arXiv subjects

E. Mason

Publications and source records attributed to E. Mason.

At least 55 records · Page 3Linked to original sources

arXiv2008

High-redshift blazar identification for Swift J1656.3-3302

We report on the high-redshift blazar identification of a new gamma-ray source, Swift J1656.3-3302, detected with the BAT imager onboard the Swift satellite and the IBIS instrument on the INTEGRAL satellite. Follow-up optical spectroscopy has allowed us to identify the counterpart as an R-band 19 mag source that shows broad Lyman-alpha, Si IV, He II, C IV, and C III] emission lines at redshift z = 2.40+-0.01. Spectral evolution is observed in X-rays when the INTEGRAL/IBIS data are compared to the Swift/BAT results, with the spectrum steepening when the source gets fainter. The 0.7-200 keV X-ray continuum, observed with Swift/XRT and INTEGRAL/IBIS, shows the power law shape typical of radio loud (broad emission line) active galactic nuclei (with a photon index around 1.6) and a hint of spectral curvature below 2 keV, possibly due to intrinsic absorption (N_H about 7e22 cm-2) local to the source. Alternatively, a slope change (of about 1 in terms of photon index) around 2.7 keV can describe the X-ray spectrum equally well. At this redshift, the observed 20-100 keV luminosity of the source is about 1e48 erg s-1 (assuming isotropic emission), making Swift J1656.3-3302 one of the most X-ray luminous blazars. This source is yet another example of a distant gamma-ray loud quasar discovered above 20 keV. It is also the farthest object, among the previously unidentified INTEGRAL sources, whose nature has been determined a posteriori through optical spectroscopy.

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arXiv2007

Life on the fast lane: the burst mode at the VLT at present and in the future

The recent implementation of the high-speed burst mode at the ISAAC instrument on UT1, and its propagation to other ESO instruments, has opened the door to observational capabilities which hold the potential for a wealth of novel results. In the ELT era, when the accent will likely be on lengthy programs aimed at the best sensitivity and angular resolution, the VLT telescopes could continue to play a significant and largely unique role by performing routinely observations of transient events at high temporal resolution. In our contribution, we provide details on two such kinds of observations, namely lunar occultations of stars and of asteroids. For the first ones, we report on two passages of the Moon in regions with high stellar density as the Galactic Center. The VLT-UT1 telescope was used for the first time to record successfully 53 and 71 occultations on March 22 and August 6, 2006, with an angular resolution of 0.5-1 milliarcsecond and $K\sim12.5$ limiting magnitude. We note that the angular resolution is superior to that achieved at present by Adaptive Optics on any telescope, and also superior to that foreseen for the ELT at the same wavelength. LO are also very efficient in terms of telescope time. We present some of the results, including the discovery of close binaries, and the detection and study of compact circumstellar components of cool giants, AGB stars and embedded IR sources. Rest of the abstract follows at the paper

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arXiv2007

The remarkable properties of the symbiotic star AE Circinus

We present new optical spectroscopy and photometry, 2MASS infrared observations and 24 years of combined AAVSO and AFOEV photometry of the symbiotic star candidate æ. The long-term light curve is characterized by outbursts lasting several years and having a slow decline of $\sim 2 \times 10^{-4}$ mag/day. The whole range of variability of the star in the $V$ band is about 4 magnitudes. The periodogram of the photometric data reveals strong signals at $\sim$ 342 and 171 days. The presence of the emission feature at $λ$ 6830 Åat minimum and the detection of absorption lines of a $\sim$ K5 type star confirm the symbiotic classification and suggest that AE Cir is a new member of the small group of s-type yellow symbiotic stars. We estimate a distance of 9.4 kpc. Our spectrum taken at the high state shows a much flatter spectral energy distribution, the disappearance of the $λ$ 6830 Åemission feature and the weakness of the He II 4686 emission relative to the Balmer emission lines. Our observations indicate the presence of emission line flickering in time scales of minutes in 2001. The peculiar character of æis revealed in the visibility of the secondary star at the high and low state, the light curve resembling a dwarf nova superoutburst and the relatively short low states. The data are hard to reconciliate with standard models for symbiotic star outbursts.

