only fast variability is not observed possibly smoothed by a dense circumstellar envelope. A physical scenario is consistently modeled that also gives credibility to its likely connection with LHAASO J1831-1007u*, an ultra-high-energy source in its immediate vicinity. Conclusions. Our reported identification not only helps to fill a gap in Galactic taxonomy, but also potentially strengthens the role of the microblazar and microquasar families in our understanding of the most energetic Milky Way phenomena.

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arXiv2026

Constraints on cyclotron features and accretion regime in the high-mass X-ray binary 4U 1700-37 from NuSTAR

4U 1700-37 is a wind-fed high-mass X-ray binary hosting a compact object, likely a neutron star, accreting from O6.5 Iaf+ supergiant HD 153919. Coherent pulsations not firmly detected; magnetic field strength remains uncertain. We analyze NuSTAR observations to characterize hard X-ray timing and spectral properties, test robustness of candidate cyclotron features, and constrain magnetic field and accretion regime. We perform timing and spectral analysis of two observations, modeling spectra with continua used for accreting pulsars, and use simulations to assess significance of features. No coherent pulsations detected; pulsed fraction constrained below 1.5\%. Spectra are well described by absorbed blackbody plus cutoff power-law continuum, showing residuals around 20 keV and 40--50 keV. Features improve fits but do not constitute firm cyclotron detections. Intensity-resolved spectroscopy suggests possible shifts of apparent line centroid. Results favor neutron-star magnetic field of 1.7--4.4 $\times 10^{12}$ G and quasi-spherical subsonic accretion regime with equilibrium spin period $\sim 1.9$ ks. Analysis provides quantitative constraints on magnetic field and accretion physics, helping reconcile discrepant line-energy measurements.

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arXiv2026

Unmasking LHAASO J2108+5157: Near Infrared Insights into a Mysterious TeV Source

LHAASO J2108+5157 is one of the few ultra-high energy gamma-ray sources in the LHAASO catalogue without secure counterpart at longer wavelengths. Several Galactic scenarios have been proposed, including an evolved supernova remnant and a pulsar wind nebula. Yet, no shocked gas, shell-like structure, or compact pulsar candidate has been identified. Follow-up observations with VERITAS and the LST-1 prototype have not firmly clarified its nature. A recent microquasar candidate from GMRT radio data remains uncertain. Here we present the first dedicated near-infrared study of the field, combining deep JHKs imaging with narrow band observations targeting the H2 v=1-0 S(1) line. Our observations were initially planned to encompass the full source region, but now only partially cover the latest updated position and size of LHAASO J2108+5157. We find no evidence of shocked emission, extended nebular structures, or an accreting compact object signature in the covered field. The GMRT radio source, despite its jet-like morphology, exhibits near-infrared properties incompatible with both a Galactic microquasar and a nearby radio galaxy, discouraging an association with the gamma-ray emission. Our analysis reveals no convincing counterpart consistent within the positional uncertainty, leaving LHAASO J2108+5157 as an enigmatic ultra-high energy emitter that requires deeper observations.

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arXiv2025

Alignment of radio jets in the microquasar V4641 Sagittarii with its high-energy structures

V4641 Sagittarii (V4641 Sgr) is a unique Galactic microquasar system featuring a stellar-mass black hole accreting matter from a massive companion. One of its intriguing features is the presence of relativistic radio jets almost perpendicular to the observed extended gamma-ray emission, implying significant propagation effects or interactions with the Galactic magnetic field. Here we report observational evidence that the radio jet and the very high-energy (VHE) and ultra high-energy (UHE) gamma-ray emission could be aligned along a common axis, indicating a co-spatial or co-directional origin. This alignment supports a model where synchrotron radio emission, VHE and UHE gamma rays are produced within a single, highly collimated relativistic outflow. Our findings favor scenarios of in-situ particle acceleration up to hundreds of TeV, challenge previous interpretations involving large-scale particle diffusion, and simplify the geometric modeling of the source. This case highlights the potential of V4641 Sgr as a PeVatron candidate within our Galaxy and provides a benchmark for understanding jet composition and magnetic structure in microquasars.

