Browsing by Author "Bird, A J"
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- ItemOpen AccessAccurate classification of 29 objects detected in the 39 month Palermo Swift /BAT hard X-ray catalogue(2012) Parisi, P; Masetti, N; Jiménez-Bailón, E; Chavushyan, V; Palazzi, E; Landi, R; Malizia, A; Bassani, L; Bazzano, A; Bird, A J; Charles, P A; Galaz, G; Mason, E; McBride, V A; Minniti, D; Morelli, L; Schiavone, F; Ubertini, PThrough an optical campaign performed at 4 telescopes located in the northern and the southern hemispheres, plus archival data from two on-line sky surveys, we have obtained optical spectroscopy for 29 counterparts of unclassified or poorly studied hard X-ray emitting objects detected with Swift/BAT and listed in the 39 months Palermo catalogue. All these objects have also observations taken with Swift/XRT or XMM-EPIC which not only allow us to pinpoint their optical counterpart, but also to study their X-ray spectral properties (column density, power law photon index and F2-10 keV flux). We find that 28 sources in our sample are AGN; 7 are classified as type 1 while 21 are of type 2; the remaining object is a galactic cataclysmic variable. Among our type 1 AGN, we find 5 objects of intermediate Seyfert type (1.2-1.9) and one Narrow Line Seyfert 1 galaxy; for 4 out of 7 sources, we have been able to estimate the central black hole mass. Three of the type 2 AGN of our sample display optical features typical of the LINER class and one is a likely Compton thick AGN. All galaxies classified in this work are relatively nearby objects since their redshifts lie in the range 0.008-0.075; the only galactic object found lies at an estimated distance of 90 pc. We have also investigated the optical versus X-ray emission ratio of the galaxies of our sample to test the AGN unified model. For them, we have also compared the X-ray absorption (due to gas) with the optical reddening (due to dust): we find that for most of our sources, specifically those of type 1.9-2.0 the former is higher than the latter confirming early results by Maiolino et al. (2001); this is possibly due to the properties of dust in the circumnuclear obscuring torus of the AGN.
- ItemOpen AccessThe nature of 50 Palermo Swift -BAT hard X-ray objects through optical spectroscopy(2017) Rojas, A F; Masetti, N; Minniti, D; Jiménez-Bailón, E; Chavushyan, V; Hau, G; McBride, V A; Bassani, L; Bazzano, A; Bird, A J; Galaz, G; Gavignaud, I; Landi, R; Malizia, A; Morelli, L; Palazzi, E; Patiño-Álvarez, V; Stephen, J B; Ubertini, PWe present the nature of 50 hard X-ray emitting objects unveiled through an optical spectroscopy campaign performed at seven telescopes in the northern and southern hemispheres. These objects were detected with the Burst Alert Telescope (BAT) instrument onboard the Swift satellite and listed as of unidentified nature in the 54-month Palermo BAT catalogue. In detail, 45 sources in our sample are identified as active galactic nuclei of which, 27 are classified as type 1 (with broad and narrow emission lines) and 18 are classified as type 2 (with only narrow emission lines). Among the broad-line emission objects, one is a type 1 high-redshift quasi-stellar object, and among the narrow-line emission objects, one is a starburst galaxy, one is a X-ray bright optically normal galaxy, and one is a low ionization nuclear emission line region. We report 30 new redshift measurements, 13 confirmations and 2 more accurate redshift values. The remaining five objects are galactic sources: three are Cataclismic Variables, one is a X-ray Binary probably with a low mass secondary star, and one is an active star.
