arXiv subjects

Maria Massi

Publications and source records attributed to Maria Massi.

15 recordsLinked to original sources

arXiv2018

Simultaneous long-term monitoring of LS I +61{\deg}303 by OVRO and Fermi-LAT

Previous long-term monitorings of the gamma-ray-loud X-ray binary LS I +61{\deg}303 have revealed the presence of a long-term modulation of ~4.5 years. After nine years of simultaneous monitoring of LS I +61{\deg}303 by the Owens Valley Radio Observatory and the Fermi-LAT, two cycles of the long-term period are now available. Here we perform timing-analysis on the radio and the gamma-ray light curves. We confirm the presence of previously detected periodicities at both radio and GeV gamma-ray wavelengths. Moreover, we discover an offset of the long-term modulation between radio and gamma-ray data which could imply different locations of the radio (15 GHz) and GeV emission along the precessing jet.

astro-ph.HE↗

arXiv2018

Hour time-scale QPOs in the X-ray and radio emission of LS I +61$^{\circ}$303

LS I +61$^{\circ}$303 is an X-ray binary with a radio outburst every ~27 days. Previous studies of the stellar system revealed radio microflares superimposed on the large radio outburst. We present here new radio observations of LS I +61$^{\circ}$303 at 2.2 GHz with the Westerbork Synthesis Radio Telescope (WSRT). Using various timing analysis methods we find significant Quasi-Periodic Oscillations (QPOs) of 55 minutes stable over the duration of 4 days. We also use archival data obtained from the Suzaku satellite at X-ray wavelengths. We report here for the first time significant X-ray QPOs of about 2 hours present over the time span of 21 hours. We compare our results with the previously reported QPO observations and we conclude that the QPOs seem to be associated with the radio outburst, independent of the amplitude of the outburst. Finally, the different QPO time-scales are discussed in the context of magnetic reconnection.

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arXiv2017

Radio QPO in the ${\gamma}$-ray-loud X-ray binary LS I +61${^\circ}$303

LS I +61${^\circ}$303 is a $\gamma$-ray emitting X-ray binary with periodic radio outbursts with time scales of one month. Previous observations have revealed microflares superimposed on these large outbursts with periods ranging from a few minutes to hours. This makes LS I +61${^\circ}$303, along with Cyg X-1, the only TeV emitting X-ray binary exhibiting radio microflares. To further investigate these microflaring activity in LS I +61${^\circ}$303 we observed the source with the 100-m Effelsberg radio telescope at 4.85, 8.35, and 10.45 GHz and performed timing analysis on the obtained data. Radio oscillations of 15 hours time scales are detected at all three frequencies. We also compare the spectral index evolution of radio data to that of the photon index of GeV data observed by Fermi-LAT. We conclude that the observed QPO could result from multiple shocks in a jet.

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arXiv2015

Origin of the long-term modulation of radio emission of LS I +61 303

One of the most unusual aspects of the X-ray binary LSI +61 303 is that at each orbit (P1=26.4960 \pm 0.0028 d) one radio outburst occurs whose amplitude is modulated with Plong, a long-term period of more than 4 yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation. We study here the stability of Plong. Such a stability is expected if Plong is due to periodic (P2) Doppler boosting of periodic (P1) ejections from the accreting compact object of the system. On the contrary it is not expected if Plong is related to variations in the mass loss of the companion Be star. We built a database of 36.8 yr of radio observations of LSI +61 303 covering more than 8 long-term cycles. We performed timing and correlation analysis. In addition to the two dominant features at P1 and P2, the timing analysis gives a feature at Plong=1628 \pm 48 days. The determined value of Plong agrees with the beat of the two dominant features, i.e. Pbeat=1/(\nu1 -\nu2)=1626 \pm 68 d. The correlation coefficient of the radio data oscillates at multiples of Pbeat. Cycles in varying Be stars change in length and disappear after 2-3 cycles following the well-studied case of the binary system zeta Tau. On the contrary, in LSI +61 303 the long-term period is quite stable and repeats itself over the available 8 cycles. The long-term modulation in LSI +61 303 accurately reflects the beat of periodical Doppler boosting (induced by precession) with the periodicity of the ejecta. The peak of the long-term modulation occurs at the coincidence of the maximum number of ejected particles with the maximum Doppler boosting of their emission; this coincidence occurs every 1/(\nu1 - \nu2) and creates the long-term modulation observed in LSI +61 303.

