arXiv subjects

Zulema Abraham

Publications and source records attributed to Zulema Abraham.

At least 19 recordsLinked to original sources

arXiv2023

A two-component clumpy model for the shell evolution of classical novae: the case of V5668 Sgr

The shell of the classical nova V5668 Sgr was resolved by ALMA at the frequency of 230 GHz 927 days after eruption, showing that most of the continuum bremsstrahlung emission originates in clumps with diameter smaller than $10^{15}$ cm. Using VLA radio observations, obtained between days 2 and 1744 after eruption, at frequencies between 1 and 35 GHz, we modeled the nova spectra, assuming first that the shell is formed by a fixed number of identical clumps, and afterwards with the clumps having a power law distribution of sizes, and were able to obtain the clump's physical parameters (radius, density and temperature). We found that the density of the clumps decreases linearly with the increase of the shell's volume, which is compatible with the existence of a second media, hotter and thinner, in pressure equilibrium with the clumps. We show that this thinner media could be responsible for the emission of the hard X-rays observed at the early times of the nova eruption, and that the clump's temperature evolution follows that of the super-soft X-ray luminosity. We propose that the clumps were formed in the radiative shock produced by the collision of the fast wind of the white dwarf after eruption, with the slower velocity of the thermonuclear ejecta. From the total mass of the clumps, the observed expansion velocity and thermonuclear explosion models, we obtained an approximate value of 1.25 M$_{\odot}$ for the mass of the white dwarf, a central temperature of $10^7$ K and an accretion rate from the secondary star of $10^{-9}-10^{-8}$ M$_{\odot}$ y$^{-1}$.

astro-ph.GA↗

arXiv2022

Telluric absorption lines in the ALMA spectra of $η$ Car

The massive binary system formed by $η$ Car and an unknown companion is a strong source at millimetre and submillimetre wavelengths. Close to the stars, continuum bremsstrahlung and radio recombination lines originate in the massive ionized wind of $η$ Car and in several compact sources of high density plasma. Molecular lines are also detected at these wavelengths, some of them are seen in absorption towards the continuum emission region, many of them revealed by ALMA observations. However, because the ALMA atmospheric calibration is performed in a low spectral resolution mode, telluric lines can still be present in some high-resolution spectra of scientific products, which could lead to a false identification of molecules. Since $η$ Carinae is a bright source, the issue is even more critical because some features can be highlighted. In this work, we explore three different sets of ALMA archive data of $η$ Car, including high resolution (0.065" x 0.043") observations recently published by our group, to verify which of these absorption lines are real and discuss their origin. We conclude that some of them truly originate in clouds close to the binary system, while others are artifacts of a faulty elimination of telluric lines during ALMA calibration procedure. We found that these absorption lines are not present in the phase calibrators because they are much weaker than $η$ Car, where the absorption line appears because the high intensity continuum enhances the small individual systematic calibration errors.

astro-ph.SR↗

arXiv2021

The relativistic parsec-scale jets of the blazars TXS 0506+056 and PKS 0502+049 and their possible association with gamma-ray flares and neutrino production

The physical nature of the mechanism responsible for the emission of neutrinos in active galactic nuclei (AGN) has been matter of debate in the literature, with relativistic jets of radio-loud AGNs as possible candidates to be the sources of high energy neutrinos. The most prominent candidate so far is the blazar TXS 0506+056, which is found to be associated with the neutrino event IceCube-170922A. Furthermore, the IceCube reported an excess of neutrinos towards TXS 0506+056 between September 2014 and March 2015, even though this association needs additional investigation, considering the presence of a nearby gamma-ray source, the quasar PKS 0502+049. Motivated by this, we studied the parsec-scale structures of TXS 0506+056 and PKS 0502+049 through radio interferometry at 8 and 15\,GHz. We identified twelve jet components in TXS 0506+056 and seven components in PKS 0502+049. The most reliable jet components show superluminal speeds ranging from 9.5c to 66c in the case of TXS 0506+056, and from 14.3c to 59c for PKS 0502+049, which were used to estimate a lower (upper) limit for the Lorentz factor (jet viewing angle) for both sources. A novel approach using simultaneously the brightness temperature of the core region and the apparent speeds of the jet components allowed us to infer basic jet parameters for TXS 0506+056 at distinct epochs. We also found that the emergence of new jet components coincides with the occurrence of gamma-ray flares. Interestingly, two of these coincidences in the case of PKS 0502+049 and one for TXS 0506+056 seems to be correlated with neutrino events detected by the IceCube Observatory.