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arXiv2024

A radio loud semiregular variable

As a byproduct of our search for Galactic stellar systems with gamma-ray emission, we have identified an unrelated cool and evolved star (IRC-10412) that attracted our attention due to its strong radio emission level with a spectral index matching, almost perfectly, the canonical +0.6 value expected from an ionized stellar wind. A follow-up observational analysis was undertaken given that these two properties are hard to reconcile as originating in the same stellar object. As a result, IRC-10412 has been classified as a new semiregular variable of SRb type in the asymptotic giant branch, and different but consistent estimates of its mass-loss parameter are reported. We propose that its unusually high radio emission arises from a ~ 1E-5 Msun/yr stellar wind exposed to an external source of ionizing photons, possibly coming from nearby OB associations.

astro-ph.SR↗

arXiv2024

Simultaneous NICER and NuSTAR observations of the Ultraluminous source NGC 4190 ULX-1

We present an X-ray analysis of three different XMM-Newton observations together with simultaneous NICER and NuSTAR observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source. We performed a temporal and spectral analyses in the 0.4--30 keV energy range where the source is significantly detected in dedicated XMM-Newton, NICER and NuSTAR observations. The temporal analysis shows no flaring activity in the light curves. No pulsation is detected throughout. The source exhibits a typical ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity-temperature relation of each thermal spectral component is consistent with the $L \propto T^{2}$ relation expected from an advection-dominated supercritical disk. We interpret these results as a super-Eddington accreting black hole seen almost face-on. A dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of Comptonization of soft photons by the electrons ejected through the funnel.

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arXiv2023

A runaway T-Tauri star leaving an extended trail

Aims. We address the problem of young stellar objects that are found too far away from possible star formation sites. Different mechanisms have been proposed before to explain this unexpected circumstance. The idea of high-velocity protostars is one of these mechanisms, although observational support is not always easy to obtain. We aim to shed light on this issue after the serendipitous discovery of a related stellar system. Methods. Following the inspection of archival infrared data, a peculiar anonymous star was found that apparently heads a long tail that resembles a wake-like feature. We conducted a multiwavelength analysis including photometry, astrometry, and spectroscopy. Together with theoretical physical considerations, this approach provided a reasonable knowledge of the stellar age and kinematic properties, together with compelling indications that the extended feature is indeed the signature of a high-velocity, or runaway, newborn star. Results. Our main result is the discovery of a low-mass young stellar object that fits the concept of a runaway T-Tauri star that was hypothesized several decades ago. In this peculiar star, nicknamed UJT-1, the interaction of the stellar wind with the surrounding medium becomes extreme. Under reasonable assumptions, this unusual degree of interaction has the potential to encode the mass-loss history of the star on timescales of several $10^5$ years

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arXiv2023

Relativistic X-Ray Reflection and Photoionised Absorption in the Neutron-Star Low-Mass X-ray Binary GX 13+1

We analysed a dedicated NuSTAR observation of the neutron-star low-mass X-ray binary Z-source GX 13+1 to study the timing and spectral properties of the source. From the colour-colour diagram, we conclude that during that observation the source transitioned from the normal branch to the flaring branch. We fitted the spectra of the source in each branch with a model consisting of an accretion disc, a Comptonised blackbody, relativistic reflection (relxillNS), and photo-ionised absorption (warmabs). Thanks to the combination of the large effective area and good energy resolution of NuSTAR at high energies, we found evidence of relativistic reflection in both the Fe K line profile, and the Compton hump present in the 10--25 keV energy range. The inner disc radius is $R_{\rm in} \lesssim 9.6~r_g$, which allowed us to further constrain the magnetic field strength to $B \lesssim 1.8 \times 10^8$ G. We also found evidence for the presence of a hot wind leading to photo-ionised absorption of Fe and Ni, with a Ni overabundance of $\sim$6 times solar. From the spectral fits, we find that the distance between the ionising source and the slab of ionised absorbing material is $\sim 4-40 \times 10^5$ km. We also found that the width of the boundary layer extends $\sim$3 km above the surface of a neutron star, which yielded a neutron-star radius $R_{\rm NS}\lesssim 16$ km. The scenario inferred from the spectral modelling becomes self-consistent only for high electron densities in the accretion disk, $n_e \sim 10^{22}-10^{23}$ cm$^{-3}$, as expected for a Shakura-Sunyaev disc, and significantly above the densities provided by relxillNS models. These results have implications for our understanding of the physical conditions in GX 13+1.

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arXiv2022

A blazar candidate for the Fermi source 4FGL J1848.7-0129

The Fermi source 4FGL J1848.7-0129 has been historically related to the globular cluster GLIMPSE-C01 since its very first detection. Although this association is widely accepted, as it appears in the most recent Fermi catalog, it deserves to be revisited given the multi-wavelength evidences and the recent discovery of variable X-ray sources in the Fermi source region. In particular, low frequency radio maps from the Giant Metre Radio Telescope in Pune (India) have been carefully inspected which, together with X-ray data re-analysis from Chandra, lead us to get a deep insight into the candidates to be associated to 4FGL J1848.7-0129. This results in the discovery of a new X-ray variable point source coincident with an unreported non-thermal radio emitter, both of them well inside the 4FGL J1848.7-0129 error ellipse. We analyze and discuss all these observational facts, and we propose now a newly discovered blazar candidate as the most promising responsible for the gamma ray emission in the Fermi source. If confirmed, this result would set constrains on the number of millisecond pulsars in GLIMPSE-C01 or their gamma-ray emission properties.