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arXiv2015

Transient sources at the highest angular resolution

By definition transients are sudden events, some, like supernovae, are catastrophic, while others might be due to recurrent phenomena. The aim of studying transients is to reveal the physical conditions causing them, in this sense ideal targets for monitoring are transients in binary systems. In these systems the physical process responsible for the transient depends directly or indirectly on the interaction of the two components of the system. Here I report on transients in stellar binary systems at two extremes of stellar evolution: a T~Tauri system formed by two young low mass stellar objects, and X-ray binary systems formed by a star and a neutron star or a black hole, i.e., end points in the life of massive stars. VLBI observations of the young binary system V773 Tau A resolve the binary separation and can be overlapped with the optical frame. Consecutive VLBI observations showing the evolution of the radio emission with respect to the two stellar objects are an unvaluable tool for a better understanding of the magnetic field topology of T Tauri stars. The characteristics of radio jets in X-ray binaries are summarised and compared with those of the radio emission of the gamma-ray binary LS I +61303. Timing analysis of radio and Fermi-LAT observations provide constraints for theoretical models that can be tested by VLBI observations.

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arXiv2015

The apparent discontinuity in the periodicity of the GeV emission from LS I +61°303

The gamma-ray binary LS I +61°303 shows a discontinuity of the periodicity in its GeV emission. In this paper, we show that during the epochs when the timing analysis fails to determine the orbital periodicity, the periodicity is in fact present in the two orbital phase intervals $Φ= 0.0-0.5$ and $Φ= 0.5-1.0$. That is, there are two periodic signals, one towards periastron (i.e., $Φ= 0.0-0.5$) and another one towards apastron ($Φ= 0.5-1.0$). The apastron peak shows the same orbital shift as the radio outburst and, in addition, reveals the same two periods $P_1$ and $P_2$ that are present in the radio data. The gamma-ray emission of the apastron peak normally just broadens the emission of the peak around periastron. Only when it appears at $Φ= 0.8-1.0$, because of the orbital shift, it is detached enough from the first peak to become recognizable as a second orbital peak, which is the reason why the timing analysis fails. Two gamma-ray peaks along the orbit are predicted by the two-peak accretion model for an eccentric orbit that was proposed by several authors for LS I +61°303.

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arXiv2012

Microblazar properties of LSI+61303

In this paper we describe the gamma-ray binary LSI+61303 as a blazar like object, based on variable Doppler boosting and core-shift effect considerations.

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arXiv2011

Implications of the radio spectral index transition in LS I +61°303 for its INTEGRAL data analysis

The TeV emitting X-ray binary LS I +61°303 has two radio periodicities that correspond to a large periodic outburst with the same period as the orbit, 26.5 days (phase Φ), and a second periodicity of 1667 days (phase Θ), which modulates the orbital phase and amplitude of the large outburst. Analyses of the radio spectral index revealed in LS I +61°303 the presence of the critical transition typical for microquasars from optically thick emission (related to a steady jet) to an optically thin outburst (related to a transient jet), and found that it occurs at Φ_{crit}, which is modulated by Θ: Φ_{crit}=f(Θ). We examine the possible implications of averaging high energy data over large Θand Φintervals in the light of puzzling published INTEGRAL results, which differ for different averaging of the data. In microquasars, a simultaneous transition between two X-ray states occurs at the switch from optically thick radio emission to an optically thin radio outburst, from the low/hard to the steep power-law state. Assuming that the same transition occurs in LS I +61°303 at Φ_{crit}, we can show qualitatively the effect of averaging high energy data on Θ, by analysing the effects of averaging radio spectral index data across the same Θinterval. We then model the two X-ray states, low/hard and steep power-law state, and show quantitatively how their mixing can affect the results. When folded over too large a Θinterval, spectral data from INTEGRAL can yield a false picture of the emission behaviour of the source along the orbit because it may be mixing two spectral states. Furthermore, averaging the data along the orbit may result in a dominant low/hard spectral state, which, for insufficiently extended sampling, might appear without a cut-off.

