The binary millisecond radio pulsar PSR J1023+0038 has been recently the subject of multiwavelength monitoring campaigns which revealed that an accretion disc has formed around the neutron star ...(since 2013 June). We present here the results of X-ray and UV observations carried out by the Swift satellite between 2013 October and 2014 May, and of optical and NIR observations performed with the REM telescope, the Liverpool Telescope, the 2.1-m telescope at the San Pedro Mártir Observatory and the 1.52-m telescope at the Loiano observing station. The X-ray spectrum is well described by an absorbed power law, which is softer than the previous quiescent epoch (up to 2013 June). The strong correlation between the X-ray and the UV emissions indicates that the same mechanism should be responsible for part of the emission in these bands. Optical and infrared photometric observations show that the companion star is strongly irradiated. Double-peaked emission lines in the optical spectra provide compelling evidence for the presence of an outer accretion disc too. The spectral energy distribution from IR to X-rays is well modelled if the contributions from the companion, the disc and the intrabinary shock emission are all considered. Our extensive data set can be interpreted in terms of an engulfed radio pulsar: the radio pulsar is still active, but undetectable in the radio band due to a large amount of ionized material surrounding the compact object. X-rays and gamma-rays are produced in an intrabinary shock front between the relativistic pulsar wind and matter from the companion and an outer accretion disc. The intense spin-down power irradiates the disc and the companion star, accounting for the UV and optical emissions.
ABSTRACT The Fermi-Large Area Telescope (LAT) First Source Catalog (1FGL) was released in 2010 February and the Fermi-LAT 2-Year Source Catalog (2FGL) appeared in 2012 April, based on data from 24 ...months of operation. Since they were released, many follow up observations of unidentified γ-ray sources have been performed and new procedures for associating γ-ray sources with potential counterparts at other wavelengths have been developed. Here we review and characterize all of the associations as published in the 1FGL and 2FGL catalogs on the basis of multifrequency archival observations. In particular, we located 177 spectra for the low-energy counterparts that were not listed in the previous Fermi catalogs, and in addition we present new spectroscopic observations of eight γ-ray blazar candidates. Based on our investigations, we introduce a new counterpart category of "candidate associations" and propose a refined classification for the candidate low-energy counterparts of the Fermi sources. We compare the 1FGL-assigned counterparts with those listed in 2FGL to determine which unassociated sources became associated in later releases of the Fermi catalogs. We also search for potential counterparts to all of the remaining unassociated Fermi sources. Finally, we prepare a refined and merged list of all of the associations of 1FGL plus 2FGL that includes 2219 unique Fermi objects. This is the most comprehensive and systematic study of all the associations collected for the γ-ray sources available to date. We conclude that 80% of the Fermi sources have at least one known plausible γ-ray emitter within their positional uncertainty regions.
The Lyman alpha reference sample Guaita, L; Melinder, J; Hayes, M ...
Astronomy and astrophysics (Berlin),
04/2015, Letnik:
576
Journal Article
Recenzirano
Odprti dostop
The transport of Ly alpha photons in galaxies is a complex process and the conditions under which Ly alpha photons manage to escape from certain galaxies is still under investigation. The Lyman alpha ...reference sample (LARS) is a sample of 14 local starforming galaxies, designed to study Ly alpha in detail and relate it to restframe UV and optical emission. With the aim of identifying restframe UV and optical properties, which are typical of Ly alpha emitters at both low and high redshift, we investigated the morphological properties of the LARS galaxies, in particular the ones that exhibit intense Ly alpha radiation. We measured sizes and morphological parameters in the continuum, Ly alpha , and H alpha images. We studied morphology by using the Gini coefficient vs. M20 and asymmetry vs. concentration diagrams. The stack of LARSLAE Ly alpha images is peaked in the centre, indicating that the conditions, which make a galaxy an LAE, tend to produce a concentrated surface brightness profile. On the other hand, the stack of all LARS galaxies is shallower and more extended.