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arXiv2022

A NuSTAR view of SS433: Precessional evolution of the jet-disk system

SS433 is a Galactic microquasar with powerful outflows (double jet, accretion disk and winds) with well known orbital, precessional and nutational period. In this work we characterise different outflow parameters throughout the precessional cycle of the system. We analyse 10 NuSTAR ($3-70$ keV) observations of $\sim$30~ks that span $\sim$1.5 precessional cycles. We extract averaged spectra and model them using a combination of a double thermal jet model (bjet) and pure neutral and relativistic reflection (xillverCp and relxilllpCp) over an accretion disk. We find an average jet bulk velocity of $β= v/c \sim0.29$ with an opening angle of $\lesssim$6~degrees. Eastern jet kinetic power ranges from 1 to $10^{39}$~erg/s, with base "coronal" temperatures $T_o$ ranging between 14 and 18 keV. Nickel to iron abundances remain constant at $\sim$9 (within 1$σ$). The western to eastern jet flux ratio becomes $\sim1$ on intermediate phases, about 35% of the total precessional orbit. The $3-70$ keV total unabsorbed luminosity of the jet and disk ranges from 2 to 20 $\times$10$^{37}$~erg/s, with the disk reflection component contributing mainly to the hard $20-30$ keV excess and the stationary 6.7 keV ionized Fe line complex. At low opening angles $Θ$ we find that the jet expands sideways following an adiabatic expansion of a gas with temperature $T_o$. Finally, the central source and lower parts of the jet could be hidden by an optically thick region of $τ> 0.1$ and size $R\sim N_H/n_{e0}\sim1.5\times10^9$~cm$\sim$1700~$r_g$ for $M_{BH}=3~M_{\odot}$

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arXiv2022

Testing source confusion and identification capability in Cherenkov Telescope Array data

The Cherenkov Telescope Array will provide the deepest survey of the Galactic Plane performed at very-high-energy gamma-rays. Consequently, this survey will unavoidably face the challenge of source confusion, i.e., the non-unique attribution of signal to a source due to multiple overlapping sources. Among the known populations of Galactic gamma-ray sources and given their extension and number, pulsar wind nebulae (PWNe, and PWN TeV halos) will be the most affected. We aim to probe source confusion of TeV PWNe in forthcoming CTA data. For this purpose, we performed and analyzed simulations of artificially confused PWNe with CTA. As a basis for our simulations, we applied our study to TeV data collected from the H.E.S.S. Galactic Plane Survey for ten extended and two point-like firmly identified PWNe, probing various configurations of source confusion involving different projected separations, relative orientations, flux levels, and extensions among sources. Source confusion, defined here to appear when the sum of the Gaussian width of two sources is larger than the separation between their centroids, occurred in $\sim$30% of the simulations. For this sample and 0.5$°$ of average separation between sources, we found that CTA can likely resolve up to 60% of those confused sources above 500 GeV. Finally, we also considered simulations of isolated extended sources to see how well they could be matched to a library of morphological templates. The outcome of the simulations indicates a remarkable capability (more than 95% of the cases studied) to match a simulation with the correct input template in its proper orientation.

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arXiv2022

Optical microflares in LS I +61 303 and the search for their multiwavelength counterpart

Stellar sources of gamma rays are one of the front lines in modern astrophysics whose understanding can benefit from observational tools not originally designed for their study. We take advantage of the high precision photometric capabilities of present-day space facilities to obtain a new perspective on the optical behavior of the X-ray and gamma-ray binary LS I +61 303. Previously unknown phenomena whose effects manifest with amplitudes below 0.01 magnitude can now be clearly observed and studied. Our work is mainly based on the analysis of optical and gamma-ray archival data and uses the tools recommended by the different collaborations that provide these valuable observational resources (in particular, the TESS and Fermi orbiting observatories). In addition, complementary ground-based optical spectroscopy has also been conducted. We report the discovery of small-amplitude optical flares on timescales of a day in the LS I +61 303 light curve. Different alternative scenarios to explain their origin are tentatively proposed.