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arXiv2007

Searching for links between magnetic fields and stellar evolution. II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations

The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars. We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands. We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T_eff, log(L/L_o), and M/M_o. With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_o the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3 10^7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10^8 yr. Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise RMS fields, will help to clarify the results obtained so far. Further field observations are in progress.

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arXiv2007

First detection of Zeeman absorption lines in the polar VV Pup

We investigated the low state of the polar VV Pup by collecting high S/N time series spectra. We monitored VV Pup with VLT+FORS1 and analyzed the evolution of its spectroscopic features across two orbits. We report the first detection of photospheric Zeeman lines in VV Puppis. We argue that the photospheric field structure is inconsistent with the assumption that the accretion shocks are located close to the foot points of a closed field line in a dipolar field distribution. A more complex field structure and coupling process is implied making VV Puppis similar to other well studied AM Herculis type systems.

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arXiv2006

Standard stars for linear polarization

We present the analysis of the observations of standard stars for linear polarization obtained from 1999 to 2005 within the context of the calibration plan of the FORS1 instrument of the ESO VLT. We have considered observations carried out both in imaging polarimetric and in spectropolarimetric mode. Broadband polarization was obtained in the Bessel BVRI filters; spectropolarimetry was obtained with various grisms covering different optical ranges and with a typical resolution of a few hundreds. Spectropolarimetric data have been convolved with the transmission functions of the Bessel filters, which enabled us to calculate polarization values equivalent to broadband polarization measurements in imaging mode. Finally, for each star, instrument mode, and for each Bessel filter band, we have calculated an average polarization value obtained after filtering all available data with a k-sigma clipping algorithm.

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arXiv2006

Early spectral evolution of Nova Sgr 2004 (V5114 Sgr)

We present optical and near-infrared spectral evolution of the Galactic nova V5114 Sgr (2004) during few months after the outburst. We use multi-band photometry and line intensities derived from spectroscopy to put constrains on the distance and the physical conditions of the ejecta of V5114 Sgr. The nova showed a fast decline (t_2 \simeq 11 days) and spectral features of FeII spectroscopic class. It reached M_V = -8.7 \pm 0.2 mag at maximum light, from which we derive a distance of 7700 \pm 700 kpc and a distance from the galactic plane of about 800 pc. Hydrogen and Oxygen mass of the ejecta are measured from emission lines, leading to 10^{-6} and 10^{-7} M_\odot, respectively. We compute the filling factor of the ejecta to be in the range 0.1 -- 10^{-3} . We found the value of the filling factor to decrease with time. The same is also observed in other novae, then giving support to the idea that nova shells are not homogeneously filled in, rather being the material clumped in relatively higher density blobs less affected by the general expanding motion of the ejecta.

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arXiv2006

Investigation of the magnetic field characteristics of Herbig Ae/Be stars: Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars

We are investigating the magnetic characteristics of pre-main sequence Herbig Ae/Be stars, with the aim of (1) understanding the origin and evolution of magnetism in intermediate-mass stars, and (2) exploring the influence of magnetic fields on accretion, rotation and mass-loss at the early stages of evolution of A, B and O stars. We have begun by conducting 2 large surveys of Herbig Ae/Be stars, searching for direct evidence of photospheric magnetic fields via the longitudinal Zeeman effect. From observations obtained using FORS1 at the ESO-VLT and ESPaDOnS at the Canada-France-Hawaii Telescope, we report the confirmed detection of magnetic fields in 4 pre-main sequence A- and B-type stars, and the apparent (but as yet unconfirmed) detection of fields in 2 other such stars. We do not confirm the detection of magnetic fields in several stars reported by other authors to be magnetic: HD 139614, HD 144432 or HD 31649. One of the most evolved stars in the detected sample, HD 72106A, shows clear evidence of strong photospheric chemical peculiarity, whereas many of the other (less evolved) stars do not. The magnetic fields that we detect appear to have surface intensities of order 1 kG, seem to be structured on global scales, and appear in about 10% of the stars studied. Based on these properties, these magnetic stars appear to be pre-main sequence progenitors of the magnetic Ap/Bp stars.