astro-ph.HE↗

arXiv2019

Sulphur-Bearing and Complex Organic Molecules in an Infrared Cold Core

Since the start of ALMA observatory operation, new and important chemistry of infrared cold core was revealed. Molecular transitions at millimeter range are being used to identify and to characterize these sources. We have investigated the 231 GHz ALMA archive observations of the infrared dark cloud region C9, focusing on the brighter source that we called as IRDC-C9 Main. We report the existence of two sub-structures on the continuum map of this source: a compact bright spot with high chemistry diversity that we labelled as core, and a weaker and extended one, that we labelled as tail. In the core, we have identified lines of the molecules OCS(19-18), $^{13}$CS(5-4) and CH$_{3}$CH$_{2}$CN, several lines of CH$_{3}$CHO and the k-ladder emission of $^{13}$CH$_{3}$CN.We report two different temperature regions: while the rotation diagram of CH$_{3}$CHO indicates a temperature of 25 K, the rotation diagram of $^{13}$CH$_{3}$CN indicates a warmer phase at temperature of $\sim450$K. In the tail, only the OCS(19-18) and $^{13}$CS(5-4) lines were detected. We used the $Nautilus$ and the \textsc{Radex} codes to estimate the column densities and the abundances. The existence of hot gas in the core of IRDC-C9 Main suggests the presence of a protostar, which is not present in the tail.

astro-ph.SR↗

arXiv2019

Multiwavelength analysis of brightness variations of 3C~279: Probing the relativistic jet structure and its evolution

We studied the correlation between brightness and polarization variations in 3C~279 at different wavelengths, over time intervals long enough to cover the time lags due to opacity effects. We used these correlations together with VLBI images to constrain the radio and high energy source position.We made 7 mm radio continuum and $R$-band polarimetric observations of 3C~279 between 2009 and 2014. The radio observations were performed at the Itapetinga Radio Observatory, while the polarimetric data were obtained at Pico dos Dias Observatory, both in Brazil. We compared our observations with the $γ$-ray Fermi/LAT and $R$-band SMARTS light curves. We found a good correlation between 7~mm and $R$-band light curves, with a delay of $170 \pm 30$ days in radio, but no correlation with the $γ$ rays. However, a group of several $γ$-ray flares in April 2011 could be associated with the start of the 7 mm strong activity observed at the end of 2011.We also detected an increase in $R$-band polarization degree and rotation of the polarization angle simultaneous with these flares. Contemporaneous VLBI images at the same radio frequency show two new strong components close to the core, ejected in directions very different from that of the jet.The good correlation between radio and $R$-band variability suggests that their origin is synchrotron radiation. The lack of correlation with $γ$-rays produced by the Inverse Compton process on some occasions could be due to the lack of low energy photons in the jet direction or to absorption of the high energy photons by the broad line region clouds. The variability of the polarization parameters during flares can be easily explained by the combination of the jet polarization parameters and those of newly formed jet components.

astro-ph.HE↗

arXiv2018

The structure of a recent nova shell as observed by ALMA

High resolution ALMA observations of the recent (2.52 yrs old) shell of Nova V5668 Sgr (2015) show a highly structured ionised gas distribution with small (10$^{15}$ cm) clumps. These are the smallest structures ever observed in the remnant of a stellar thermonuclear explosion. No extended contiguous emission could be found above the 2.5 $σ$ level in our data, while the peak hydrogen densities in the clumps reach 10$^6$ cm$^{-3}$. The millimetre continuum image suggests that large scale structures previously distinguished in other recent nova shells may result from the distribution of bright unresolved condensations.

astro-ph.SR↗

arXiv2018

Herschel Planetary Nebula Survey (HerPlaNS): Hydrogen Recombination Laser Lines in Mz 3