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arXiv2021

Orbital and sub-orbital period determination of the candidate high-mass X-ray binary HD 3191

The final aim of this paper is to expand the sparse group of X-ray binaries with gamma-ray counterparts as laboratories to study high-energy processes under physical conditions that periodically repeat. A follow-up of a candidate system has been carried out. We applied both photometric and spectroscopic techniques in the optical domain together with a period analysis using the phase dispersion minimization and CLEAN methods. A tentative period search was also conducted in the gamma-ray domain. Our main result is having established the binary nature of the optical star and X-ray emitter HD 3191 towards the Fermi gamma-ray source 4FGL J0035.8+6131, the last one proposed to be associated with a blazar of an unknown type. An orbital period close to 16 days is reported for HD 3191 together with a likely rotation, or pulsation, period of about 0.6 d. Although no convincing evidence for the orbital cycle has been found in the Fermi light curve up to now, the confirmed presence of a high-mass X-ray binary towards 4FGL J0035.8+6131 now strengthens the need for caution about its true nature.

astro-ph.HE↗

arXiv2020

Effects of precession versus instabilities on the jets of GRS 1758-258

Aims. The prototypical microquasar GRS 1758-258 exhibits large-scale morphological changes in radio maps over time which have been attributed to the rise of instabilities. Here, we investigate whether these effects could be attributed to jet precession instead. Methods.We used new and archival radio maps to fit a kinematic jet precession model. The value of the parameters thus obtained were analysed in order to get constraints on the physical properties of the GRS 1758-258 system. Their consistency with different theories of the origins for the jet precession, such as Lense-Thirring effect and tidal torques induced by the secondary star, has previously been studied. Alternatively, we also assessed the possibility that observations are compatible with eventual jet instabilities. Results. The new radio data presented here confirm that the large-scale radio morphology of GRS 1758-258 is changing over time. Our study shows that the 18.48 day period could plausibly be ascribed to precession, thus implying a reinterpretation of assumptions made for the orbital period to date. However, the observed structural changes in radio jets cannot be easily attributed to jet precession according to our analysis. In contrast, the growth of instabilities certainly appears to be a more realistic explanation of these effects.

astro-ph.HE↗

arXiv2020

A blazar as the likely counterpart to 4FGL J0647.7-4418 instead of a gamma-ray binary

The persistent gamma-ray source 4FGL J0647.7-4418 is tentatively associated in the latest Fermi catalogue with the sub-dwarf O-type X-ray binary HD 49798. However, an AGN candidate is also mentioned as an alternative identification in updated versions of the catalogue accompanying paper. If the first association were correct, this would add HD 49798 to the handful of currently known gamma-ray binaries and, therefore, represent a significant breakthrough not only because of a new member addition, but also because of the apparent white dwarf companion in this system. Despite these perspectives, here we show that the stellar association is likely wrong and that the proposed AGN object, well inside the Fermi 95% confidence ellipse, is a more conceivable counterpart candidate to the Fermi source due to its strong blazar similarities.

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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

GRS 1758-258: the first winged microquasar

The family links between radio galaxies and microquasars have been strongly strengthened thanks to a new common phenomenon: the presence of extended winged features. The first detection of such structures in a Galactic microquasar, recently reported in Nature Communications (http://rdcu.be/zgX8), widens the already known analogy between both kinds of outflow sources (Martí et al. 2017). This observational result also has potential implications affecting the black hole merger scenarios that contribute to the gravitational wave background.

astro-ph.GA↗

arXiv2017

The Highly Collimated Radio Jet of HH 80-81: Structure and Non-Thermal Emission

Radio emission from protostellar jets is usually dominated by free-free emission from thermal electrons. However, in some cases, it has been proposed that non-thermal emission could also be present. This additional contribution from non-thermal emission has been inferred through negative spectral indices at centimeter wavelengths in some regions of the radio jets. In the case of HH 80-81, one of the most powerful protostellar jets known, linearly polarized emission has also been detected, revealing that the non-thermal emission is of synchrotron nature from a population of relativistic particles in the jet. This result implies that an acceleration mechanism should be taking place in some parts of the jet. Here, we present new high sensitivity and high angular resolution radio observations at several wavelengths (in the 3-20 cm range) of the HH80-81 radio jet. These new observations represent an improvement in sensitivity and angular resolution by a factor of $\sim$10 with respect to previous observations. This allows us to resolve the morphology of the radio jet, and to study the different emission mechanisms involved through spectral index maps. We conclude that synchrotron emission in this jet arises from an extended component detected at low frequencies and from the termination points of the jet, where strong shocks against the ambient medium can produce efficient particle acceleration.

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