- ItemOpen AccessThe VLT-FLAMES Tarantula survey: XX. The nature of the X-ray bright emission-line star VFTS 399⋆(2015) Clark, J S; Bartlett, E S; Broos, P S; Townsley, L K; Taylor, W D; Walborn, N R; Bird, A J; Sana, H; de Mink, S E; Dufton, P L; Evans, C J; Langer, N; Maíz Apellániz, J; Schneider, F R N; Soszyński, IContext: The stellar population of the 30 Doradus star-forming region in the Large Magellanic Cloud contains a subset of apparently single, rapidly rotating O-type stars. The physical processes leading to the formation of this cohort are currently uncertain. Aims. One member of this group, the late O-type star VFTS 399, is found to be unexpectedly X-ray bright for its bolometric luminosity − in this study we aim to determine its physical nature and the cause of this behaviour. Methods. To accomplish this we performed a time-resolved analysis of optical, infrared and X-ray observations. Results. We found VFTS 399 to be an aperiodic photometric variable with an apparent near-IR excess. Its optical spectrum demonstrates complex emission profiles in the lower Balmer series and select He i lines − taken together these suggest an OeBe classification. The highly variable X-ray luminosity is too great to be produced by a single star, while the hard, non-thermal nature suggests the presence of an accreting relativistic companion. Finally, the detection of periodic modulation of the X-ray lightcurve is most naturally explained under the assumption that the accretor is a neutron star. Conclusions. VFTS 399 appears to be the first high-mass X-ray binary identified within 30 Dor, sharing many observational characteristics with classical Be X-ray binaries. Comparison of the current properties of VFTS 399 to binary-evolution models suggests a progenitor mass 25 M for the putative neutron star, which may host a magnetic field comparable in strength to those of magnetars. VFTS 399 is now the second member of the cohort of rapidly rotating “single” O-type stars in 30 Dor to show evidence of binary interaction resulting in spin-up, suggesting that this may be a viable evolutionary pathway for the formation of a subset of this stellar population.
- ItemOpen AccessUnveiling the nature of INTEGRAL objects through optical spectroscopy: III. Observations of seven southern sources(2006) Masetti, N; Pretorius, M L; Palazzi, E; Bassani, L; Bazzano, A; Bird, A J; Charles, P A; Dean, A J; Malizia, A; Nkundabakura, P; Stephen, J B; Ubertini, P. The detection, thanks to the
- ItemOpen AccessUnveiling the nature of INTEGRAL objects through optical spectroscopy: X. A new multi-year, multi-observatory campaign(2013) Masetti, N; Parisi, P; Palazzi, E; Jiménez-Bailón, E; Chavushyan, V; McBride, V; Rojas, A F; Steward, L; Bassani, L; Bazzano, A; Bird, A J; Charles, P A; Galaz, G; Landi, R; Malizia, A; Mason, E; Minniti, D; Morelli, L; Schiavone, F; Stephen, J B; Ubertini, PWithin the framework of our program (running since 2004) of identification of hard X-ray INTEGRAL sources through optical spectroscopy, we present the results concerning the nature of 33 high-energy objects. The data were acquired with the use of six telescopes of different sizes and from one on-line archive. The results indicate that the majority of these objects (23 out of 33) are active galactic nuclei (AGNs), whereas 10 are sources in the local Universe with eight of which in the Galaxy and two in the Small Magellanic Cloud (SMC). Among the identified AGNs, 13 are of Type 1 (i.e., with broad emission lines), eight are of Type 2 (with narrow emissions only), and two are X-ray bright, optically normal galaxies with no apparent nuclear activity in the optical. Six of these AGNs lie at high redshift (z > 0.5). Concerning local objects, we found that five of them are Galactic cataclysmic variables, three are high-mass X-ray binaries (two of which lying in the SMC), one is a low-mass X-ray binary, and one is classified as a flare star that is likely of RS CVn type. The main optical properties and inferred physical characteristics of these sources are presented and discussed.
- ItemOpen AccessX-ray pulsations from the region of the supergiant fast X-ray transient IGR J17544−2619(2012) Drave, S P; Bird, A J; Townsend, L J; Hill, A B; McBride, V A; Sguera, V; Bazzano, A; Clark, D JPhase-targeted RXTE observations have allowed us to detect a transient 71.49 pm 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544-2619. The phase-folded light curve shows a possible double-peaked structure with a pulsed flux of ~4.8*10-12 erg cm-2 s-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544-2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant fast X-ray transients to span across both of the original classes of HMXB in Porb - Pspin space.