The bipolar nebula Menzel 3 (Mz 3) was observed as part of the \textit{Herschel} Planetary Nebula Survey (\textit{HerPlaNS}), which used the PACS and SPIRE instruments aboard the \textit{Herschel Space Observatory} to study a sample of planetary nebulae (PNe). In this paper, one of the series describing \textit{HerPlaNS} results, we report the detection of H I recombination lines (HRLs) in the spectrum of Mz 3. Inspection of the spectrum reveals the presence of 12 HRLs in the 55 to 680 $μ$m range covered by the PACS and SPIRE instruments (H11$α$ to H21$α$ and H14$β$). The presence of HRLs in this range is unusual for PNe and has not been reported in Mz 3 before. Our analysis indicates that the HRLs we observed are enhanced by laser effect occurring in the core of Mz 3. Our arguments for this are: (i) the available Mz 3 optical to submillimetre HRL $α$ line intensity ratios are not well reproduced by the spontaneous emission of optically thin ionized gas, as would be typical for nebular gas in PNe; (ii) the compact core of Mz 3 is responsible for a large fraction of the Herschel HRLs emission; (iii) the line intensity ratios for Mz 3 are very similar to those in the core emission of the well known star MWC 349A, where laser effect is responsible for the enhancement of HRLs in the Herschel wavelength range; (iv) the physical characteristics relevant to cause laser effect in the core of MWC 349A are very similar to those in the core of Mz 3.

astro-ph.GA↗

arXiv2017

Jet precession driven by a supermassive black hole binary system in the BL Lac object PG 1553+113

The recent discovery of a roughly simultaneous periodic variability in the light curves of the BL Lac object PG 1553+113 at several electromagnetic bands represents the first case of such odd behavior reported in the literature. Motivated by this, we analyzed 15 GHz interferometric maps of the parsec-scale radio jet of PG 1553+113 to verify the presence of a possible counterpart of this periodic variability. We used the Cross-Entropy statistical technique to obtain the structural parameters of the Gaussian components present in the radio maps of this source. We kinematically identified seven jet components formed coincidentally with flare-like features seen in the $γ$-ray light curve. From the derived jet component positions in the sky plane and their kinematics (ejection epochs, proper motions, and sky position angles), we modeled their temporal changes in terms of a relativistic jet that is steadily precessing in time. Our results indicate a precession period in the observer's reference frame of $2.24\pm0.03$ years, compatible with the periodicity detected in the light curves of PG 1553+113. However, the maxima of the jet Doppler boosting factor are systematically delayed relative to the peaks of the main $γ$-ray flares. We propose two scenarios that could explain this delay, both based on the existence of a supermassive black hole binary system in PG 1553+113. We estimated the characteristics of this putative binary system that also would be responsible for driving the inferred jet precession.

astro-ph.GA↗

arXiv2017

Multiwavelength flaring activity of PKS1510-089

Aims: In this work, we analyse the multiwavelength brightness variations and flaring activity of FSRQ PKS1510-089, aiming to constrain the position of the emission sources. Methods: We report 7 mm (43 GHz) radio and R-band polarimetric observations of PKS1510-089. The radio observations were performed at the Itapetinga Radio Observatory, while the polarimetric data were obtained at the Pico dos Dias Observatory. The 7 mm observations cover the period between 2011 and 2013, while the optical polarimetric observations were made between 2009 and 2012. Results: At 7 mm, we detected a correlation between four radio and $γ$-ray flares with a delay of about 54 days between them; the higher frequency counterpart occurred first. Using optical polarimetry, we detected a large variation in polarization angle (PA) within two days associated with the beginning of a $γ$-ray flare. Complementing our data with other data obtained in the literature, we show that PA presented rotations associated with the occurrence of flares. Conclusions: Our results can be explained by a shock-in-jet model, in which a new component is formed in the compact core producing an optical and/or $γ$-ray flare, propagates along the jet, and after some time becomes optically thin and is detected as a flare at radio frequencies. The variability in the polarimetric parameters can also be reproduced; we can explain large variation in both PA and polarization degree (PD), in only one of them, or in neither, depending on the differences in PA and PD between the jet and the new component.