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arXiv2010

The Two-Peak Model of LS I +61303: Radio Spectral Index Analysis

The most puzzling aspect of the radio emission from LSI+61303 is that the large periodic radio outburst, with period equal to the orbital one, occurs very displaced from periastron passage, nearly at apoastron. In 1992, Taylor, one of the discoverers of this source, together with his collaborators proposed a model of a compact object in an eccentric orbit accreting from the equatorial wind of the Be star primary. The application of this model by Marti & Paredes (1995) predicts one ejection at periastron and a second more displaced ejection along the orbit. The first ejection should correspond to weak radio emission, because of strong inverse Compton losses of the emitting electrons due to the proximity to the hot Be star, whereas the second ejection, quite displaced from the star, would correspond to a strong radio outburst, that one indeed observed. Corroborated along the years by numerical computations, simulations and gamma-ray observations, until now this two-peak model could not be proved in the radio band, because of the negligible emission around periastron. We show here, that the radio spectral index based on the ratio of flux densities is the unique tool to monitor activity of LSI+61303 in the radio band around periastron. The analysis of the radio spectral index over almost 7 years of Green Bank Interferometer data results in a clear double-peaked spectral index curve along the orbit. This result gives finally observational support at radiowavelengths to the two-peak accretion/ejection model for LSI+61303. Moreover, the here shown comparison of the two-peak curves - the radio spectral index curve and the Fermi-LAT gamma-ray curve - indicates a new interesting hypothesis on the electron population responsible for the gamma-ray emission.

astro-ph.HE↗

arXiv2007

Searching for coronal radio emission from protostars using Very-Long-Baseline Interferometry

In order to directly study the role of magnetic fields in the immediate vicinity of protostars, we use Very-Long-Baseline Interferometry (VLBI), aiming at the detection of non-thermal centimetric radio emission. This is technically the only possibility to study coronal emission at sub-AU resolution. We performed VLBI observations of the four nearby protostars HL Tau, LDN 1551 IRS5, EC 95, and YLW 15 in order to look for compact non-thermal centimetric radio emission. For maximum sensitivity, we used the High Sensitivity Array (HSA) where possible, involving the Very Long Baseline Array (VLBA), the phased Very Large Array (VLA), as well as the Arecibo, Green Bank, and Effelsberg radio telescopes. While all four protostars were detected in VLA-only data, only one source (YLW 15 VLA 2) was detected in the VLBI data. The possibility of non-detections due to free-free absorption, possibly depending on source geometry, is considered. For YLW 15 VLA 2, the prospects for an accurate orbit determination appear to be good.

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arXiv2007

The Flaring Corona of UX Arietis

Here we present observational results on the RS CVn star UX Arietis from four very-long-baseline interferometry observations distributed in time to cover the rotational period of 6.44 days. The data are better fit by two Gaussian components than by the usual core-plus-halo model. In the first three days the sizes of the two components did not change much from hour to hour but their relative position and orientation changed from day to day. The origin of this evolution can be explained by geometrical factors (i.e., star rotation). The fourth day a large flare occurred and dramatic changes in the sizes of the Gaussian components were seen.

