47 research outputs found
Optical spectroscopy of the microquasar GRS 1758-258: a possible intermediate mass system?
Context. GRS 1758-258 is one of two prototypical microquasars towards the
Galactic Center direction discovered almost a quarter of a century ago. The
system remains poorly studied in the optical domain due to its counterpart
being a very faint and absorbed target in a crowded region of the sky. Aims.
Our aim is to investigate GRS 1758-258 in order to shed light on the nature of
the stellar binary components. In particular, the main physical parameters of
the donor star, such as the mass or the spectral type, are not yet well
constrained. Methods. GRS 1758-258 has remained so far elusive to optical
spectroscopy owing to its observational difficulties. Here, we use this
traditional tool of stellar astronomy at low spectral resolution with a 10 m
class telescope and a long slit spectrograph. Results. An improved spectrum is
obtained as compared to previous work. The quality of the data does not allow
the detection of emission or absorption features but, nevertheless, we manage
to partially achieve our aims comparing the de-reddened continuum with the
spectral energy distribution expected from an irradiated disc model and
different donor star templates. Conclusions. We tentatively propose that GRS
1758-258 does not host a giant star companion. Instead, a main sequence star
with mid-A spectral type appears to better agree with our data. The main
impacts of this finding are the possibility that we are dealing with an
intermediate mass system and, in this case, the prediction of an orbital period
significantly shorter than previously proposed.Comment: 5 pages, 6 figures, accepted for publication in A&
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.Comment: Accepted for publication in Astronomy and Astrophysic
Prediction of CpG-island function: CpG clustering vs. sliding-window methods
Background Unmethylated stretches of CpG dinucleotides (CpG islands) are an outstanding property of mammal genomes. Conventionally, these regions are detected by sliding window approaches using %G + C, CpG observed/expected ratio and length thresholds as main parameters. Recently, clustering methods directly detect clusters of CpG dinucleotides as a statistical property of the genome sequence. Results We compare sliding-window to clustering (i.e. CpGcluster) predictions by applying new ways to detect putative functionality of CpG islands. Analyzing the co-localization with several genomic regions as a function of window size vs. statistical significance (p-value), CpGcluster shows a higher overlap with promoter regions and highly conserved elements, at the same time showing less overlap with Alu retrotransposons. The major difference in the prediction was found for short islands (CpG islets), often exclusively predicted by CpGcluster. Many of these islets seem to be functional, as they are unmethylated, highly conserved and/or located within the promoter region. Finally, we show that window-based islands can spuriously overlap several, differentially regulated promoters as well as different methylation domains, which might indicate a wrong merge of several CpG islands into a single, very long island. The shorter CpGcluster islands seem to be much more specific when concerning the overlap with alternative transcription start sites or the detection of homogenous methylation domains. Conclusions The main difference between sliding-window approaches and clustering methods is the length of the predicted islands. Short islands, often differentially methylated, are almost exclusively predicted by CpGcluster. This suggests that CpGcluster may be the algorithm of choice to explore the function of these short, but putatively functional CpG islands
Real-time evolution of a large-scale relativistic jet
Context. Astrophysical jets are ubiquitous in the Universe on all scales, but
their large-scale dynamics and evolution in time are hard to observe since they
usually develop at a very slow pace.
Aims. We aim to obtain the first observational proof of the expected
large-scale evolution and interaction with the environment in an astrophysical
jet. Only jets from microquasars offer a chance to witness the real-time,
full-jet evolution within a human lifetime, since they combine a 'short', few
parsec length with relativistic velocities.
Methods. The methodology of this work is based on a systematic recalibraton
of interferometric radio observations of microquasars available in public
archives. In particular, radio observations of the microquasar GRS 1758-258
over less than two decades have provided the most striking results.