astro-ph.HE↗

arXiv2014

$η$ Carinae Baby Homunculus Uncovered by ALMA

We report observations of $η$ Carinae obtained with ALMA in the continuum of 100, 230, 280 and 660 GHz in 2012 November, with a resolution that varied from 2.88" to 0.45" for the lower and higher frequencies respectively. The source is not resolved, even at the highest frequency; its spectrum is characteristic of thermal bremsstrahlung of a compact source, but different from the spectrum of optically thin wind. The recombination lines H42$α$, He42$α$, H40$α$, He40$α$, H50$β$, H28$α$, He28$α$, H21$α$ and He21$α$ were also detected and their intensities reveal non local thermodynamic equilibrium (NLTE) effects. We found that the line profiles could only be fit by an expanding shell of dense and ionized gas, which produces a slow shock in the surroundings of $η$ Carinae. Combined with fittings to the continuum, we were able to constrain the shell size, radius, density, temperature and velocity. The detection of the He recombination lines is compatible with the high temperature gas and requires a high energy ionizing photon flux, which must be provided by the companion star. The mass loss rate and wind velocity, necessary to explain the formation of the shell, are compatible with a LBV eruption. The position, velocity and physical parameters of the shell coincide with those of the Weigelt blobs. The dynamics found for the expanding shell corresponds to matter ejected by $η$ Carinae in 1941, in an event similar to that which formed the Little Homunculus ; for that reason we called the new ejecta the "Baby Homunculus".

astro-ph.SR↗

arXiv2014

Kinematic study of the parsec-scale jet of the quasar PKS 1741--03

We present 23 interferometric images of parsec-scale jet of the quasar PKS 1741--03 at 15, 24 and 43 GHz spanning about 13 yr. We model the images as a superposition of discrete two--dimensional elliptical Gaussian components, with parameters determined by the cross--entropy technique. All the images present a spatially unresolved component (core) and usually two or three components receding from it. The same components were found in simultaneous 24 and 43 GHz maps, showing the robustness of our model-fitting. The core-shift opacity effect between these frequencies is weak. We have identified seven components moving along straight lines at constant apparent superluminal speeds ($3.5\leβ_\mathrm{obs}\le 6.1$), with different sky position angles ($-186°\le η\le -125°$). The core flux density tracks quite well the fluctuations seen in the historical single-dish light curve at 14.5 GHz, with no measurable delay. The total flux density from the moving jet components is delayed $\sim$2 yr in relation to the core light curve, roughly the same as the lag between the ejection epoch and the maximum flux density in the light curves of the jet components. We propose that there are three non-exclusive mechanisms for producing these delays. From the kinematics of the most robust jet components and the core brightness temperature, we determined the bulk Lorentz factor ($4.8\leγ\le 24.5$) and the jet viewing angle ($0.°35 \le θ\le 4.°2$); these values agree with previous estimates from the spectral energy distribution of PKS 1741--03 and its radio variability.

astro-ph.HE↗

arXiv2013

3C273 variability at 7 mm: Evidences of shocks and precession in the jet

We report 4 years of observations of 3C273 at 7 mm obtained with the Itapetinga Radiotelescope, in Brazil, between 2009 and 2013. We detected a flare in 2010 March, when the flux density increased by 50% and reached 35 Jy. After the flare, the flux density started to decrease and reached values lower than 10 Jy. We suggest that the 7 mm flare is the radio counterpart of the $γ$-ray flare observed by Fermi/LAT in 2009 September, in which the flux density at high energies reached a factor of fifty of its average value. A delay of 170 days between the radio and $γ$-ray flares was revealed using the Discrete Correlation Function (DCF) that can be interpreted in the context of a shock model, in which each flare corresponds to the formation of a compact superluminal component that expands and becomes optically thin at radio frequencies at latter epochs. The difference in flare intensity between frequencies and at a different times, is explained as a consequence of an increase in the Doppler factor $δ$, as predicted by the 16 year precession model proposed by Abraham & Romero, which has a large effect on boosting at high frequencies while does not affect too much the observed optically thick radio emission. We discuss other observable effects of the variation in $δ$, as the increase in the formation rate of superluminal components, the variations in the time delay between flares and the periodic behaviour of the radio light curve that we found compatible with changes in the Doppler factor.