The systematic patterns of separations between frequencies of modes of different degree and order are a characteristic of p-mode oscillations of stars. The frequency separations depend on the ...internal structure of the star and so measuring them in the observed oscillation spectra of variable stars gives valuable diagnostics of the interior of a star. Roxburgh & Vorontsov proposed using the ratio of the so-called small frequency separation to the large frequency separation as a diagnostic of the stellar interior, and demonstrated that this ratio was less sensitive than the individual frequency separations themselves to uncertain details of the near-surface structure. Here we derive kernels relating the frequency separation ratio to structure, and show why the ratio is relatively insensitive to the near-surface structure in terms of the very small amplitude of the kernels in the near-surface layers. We also investigate the behaviour of the separation ratio for stars of different masses and ages, and demonstrate the usefulness of the ratio in the so-called asteroseismic Hertzsprung–Russell diagram.
Context. To derive the history of star formation in the Universe a set of calibrated star formation rate tracers at different wavelengths is required. The calibration has to consistently take into ...account the effects of extinction, star formation regime (short or long-lived) and the evolutionary state to avoid biases at different redshift ranges. Aims. We use evolutionary synthesis models optimized for intense episodes of star formation to compute a consistent calibration of the most usual star formation rate tracers at different energy ranges, from X-ray to radio luminosities. Methods. We have computed the predicted evolution of the different estimators taking into account nearly-instantaneous and continuous star formation regimes and the effect of interstellar extinction (attenuation at high energies, thermal reradiation in the far infrared). We have also considered the effect of metallicity on the calibration of the different estimators. Results. A consistent calibration of a complete set of star formation rate tracers is presented, computed for the most usual star-forming regions conditions in terms of evolutionary state, star formation regime, interstellar extinction and initial mass function. We discuss the validity of the different tracers in different star formation scenarios and compare our predictions with previous calibrations of general use. Conclusions. In order to measure the intensity of star formation episodes we should distinguish between nearly-instantaneous and continuous star formation regimes. While the star formation strength ($M_\odot$) should be used for the former, the more common star formation rate ($M_\odot$ yr-1) is only valid for episodes forming stars at a constant rate during extended periods of time. Moreover, even for extended star formation episodes, the evolutionary state should be taken into account, since most SFR tracers stabilize only after 100 Myr of evolution.
We use integral field spectroscopy to study in detail the Wolf–Rayet (WR) population in NGC 3310, spatially resolving 18 star-forming knots with typical sizes of 200–300 pc in the disc of the galaxy ...hosting a substantial population of WRs. The detected emission in the so-called blue bump is attributed mainly to late-type nitrogen WRs (WNL), ranging from a few dozens to several hundreds of stars per region. Our estimated WNL/(WNL+O) ratio is comparable to reported empirical relations once the extinction-corrected emission is further corrected by the presence of dust grains inside the nebula that absorb a non-negligible fraction of UV photons. Comparisons of observables with stellar population models show disagreement by factors larger than 2–3. However, if the effects of interacting binaries and/or photon leakage are taken into account, observations and predictions tend to converge. We estimate the binary fraction of the H ii regions hosting WRs to be significant in order to recover the observed X-ray flux, hence proving that the binary channel can be critical when predicting observables. We also explore the connection of the environment with the current hypothesis that WRs can be progenitors to long-duration gamma-ray bursts (GRBs). Galaxy interactions, which can trigger strong episodes of star formation in the central regions, may be a plausible environment where WRs may act as progenitors of GRBs. Finally, even though the chemical abundance is generally homogeneous, we also find weak evidence for rapid N pollution by WR stellar winds at scales of ∼200 pc.
ULX contribution to stellar feedback Oskinova, L. M.; Bik, Adrianus; Mas-Hesse, J. M. ...