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arXiv2005

Introduction to Astrophysics of Microquasars

The Astrophysics of microquasars - galactic miniatures of the far distant quasars - has become one of the most active fields of modern Astronomy in recent years. Here I review the astronomical methods used for the investigation of these objects, from Gamma-rays over X-rays and optical to radio wavelengths. The description of each astronomical method is always followed by an examplary application on the source LS I+61 303, one of the most observed Be/X-ray binary systems

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arXiv2005

Radio-loud and radio-quiet X-ray binaries: LSI+61o303 Galaxies

The three basic ingredients - a spinning compact object, an accretion disc and a collimated relativistic jet - make microquasars a galactic scaled-down version of the radio-loud AGN. That explains the large interest attributed to this new class of objects, which up to now consists of less than 20 members. Microquasars belong to the much larger class of X-ray binary systems, where there exits a compact object together with its X-ray emitting accretion disc, but the relativistic jet is missing. When does an X-ray binary system evolve into a microquasar? Ideal for studying such kind of a transition is the periodic microquasar LSI+61deg303 formed by a compact object accreting from the equatorial wind of a Be star and with more than one event of super-critical accretion and ejection along the eccentric orbit. For ejections at periastron passage the relativistic electrons suffer severe inverse Compton losses by upscattering the UV photons of the Be star at high energy : At periastron passage Gamma-ray emission has been observed, whereas radio outbursts have never been observed in 20 years of radio flux monitoring. For ejections displaced from periastron passage the losses are less severe and radio outbursts are observed. The radio emission mapped on scales from a few AU to hundreds of AU shows a double-sided relativistic (beta=0.6c) S-shaped jet, similar to the well-known precessing jet of SS433.

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arXiv2004

Radio-loud and Radio-quiet X-ray Binaries: LSI+61303 in Context

The three basic ingredients - a spinning compact object, an accretion disc and a collimated relativistic jet - make microquasars a galactic scaled-down version of the radio-loud AGN. That explains the large interest attributed to this new class of objects, which up to now consists of less than 20 members. Microquasars belong to the much larger class of X-ray binary systems, where there exists a compact object together with its X-ray emitting accretion disc, but the relativistic jet is missing. When does an X-ray binary system evolve into a microquasar? Ideal for studying such kind of a transition is the periodic microquasar LS I +61303 formed by a compact object accreting from the equatorial wind of a Be star and with more than one event of super-critical accretion and ejection along the eccentric orbit. For ejections at periastron passage the relativistic electrons suffer severe inverse Compton losses by upscattering the UV photons of the Be star at high energy: At periastron passage Gamma-ray emission has been observed, whereas radio outbursts have never been observed in 20 years of radio flux monitoring. For ejections displaced from periastron passage the losses are less severe and radio outbursts are observed. The radio emission mapped on scales from a few AU to hundreds of AU shows a double-sided relativistic (beta=0.6c) S-shaped jet, similar to the well-known precessing jet of SS 433.

astro-ph↗

arXiv2004

LSI+61303 in the context of microquasars

LSI+61303 is one of the most observed Be/X-ray binary systems because, peculiarly, it has periodical radio and X-ray emission together with strong, variable gamma-ray emission. This source remains, however, quite enigmatic. Some properties of this system can be explained assuming that the unseen companion is a non-accreting young pulsar with a relativistic wind strongly interacting with the wind of the Be star. On the contrary, other properties of LSI+61303 fit a model where the companion is accreting even with two events of super-critical accretion along the orbit. The very recent discovery of a radio jet extending ca. 200 AU at both sides of a central core has definitely proved the occurrence of accretion--ejection processes in this system. Therefore it is of great interest to combine this result with previous observations at other wavelengths within the framework of the two-peak accretion (ejection) model. Concerning the first ejection, we show that the observed gamma-rays variations might be periodic with outbursts confined around the periastron passage (i.e. where the first accretion-rate peak occurs and high-energy emission but no radio emission is predicted). Concerning the second ejection, with radio bursts, we point out that it can be also traced in the X-ray data, both in episodes of hardening of the X-ray emission and in a transition from soft- to hard-states at the onsets of radiobursts. In fact, both hardening and transitions between spectral states are related to the dramatic change in the structure of the accretion disk preceeding the ejection. Finally, we explore the nature of the accretor and we conclude that on the basis of the present optical data a black hole cannot be ruled out.

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