Showing papers by "Luigi Stella published in 2007"
TL;DR: In this paper, the early-time afterglow light curve carries information about 0, which determines the time of the peak of the gamma-ray burst (GRB) peak.
Abstract: Context. Gamma-ray burst (GRB) emission is believed to originate in highly relativistic fireballs. Aims. Currently, only lower limits were securely set to the initia l fireball Lorentz factor 0. We aim to provide a direct measure of 0. Methods. The early-time afterglow light curve carries information about 0, which determines the time of the afterglow peak. We have obtained early observations of the near-infrared afte rglows of GRB 060418 and GRB 060607A with the REM robotic telescope. Results. For both events, the afterglow peak could be clearly singled out, allowing a firm determination of the fireball Lorentz of 0∼ 400, fully confirming the highly relativistic nature of GRB fi reballs. The deceleration radius was inferred to be Rdec≈ 10 17 cm. This is much larger than the internal shocks radius (believed to power the prompt emission), thus providing further evidence for a different origin of the prompt and afterglow stages of the GRB.
253 citations
TL;DR: In this article, the evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4 days, which they interpret as the recovery stage following a large glitch (??/? 6? 10-5).
Abstract: On 2006 September 21, an intense (~1039 erg s-1) and short (20 ms) burst was detected by Swift BAT at a position consistent with that of the candidate anomalous X-ray pulsar (AXP) CXOU J164710.2-455216, discovered by Chandra in 2005. Swift follow-up observations began ~13 hr after the event and found the source at a 1-10 keV flux level of about 4.5 ? 10-11 erg cm-2 s-1, i.e., ~300 times brighter than measured 5 days earlier by XMM-Newton. We report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried out during the first 4 months after the burst. These data are complemented with those from two XMM-Newton observations (carried out just before and after the BAT event) and four archival Chandra observations carried out between 2005 and 2007. We find a phase-coherent solution for the source pulsations after the burst. The evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4 days, which we interpret as the recovery stage following a large glitch (??/? ~ 6 ? 10-5). We also detect a quadratic component corresponding to a spin-down rate of ~ 9 ? 10-13 s s-1, implying a magnetic field strength of 1014 G. During the first Swift XRT observation taken 0.6 days after the burst, the spectrum showed a kT ~ 0.65 keV blackbody (RBB ~ 1.5 km) plus a ? ~ 2.3 power law accounting for about 60% of the 1-10 keV observed flux. Analysis of Chandra archival data, taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at energies above ~4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the neutron star as it rotates. These findings demonstrate that CXOU J164710.2-455216 is indeed an AXP; we compare them with the properties of three other AXPs which displayed similar behavior in the past.
97 citations
TL;DR: In this paper, the authors performed a spectral and timing analysis of the prompt and afterglow emission and discussed the results in the context of the standard fireball model, and found that the spectral behavior of the X-ray light curve is consistent with a jet origin of the observed break, although the optical data can not definitively confirm this.
Abstract: Context. Our knowledge of the intrinsic properties of short duration Gamma-Ray Bursts has relied, so far, only upon a few cases for which the estimate of the distance and an extended, multiwavelength monitoring of the afterglow have been obtained. Aims. We carried out multiwavelength observations of the short GRB 061201 aimed at estimating its distance and studying its properties. Methods. We performed a spectral and timing analysis of the prompt and afterglow emission and discuss the results in the context of the standard fireball model. Results. A clear temporal break was observed in the X-ray light curve about 40 min after the burst trigger. We find that the spectral and timing behaviour of the X-ray afterglow is consistent with a jet origin of the observed break, although the optical data can not definitively confirm this and other scenarios are possible. No underlying host galaxy down to R ∼ 26 mag was found after fading of the optical afterglow. Thus, no secure redshift could be measured for this burst. The nearest galaxy is at z = 0.111 and shows evidence of star formation activity. We discuss the association of GRB 061201 with this galaxy and with the ACO S 995 galaxy cluster, from which the source is at an angular distance of 17 �� and 8.5 � , respectively. We also test the association with a possible undetected, positionally consistent galaxy at z ∼ 1. In all these cases, in the jet interpretation, we find a jet opening angle of 1–2 degrees.
92 citations
TL;DR: In this article, optical observations of the afterglow of GRB 050724 and of its host galaxy, significantly expanding the existing dataset for this event, were presented, complementing them with available measurements from the literature.