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arXiv2006

Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1

About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate. With this work we provide observational material to study how magnetic fields change with the evolution of stars on the main sequence, and to constrain theory explaining the presence of magnetic fields in A and B-type stars. Using FORS1 in spectropolarimetric mode at the ESO VLT, we have carried out a survey of magnetic fields in early-type stars belonging to open clusters and associations of various ages. We have measured the magnetic field of 235 early-type stars with a typical uncertainty of about 100 G. In our sample, 97 stars are Ap or Bp stars. For these targets, the median error bar of our field measurements was about 80 G. A field has been detected in about 41 of these stars, 37 of which were not previously known as magnetic stars. For the 138 normal A and B-type stars, the median error bar was 136 G, and no field was detected in any of them.

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arXiv2005

Unveiling the nature of INTEGRAL objects through optical spectroscopy. II. The nature of four unidentified sources

We present the results of our optical spectrophotometric campaign ongoing at the Astronomical Observatory of Bologna in Loiano (Italy) on hard X-ray sources detected by INTEGRAL. We observed spectroscopically the putative optical counterparts of four more INTEGRAL sources, IGR J12391-1610, IGR J18406-0539, 2E 1853.7+1534 and IGR J19473+4452. These data have allowed us to determine their nature, finding that IGR J12391-1610 (=LEDA 170194) and IGR J19473+4452 are Seyfert 2 galaxies at redshifts z = 0.036 and z = 0.053, respectively, IGR J18406-0539 (=SS 406) is a Be massive X-ray binary located at about 1.1 kpc from Earth, and 2E 1853.7+1534 is a Type 1 Seyfert galaxy with z = 0.084. Physical parameters for these objects are also evaluated by collecting and discussing the available multiwavelength information. The determination of the extragalactic nature of a substantial fraction of sources inside the INTEGRAL surveys underlines the importance of hard X-ray observations for the study of background Active Galactic Nuclei located beyond the `Zone of Avoidance' of the Galactic Plane.

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arXiv2005

The extragalactic nature of the serendipitous BeppoSAX source 2MASX J14585116-1652223

As a follow-up of the X-ray serendipitous detection of the source 2MASX J14585116-1652223, medium-resolution optical spectroscopic observations collected at the 1.5-metre "Cassini" telescope of the Astronomical Observatory of Bologna were performed. This allowed us to determine the extragalactic nature of this X-ray source, which is a Type 2 Seyfert galaxy at redshift z = 0.068 +- 0.001. This result points to the fact that 2MASX J14585116-1652223 hosts one of the very few Active Galactic Nuclei detected so far in the hard X-ray band above 20 keV, in particular at z > 0.05. Other optical and X-ray properties of this object are also discussed.

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arXiv2005

Discovery of the pre-main sequence progenitors of the magnetic Ap/Bp stars?

We report the discovery, using FORS1 at the ESO-VLT and ESPaDOnS at the CFHT, of magnetic fields in the young A-type stars HD 101412, V380 Ori and HD 72106A. Two of these stars (HD 101412 and V380 Ori) are pre-main sequence Herbig Ae/Be (HAeBe) stars, while one (HD 72106A) is physically associated with a HAeBe star. Remarkably, evidence of surface abundance spots is detected in the spectra of HD 72106A. The magnetic fields of these objects display intensities of order 1 kG at the photospheric level, are ordered on global scales, and appear in approximately 10% of the stars studied. Based on these properties, the detected stars may well represent pre-main sequence progenitors of the magnetic Ap/Bp stars. The low masses inferred for these objects (2.6, 2.8 and 2.4 solar masses) represent additional contradictions to the hypothesis of Hubrig et al. (2000), who claim that magnetic fields appear in intermediate-mass stars only after 30% of their main sequence evolution is complete. Finally, we fail to confirm claims by Hubrig et al. (2004) of magnetic fields in the Herbig Ae star HD 139614.