astro-ph.HE↗

arXiv2012

SgrA* emission at 7mm: variability and periodicity

We present the result of 6 years monitoring of SgrA*, radio source associated to the supermassive black hole at the centre of the Milky Way. Single dish observations were performed with the Itapetinga radio telescope at 7 mm, and the contribution of the SgrA complex that surrounds SgrA* was subtracted and used as instantaneous calibrator. The observations were alternated every 10 min with those of the HII region SrgB2, which was also used as a calibrator. The reliability of the detections was tested comparing them with simultaneous observations using interferometric techniques. During the observing period we detected a continuous increase in the SgrA* flux density starting in 2008, as well as variability in timescales of days and strong intraday fluctuations. We investigated if the continuous increase in flux density is compatible with free-free emission from the tail of the disrupted compact cloud that is falling towards SgrA* and concluded that the increase is most probably intrinsic to SgrA*. Statistical analysis of the light curve using Stellingwerf and Structure Function methods revealed the existence of two minima, 156 +/- 10 and 220 +/- 10 days. The same statistical tests applied to a simulated light curve constructed from two quadratic sinusoidal functions superimposed to random variability reproduced very well the results obtained with the real light curve, if the periods were 57 and 156 days. Moreover, when a daily sampling was used in the simulated light curve, it was possible to reproduce the 2.3 GHz structure function obtained by Falcke in 1999, which revealed the 57 days period, while the 106 periodicity found by Zhao et al in 2001 could be a resonance of this period.

astro-ph.GA↗

arXiv2012

Parsec-scale jet precession in BL Lacertae (2200+420)

BL Lacertae is the prototype of the BL Lac class of active galactic nuclei, exhibiting intensive activity on parsec (pc) scales, such as intense core variability and multiple ejections of jet components. In particular, in previous works the existence of precession motions in the pc-scale jet of BL Lacertae has been suggested. In this work we revisit this issue, investigating temporal changes of the observed right ascension and declination offsets of the jet knots in terms of our relativistic jet-precession model. The seven free parameters of our precession model were optimized via a heuristic cross-entropy method, comparing the projected precession helix with the positions of the jet components on the plane of the sky and imposing constraints on their maximum and minimum superluminal velocities. Our optimized best model is compatible with a jet having a bulk velocity of 0.9824c, which is precessing with a period of about 12.1 yr in the observer's reference frame and changing its orientation in relation to the line of sight between 4 and 5 degrees, approximately. Assuming that the jet precession has its origin in a supermassive binary black hole system, we show that the 2.3-yr periodic variation in the structural position angle of the very-long-baseline interferometry (VLBI) core of BL Lacertae reported by Stirling et al. is compatible with a nutation phenomenon if the secondary black hole has a mass higher than about six times that of the primary black hole.

astro-ph.HE↗

arXiv2012

The young stellar cluster [DBS2003] 157 associated with the H II region GAL 331.31-00.34

We report a study of the stellar content of the Near-infrared cluster [DBS2003]\,157 embedded in the extended H\,{\sc ii} region GAL\,331.31-00.34, which is associated with the IRAS source 16085-5138. $JHK$ photometry was carried out in order to identify potential ionizing candidates, and the follow-up NIR spectroscopy allowed the spectral classification of some sources, including two O-type stars. A combination of NIR photometry and spectroscopy data was used to obtain the distance of these two stars, with the method of spectroscopic parallax: IRS\,298 (O6\,{\sc V}, $3.35 \pm 0.61$\,kpc) and IRS\,339 (O9\,{\sc V}, $3.24 \pm 0.56$\,kpc). Adopting the average distance of $3.29 \pm 0.58$\,kpc and comparing the Lyman continuum luminosity of these stars with that required to account for the radio continuum flux of the H\,{\sc ii} region, we conclude that these two stars are the ionizing sources of GAL\,331.31-00.34. Young stellar objects (YSOs) were searched by using our NIR photometry and MIR data from the GLIMPSE survey. The analysis of NIR and MIR colour-colour diagrams resulted in 47 YSO candidates. The GLIMPSE counterpart of IRAS\,16085-5138, which presents IRAS colour indices compatible with an ultra-compact H\,{\sc ii} region, has been identified. The analysis of its spectral energy distribution between 2 and $100\,μ$m revealed that this source shows a spectral index $α= 3.6$ between 2 and $25\,μ$m, which is typical of a YSO immersed in a protostellar envelope. Lower limits to the bolometric luminosity and the mass of the embedded protostar have been estimated as $L=7.7\times10^3L_{\sun}$ and $M = 10\,M_{\sun}$, respectively, which corresponds to a B0--B1\,{\sc V} ZAMS star.