Astronomy and astrophysics (Berlin),
07/2019, Letnik:
627
Journal Article
Recenzirano
Context. X-ray radiation from accreting compact objects is an important part of stellar feedback. The metal-poor galaxy ESO 338-4 has experienced vigorous starburst during the last <40 Myr and ...contains some of the most massive super star clusters in the nearby Universe. Given its starburst age and its star-formation rate, ESO 338-4 is one of the most efficient nearby manufactures of neutron stars and black holes, hence providing an excellent laboratory for feedback studies. Aims. We aim to use X-ray observations with the largest modern X-ray telescopes XMM-Newton and Chandra to unveil the most luminous accreting neutron stars and black holes in ESO 338-4. Methods. We compared X-ray images and spectra with integral field spectroscopic observations in the optical to constrain the nature of strong X-ray emitters. Results. X-ray observations uncover three ultraluminous X-ray sources (ULXs) in ESO 338-4. The brightest among them, ESO 338 X-1, has X-ray luminosity in excess of 10(40) erg s(-1). We speculate that ESO 338-4 X-1 is powered by accretion on an intermediate-mass (greater than or similar to 300 M-circle dot)black hole. We show that X-ray radiation from ULXs and hot superbubbles strongly contributes to He II ionization and general stellar feedback in this template starburst galaxy.
A significant fraction (~30%) of the gamma -ray sources listed in the second Fermi/LAT (2FGL) catalog is still of unknown origin, being not yet associated with counterparts at lower energies. Using ...the available information at lower energies and optical spectroscopy on the selected counterparts of these gamma -ray objects, we can pinpoint their exact nature. Here, we present a pilot project pointing to assess the effectiveness of the several classification methods developed to select gamma -ray blazar candidates. To this end, we report optical spectroscopic observations of a sample of five gamma -ray blazar candidates selected on the basis of their infrared Wide-field Infrared Survey Explorer (WISE) colors or of their low-frequency radio properties. Blazars come in two main classes, BL Lac objects and FSRQs, showing similar optical spectra except for the stronger emission lines of the latter. For three of our sources, the almost featureless optical spectra obtained confirm their BL Lac nature, while for the source WISEJ022051.24+250927.6 we observe emission lines with equivalent width EW ~ 31 A, identifying it as a FSRQ with z = 0.48. The source WISEJ064459.38+603131.7, although not featuring a clear radio counterpart, shows a blazar-like spectrum with weak emission lines with EW ~ 7 A, yielding a redshift estimate of z = 0.36. In addition, we report optical spectroscopic observations of four WISE sources associated with known gamma -ray blazars without a firm classification or redshift estimate. For the latter sources, we confirm a BL Lac classification, with a tentative redshift estimate for the source WISEJ100800.81+062121.2 of z = 0.65.
Context. Though Lyα emission is one of the most used tracers of massive star formation at high redshift, it is strongly affected by neutral gas radiation transfer effects. A correct understanding of ...these effects is required to properly quantify the star formation rate along the history of the Universe. Aims. We aim to parameterize the escape of Lyα photons as a function of the galaxy properties, in order to properly calibrate the Lyα luminosity as a tracer of star formation intensity at any age of the Universe. Methods. We have embarked on a program to study the properties of the Lyα emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in a number of starburst galaxies in the Local Universe. The study is based on Hubble Space Telescope spectroscopic and imaging observations at various wavelengths, X-ray data, and ground-based spectroscopy, complemented with the use of evolutionary population synthesis models. Results. We present here the results obtained for one of those sources, IRAS 08339+6517, a strong Lyα emitter in the Local Universe, which is undergoing an intense episode of massive star formation. We have characterized the properties of the starburst, which transformed 1.4 × 108 M⊙ of gas into stars around 5−6 Myr ago. The mechanical energy released by the central super stellar cluster (SSC), located in the core of the starburst, has created a cavity devoid of gas and dust around it, leaving a clean path through which the UV continuum of the SSC is observed, with almost no extinction. While the average extinction affecting the stellar continuum is significantly larger out of the cavity, with E(B − V) = 0.15 on average, we have