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arXiv2005

On the new short orbital period cataclysmic variable Halpha0242-2802

We present results from our phase-resolved spectroscopy of the newly identified cataclysmic variable Halpha0242-2802. Halpha0242-2802 was identified from a deep UK Schmidt Halpha-R band survey as a candidate cataclysmic variable. Although initial spectroscopy revealed an optical spectrum very similar to that of the famous short orbital period system WZ Sge, the analysis of our data shows that this is not the case. Halpha0242-28 differs from WZ Sge in both the accretion disk structure and the orbital period. In particular, Halpha0242-28 appears to be a system which has not reached the orbital period minimum, yet.

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arXiv2005

Early decline spectra of Nova SMC 2001 and Nova LMC 2002

We report results on the spectroscopic follow-up of Nova SMC 2001 and Nova LMC 2002 carried out at La Silla. The analysis of the spectroscopic evolution shows that these objects belong to the {\sl Fe II} class, according to the Cerro Tololo scheme. From the line fluxes and the expansion velocities, we have derived an approximate mass for the ejected shells of 2$\div3\times 10^{-4} M_\odot$. The filling factor measurements ($ε\sim 10^{-4}÷10^{-1}$) suggest a clumpy structure for the ejecta.

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arXiv2003

SN 2002lt and GRB 021211: a SN/GRB Connection at z = 1

We present spectroscopic and photometric observations of the afterglow of GRB 021211 and the discovery of its associated supernova, SN 2002lt. The spectrum shows a broad feature (FWHM = 150 A), around 3770 A (in the rest-frame of the GRB), which we interpret as Ca H+K blueshifted by 14400 km/s. Potential sources of contamination due to the host galaxy and/or residuals of telluric absorption have been analyzed and ruled out. Overall, the spectrum shows a suggestive resemblance with the one of the prototypical type-Ic SN 1994I. This might indicate that GRBs are produced also by standard type-Ic supernovae.

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arXiv2003

Evidence for Supernova Signatures in the Spectrum of the Late-time Bump of the Optical Afterglow of GRB 021211

We present photometric and spectroscopic observations of the gamma-ray burst GRB 021211 obtained during the late stages of its afterglow. The light curve shows a rebrightening occurring ~25 days after the GRB. The analysis of a VLT spectrum obtained during the bump (27 days after the GRB) reveals a suggestive resemblance with the spectrum of the prototypical type-Ic SN 1994I, obtained about ~10 days past maximum light. Particularly we have measured a strong, broad absorption feature at 3770 A, which we have identified with Ca II blueshifted by ~14400 km/s, thus indicating that a supernova (SN) component is indeed powering the `bump' in the afterglow decay. Assuming SN 1994I as a template, the spectroscopic and photometric data together indicate that the SN and GRB explosions were at most separated by a few days. Our results suggest that GRBs might be associated also to standard type-Ic supernovae.

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arXiv2003

Spectroscopic Observations of Twenty-one Faint Cataclysmic Variables Candidates

We provide the first minimum light spectroscopic observations for 21 previously known or suspected faint cataclysmic variable candidates. The sources were selected from the Downes et al. (2001) living edition catalog and the identified candidates have minimum light magnitudes of V~18-22. We confirm 15 of the candidates to be cataclysmic variables.

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arXiv2002

Discovery of Two New Faint Cataclysmic Variables

We report on the discovery of two new faint cataclysmic variables. The objects were selected as candidates from two different imaging surveys aimed at the discovery of such faint systems. One survey used color and variability while the other used color and H$_α$ emission as selection criteria. We present our spectra of the two new variables and discuss their properties. A discussion of the implication of these discoveries on the space density of faint cataclysmic variables is presented.

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