Abstract: Context. New information on short/hard gamma-ray bursts (GRBs) is being gathered thanks to the discovery of their optical and X-ray afterglows. However, some key aspects are still poorly understood, including the collimation level of the outflow, the duration of the central engine activity, and the properties of the progenitor systems. Aims. We want to constrain the physical properties of the short GRB 050724 and of its host galaxy, and make some inferences on the global short GRB population. Methods. We present optical observations of the afterglow of GRB 050724 and of its host galaxy, significantly expanding the existing dataset for this event. We compare our results with models, complementing them with available measurements from the literature. We study the afterglow light curve and spectrum including X-ray data. We also present observations of the host galaxy. Results. The observed optical emission was likely related to the large flare observed in the X-ray light curve. The apparent steep decay was therefore not due to the jet effect. Available data are indeed consistent with low collimation, in turn implying a large energy release, comparable to that of long GRBs. The flare properties also constrain the internal shock mechanism, requiring a large Lorentz factor contrast between the colliding shells. This implies that the central engine was active at late times, rather than ejecting all shells simultaneously. The host galaxy has red colors and no ongoing star formation, consistent with previous findings on this GRB. However, it is not a pure elliptical, and has some faint spiral structure. Conclusions. GRB 050724 provides the most compelling case for association between a short burst and a galaxy with old stellar population. It thus plays a pivotal role in constraining progenitors models, which should allow for long delays between birth and explosion.
74 citations
TL;DR: In this article, the authors analyzed high resolution spectroscopic observations (R = 20000-45 000, corresponding to 14 km s −1 at at 4200 A and 66 km s -1 at 9000 A) of the optical afterglow of GRB050730, obtained with UVES @VLT ∼4 h after the GRB trigger.
Abstract: Aims The aim of this paper is to study the Gamma Ray Burst (GRB) environment through the analysis of the optical absorption features due to the gas surrounding the GRB Methods To this purpose we analyze high resolution spectroscopic observations (R = 20000-45 000, corresponding to 14 km s -1 at at 4200 A and 66 km s -1 at 9000 A) of the optical afterglow of GRB050730, obtained with UVES @VLT ∼4 h after the GRB trigger Results The spectrum shows that the ISM of the GRB host galaxy at z = 3967 is complex, with at least five components contributing to the main absorption system We detect strong C II*, Si II*, O I* and Fe II* fine structure absorption lines associated to the second and third component Conclusions For the first three components we derive information on the relative distance from the site of the GRB explosion Component 1, which has the longest wavelength, highest positive velocity shift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as CIV and OVI, suggesting that this component is very close to the GRB site From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10-100 times smaller than that of component 3 We evaluated the mean metallicity of the z = 3967 system obtaining values ≈10-2 of the solar metallicity or less However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values The mean [C/Fe] ratio agrees well with that expected by single star-formation event models Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site
63 citations
TL;DR: In this article, the results of the first Suzaku observation of SGR 1806-20, together with almost simultaneous observations with XMM-Newton and INTEGRAL, were presented.
Abstract: In December 2004, the soft gamma-ray repeater SGR 1806-20 emitted the most powerful giant flare ever observed. This probably involved a large-scale rearrangement of the magnetosphere leading to observable variations in the properties of its X-ray emission. Here we present the results of the first Suzaku observation of SGR 1806-20, together with almost simultaneous observations with XMM-Newton and INTEGRAL. The source seems to have reached a state characterized by a flux close to the pre-flare level and by a relatively soft spectrum. Despite this, SGR 1806-20 also remained quite active after the giant flare, allowing us to study several short bursts observed by Suzaku in the 1-100 keV range. We discuss the broad-band spectral properties of SGR 1806-20, covering both persistent and bursting emission, in the context of the magnetar model, and consider its recent theoretical developments.
61 citations
TL;DR: In this article, the authors performed a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of 2004 December.
Abstract: We performed a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of 2004 December. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we are able to study, for the first time in this class of sources, the spin-up of a millisecond pulsar as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.
52 citations
TL;DR: In this article, the authors report on two new XMM−Newton observations of the anomalous X-ray pulsar (AXP) 4U 0142+614 performed in 2004 March and July, collecting the most accurate spectrum for this source.
Abstract: We report on two new XMM‐Newton observations of the anomalous X-ray pulsar (AXP) 4U 0142+614 performed in 2004 March and July, collecting the most accurate spectrum for this source. Furthermore, we analyse two short archival observations performed in 2002 February and 2003 January in order to study the long-term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in 4U 0142+614 spectrum as a function of energy: equivalent width in the 1‐3 keV energy range <4 and <8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse‐phase spectroscopy shows spectral variability as a function of phase. By making use of XMM‐Newton and INTEGRAL data, we successfully model the 1‐250 keV spectrum of 4U 0142+614 with three models, namely the canonical absorbed blackbody plus two power laws, a resonant cyclotron scattering model plus one power-law and two logparabolic functions.