not found any evidence for regions with very large extinctions, which could be hiding some young, massive stars not contributing to the global UV continuum. The observed soft and hard X-ray emissions are consistent with this scenario, being originated by the interstellar medium heated by the release of mechanical energy in the first case, and by a large number of active high-mass X-ray binaries (HMXBs) in the second. In addition to the central compact emission blob, we have identified a diffuse Lyα emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the Hα emission, or the Hα/Hβ ratio. Both locally and globally, the Lyα/Hα ratio is lower than the Case B predictions, even after reddening correction, with an overall Lyα escape fraction of only 4%. Conclusions. We conclude that in IRAS 08339+6517 the Lyα photons resonantly scattered by an outflowing shell of neutral gas are being smoothly redistributed over the whole central area of the galaxy. Their increased probability of being destroyed by dust would explain the low Lyα escape fraction measured. In any case, in the regions where the diffuse Lyα emission shows the largest Lyα/Hα ratios, no additional sources of Lyα emission are required, like ionization by hot plasma as proposed for Haro 2, another galaxy in our sample. These results stress again the importance of a proper correction of scattering and transfer effects when using Lyα to derive the star formation rate in high-redshift galaxies.
Context. Lyman-α emission is commonly used as star formation tracer in cosmological studies. Nevertheless, resonant scattering strongly affects the resulting luminosity, leading to variable and ...unpredictable escape fractions in different objects. Aims. To understand how the Lyα escape fraction depends on the properties of the star-forming regions, we need high spatial resolution multiwavelength studies of nearby Lyα emitters, like Haro 2. Methods. We study the Lyα emission of Haro 2 in connection with the properties of the young stellar population, the characteristics of the interstellar medium, the distribution and intensity of the Balmer emission lines and the properties of the X-ray emission. We have used HST-STIS spectral images along the major and minor axes of Haro 2 to characterize the Lyα emission, as well as FOC UV, WFPC-2 optical and NICMOS near infrared broadband-filter images to analyze the properties of the stellar population. WFPC-2 Hα image and ground-based spectroscopy allow us to study the Balmer emission lines. Finally, Chandra/ACIS X-ray images provide resolved distribution of the X-ray emission at various energy bands. The observational data are analyzed by comparison with the predictions from evolutionary synthesis models to constrain the properties of the star formation episode. Results. The UV, Hα and far infrared luminosities of the Haro 2 nuclear starburst are well reproduced assuming a young stellar population with ages ~3.5–5.0 Myr, affected by differential intestellar extinctions. A significant fraction of the stars are completely obscured in the UV, being identifiable only indirectly by their contribution to the ionization of the gas and to the far infrared emission. The diffuse soft X-ray emission extending over the whole source is attributed to gas heated by the mechanical energy released by the starburst. A compact hard X-ray emission (likely an UltraLuminous X-ray source) has been identified in a star-forming condensation to the southeast. Both compact and diffuse Lyα emission components are observed along the major and minor axes in STIS spectral images. Lyα is spatially decoupled from Balmer lines emission, Balmer decrement and UV continuum. However, the diffuse Lyα component is spatially correlated with the diffuse soft X-ray emission. Moreover, unlike the compact Lyα emission, diffuse Lyα shows luminosities larger than predicted from Hα, assuming case B recombination and considering the dust extinction as derived from Hα/Hβ. Conclusions. The Lyα emission closely associated to the massive stellar clusters is strongly affected by the properties of the surrounding neutral gas (presence of outflows, dust abundance), leading to even a range of escape fractions at different locations within the same starburst. On the other hand, we propose that the diffuse Lyα emission originates in gas ionized by the hot plasma responsible for the soft X-ray radiation, as suggested by their spatial correlation and by the measured L(Hα)/L0.4−2.4 keV ratios. Calibration of Lyα as star formation rate tracer should therefore include both effects (destruction vs. enhancement) to avoid biases in the study of galaxies at cosmological distances.
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