astro-ph.GA↗

arXiv2011

Modeling very long baseline interferometric images with the cross-entropy global optimization technique

We present a new technique for obtaining model fittings to VLBI images of astrophysical jets. The method minimizes a performance function proportional to the sum of the squared difference between the model and observed images. The model image is constructed by summing elliptical Gaussian sources characterized by six parameters: two-dimensional peak position, peak intensity, eccentricity, amplitude and orientation angle of the major axis. We present results for the fitting of two main benchmark jets: the first, constructed from three individual Gaussian sources, the second formed by five Gaussian sources. Both jets were analyzed by our cross-entropy technique in finite and infinite signal-to-noise regimes, the background noise chosen to mimic that found in interferometric radio maps. We show that our technique is capable of recovering the parameters of the sources with a similar accuracy to that obtained from the traditional AIPS task IMFIT when the image is relatively simple (e.g., few components). For more complex maps, our method displays superior performance in recovering the parameters of the jet components. Our methodology is also able to show quantitatively the number of individual components present in an image. An additional application of the cross-entropy technique to a real image of a BL Lac object is shown and discussed. Our results indicate that our cross-entropy technique must be used in situations involving the analysis of complex emission regions having more than three sources, even though it is substantially slower than current model fitting tasks (at least 10,000 times slower for a single processor, depending on the number of sources to be optimized). As in the case of any model fitting performed in the image plane, caution is required in analyzing images constructed from a poorly sampled (u,v) plane.

astro-ph.CO↗

arXiv2010

The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance

We present a map of the spiral structure of the Galaxy, as traced by molecular CS emission associated with IRAS sources which are believed to be compact HII regions. The CS line velocities are used to determine the kinematic distances of the sources, in order to investigate their distribution in the galactic plane. This allows us to use 870 objects to trace the arms, a number larger than that of previous studies based on classical HII regions. The distance ambiguity of the kinematic distances, when it exists, is solved by different procedures, including the latitude distribution and an analysis of the longitude-velocity diagram. The well defined spiral arms are seen to be confined inside the co-rotation radius, as is often the case in spiral galaxies. We identify a square-shaped sub-structure in the CS map with that predicted by stellar orbits at the 4:1 resonance (4 epicycle oscillations in one turn around the galactic center). The sub-structure is found at the expected radius, based on the known pattern rotation speed and epicycle frequency curve. An inner arm presents an end with strong inward curvature and intense star formation that we tentatively associate with the region where this arm surrounds the extremity of the bar, as seen in many barred galaxies. Finally, a new arm with concave curvature is found in the Sagitta to Cepheus region of the sky.

astro-ph.GA↗

arXiv2007

Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?

Strong evidence for the presence of a warped Keplerian accretion disc in NGC4258 (M 106) has been inferred from the kinematics of water masers detected at sub-parsec scales. Assuming a power-law accretion disc and using constraints on the disc parameters derived from observational data, we have analyzed the relativistic Bardeen-Petterson effect driven by a Kerr black hole as the potential physical mechanism responsible for the disc warping. We found that the Bardeen-Petterson radius is comparable to or smaller than the inner radius of the maser disc (independent of the allowed value for the black hole spin parameter). Numerical simulations for a wide range of physical conditions have shown that the evolution of a misaligned disc due to the Bardeen-Petterson torques usually produces an inner flat disc and a warped transition region with a smooth gradient in the tilt and twist angles. Since this structure is similar to that seen in NGC 4258, we propose that the Bardeen-Petterson effect may be responsible for the disc warping in this galaxy. We estimated the time-scale necessary for the disc inside of the Bardeen-Petterson radius to align with the black hole's equator, as a function of the black hole spin. Our results show that the Bardeen-Petterson effect can align the disc within a few billion years in the case of NGC 4258. Finally, we show that if the observed curvature of the outer anomalous arms in the galactic disc of NGC 4258 is associated with the precession of its radio jet/counterjet, then the Bardeen-Petterson effect can provide the required precession period.

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