Results. Significant morphological variations in the extended jet structure
of GRS 1758-258 are reported here that were previously missed. Its northern
radio lobe underwent a major morphological variation that rendered the hotspot
undetectable in 2001 and reappeared again in the following years. The reported
changes confirm the Galactic nature of the source. We tentatively interpret
them in terms of the growth of instabilities in the jet flow. There is also
evidence of surrounding cocoon. These results can provide a testbed for models
accounting for the evolution of jets and their interaction with the
environment.Comment: 5 pages, 3 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics Letter
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.Comment: 14 pages, 7 figures, Accepted for publication in Astronomy &
Astrophysic
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.Comment: 8 pages; 10 figures; accepted for publication in Astronomy &
Astrophysic
CpGcluster: a distance-based algorithm for CpG-island detection
BACKGROUND: Despite their involvement in the regulation of gene expression and their importance as genomic markers for promoter prediction, no objective standard exists for defining CpG islands (CGIs), since all current approaches rely on a large parameter space formed by the thresholds of length, CpG fraction and G+C content. RESULTS: Given the higher frequency of CpG dinucleotides at CGIs, as compared to bulk DNA, the distance distributions between neighboring CpGs should differ for bulk and island CpGs. A new algorithm (CpGcluster) is presented, based on the physical distance between neighboring CpGs on the chromosome and able to predict directly clusters of CpGs, while not depending on the subjective criteria mentioned above. By assigning a p-value to each of these clusters, the most statistically significant ones can be predicted as CGIs. CpGcluster was benchmarked against five other CGI finders by using a test sequence set assembled from an experimental CGI library. CpGcluster reached the highest overall accuracy values, while showing the lowest rate of false-positive predictions. Since a minimum-length threshold is not required, CpGcluster can find short but fully functional CGIs usually missed by other algorithms. The CGIs predicted by CpGcluster present the lowest degree of overlap with Alu retrotransposons and, simultaneously, the highest overlap with vertebrate Phylogenetic Conserved Elements (PhastCons). CpGcluster's CGIs overlapping with the Transcription Start Site (TSS) show the highest statistical significance, as compared to the islands in other genome locations, thus qualifying CpGcluster as a valuable tool in discriminating functional CGIs from the remaining islands in the bulk genome. CONCLUSION: CpGcluster uses only integer arithmetic, thus being a fast and computationally efficient algorithm able to predict statistically significant clusters of CpG dinucleotides. Another outstanding feature is that all predicted CGIs start and end with a CpG dinucleotide, which should be appropriate for a genomic feature whose functionality is based precisely on CpG dinucleotides. The only search parameter in CpGcluster is the distance between two consecutive CpGs, in contrast to previous algorithms. Therefore, none of the main statistical properties of CpG islands (neither G+C content, CpG fraction nor length threshold) are needed as search parameters, which may lead to the high specificity and low overlap with spurious Alu elements observed for CpGcluster predictions
A galactic microquasar mimicking winged radio galaxies
A subclass of extragalactic radio sources known as winged radio galaxies has puzzled astronomers for many years. The wing features are detected at radio wavelengths as low-surface-brightness radio lobes that are clearly misaligned with respect to the main lobe axis. Different models compete to account for these peculiar structures. Here, we report observational evidence that the parsec-scale radio jets in the Galactic microquasar GRS 1758-258 give rise to a Z-shaped radio emission strongly reminiscent of the X and Z-shaped morphologies found in winged radio galaxies. This is the first time that such extended emission features are observed in a microquasar, providing a new analogy for its extragalactic relatives. From our observations, we can clearly favour the hydrodynamic backflow interpretation against other possible wing formation scenarios. Assuming that physical processes are similar, we can extrapolate this conclusion and suggest that this mechanism could also be at work in many extragalactic cases
Hints of the photometric period in radial velocity data of the X-ray binary HD 3191 with the Joan Oró telescope
We focus our attention on the newly proposed X-ray binary system HD 3191 whose nature still lacks dynamical confirmation
and accurate orbital parameters. The positional agreement of this object with a flaring gamma-ray source was the original
motivation of this work, although the present chances of association are reduced. A long-term spectroscopic monitoring of
HD3191 was conducted using the 1-m class, robotic, Joan Oró telescope at the Observatori Astronòmic del Montsec facilities,
located in the pre-Pyrenees mountains 140 km away from Barcelona. The Doppler measurements suggest an agreement with
the previously reported 16.09 d photometric period, as expected. We also present a very preliminary radial velocity curve
based on a circular orbit solution that hints to a low velocity amplitude. Nevertheless, the instrumental limitations of our data
render necessary to carry out improved spectroscopic observations to better discriminate among the nature of the compact
companion.Funding for open access publishing: Universidad de Jaén/
CBUA. This work was mainly supported by grant PID2019-105510GBC32/
AEI/10.13039/501100011033 from State Agency for Research
of the Spanish Ministry of Science and Innovation entitled High energy
sources with outflows at different scales: observation of galactic
sources. We also acknowledge support by Consejería de Economía,
Innovación, Ciencia y Empleo of Junta de Andalucía as research group
FQM-322, as well as FEDER funds. JMP acknowledges financial support
from the State Agency for Research of the Spanish Ministry
of Science and Innovation under grant PID2019-105510GB-C31 and
through the Unit of Excellence María de Maeztu 2020-2023 award to
the Institute of Cosmos Sciences (CEX2019-000918-M), and by the
Catalan DEC grant 2017 SGR 643