49 citations
TL;DR: In this paper, the host galaxy of the long-duration gamma-ray burst (GRB) 31203 (HG 031203) offers a precious opportunity to study in detail the environment of a nearby GRB.
Abstract: Aims. The host galaxy of the long-duration gamma-ray burst (GRB) 031203 (HG 031203) offers a precious opportunity to study in detail the environment of a nearby GRB. The aim is to better characterize this galaxy and analyse the possible evolution with time of the spectroscopic quantities we derive. Methods. We analyse HG 031203 using a set of optical spectra acquired with the ESO-VLT and Keck telescope. We compare the metallicity, luminosity and star formation properties of this galaxy and of the other supernova-long gamma-ray burst hosts in the local universe (z < 0.2) against the KPNO International Spectroscopic Survey. Results. HG031203 is a metal poor, actively star forming galaxy (star formation rate of 12.9 ± 2.2 M ⊙ yr -1 ) at z = 0.1054. From the emission-line analysis we derive an intrinsic reddening E HG (B - V) ≈ 0.4. This parameter doesn't show a compelling evidence of evolution at a month time-scale. We find an interstellar medium temperature of ≈12 500 K and an electronic density of N e = 160 cm -3 . After investigating for possible Wolf-Rayet emission features in our spectra, we consider dubious the classification of HG 031203 as a Wolf-Rayet galaxy. Long gamma-ray burst (LGRB) and supernova hosts in the local universe (z < 0.2) show, on average, specific star formation rates higher than ordinary star forming galaxy at the same redshift. On the other hand, we find that half of the hosts follows the metallicity-luminosity relation found for star-burst galaxies; HG 031203 is a clear outlier, with its really low metallicity (12 + log (O/H)= 8.12 ± 0.04).
45 citations
TL;DR: In this paper, the host galaxy of the long-duration gamma-ray burst (GRB) 31203 (HG 031203) was analyzed using a set of optical spectra acquired with the ESO-VLT and Keck telescope.
Abstract: Aims. The host galaxy of the long-duration gamma-ray burst (GRB) 031203 (HG 031203) offers a precious opportunity to study in detail the environment of a nearby GRB. The aim is to better characterize this galaxy and analyse the possible evolution with time of the spectroscopic quantities we derive. Methods. We analyse HG 031203 using a set of optical spectra acquired with the ESO-VLT and Keck telescope. We compare the metallicity, luminosity and star formation properties of this galaxy and of the other supernova-long gamma-ray burst hosts in the local universe (z < 0.2) against the KPNO International Spectroscopic Survey. � � �
43 citations
University of Nevada, Las Vegas1, University of Copenhagen2, California Institute of Technology3, Tel Aviv University4, University of Texas at Austin5, University of California, Berkeley6, University of Iceland7, University of Hertfordshire8, INAF9, Russian Academy of Sciences10, Sternberg Astronomical Institute11, Ohio University12, European Space Agency13, Spanish National Research Council14, University of Milan15, Yale University16, European Southern Observatory17, Stockholm University18, Aarhus University19, University of Bonn20, Seoul National University21, Scientific and Technological Research Council of Turkey22, Middle East Technical University23, Goddard Space Flight Center24, University of Warwick25, Loyola University Chicago26, Complutense University of Madrid27, University of Denver28, University of Oslo29, University of Concepción30, University of Leicester31, Clemson University32, University of Amsterdam33
TL;DR: In this paper, the authors collected optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to September 2009, for a total of 76 GRBs, and presented an additional three pre-Swift GRBs not included in an earlier sample.
Abstract: We have gathered optical photometry data from the literature on a large sample of Swift-era gamma-ray burst (GRB) afterglows including GRBs up to September 2009, for a total of 76 GRBs, and present an additional three pre-Swift GRBs not included in an earlier sample. Furthermore, we publish 840 additional new photometry data points on a total of 42 GRB afterglows, including large data sets for GRBs 050319, 050408, 050802, 050820A, 050922C, 060418, 080413A and 080810. We analyzed the light curves of all GRBs in the sample and derived spectral energy distributions for the sample with the best data quality, allowing us to estimate the host galaxy extinction. We transformed the afterglow light curves into an extinction-corrected z=1 system and compared their luminosities with a sample of pre-Swift afterglows. The results of a former study, which showed that GRB afterglows clustered and exhibited a bimodal distribution in luminosity space, is weakened by the larger sample. We found that the luminosity distribution of the two afterglow samples (Swift-era and pre-Swift) are very similar, and that a subsample for which we were not able to estimate the extinction, which is fainter than the main sample, can be explained by assuming a moderate amount of line-of-sight host extinction. We derived bolometric isotropic energies for all GRBs in our sample, and found only a tentative correlation between the prompt energy release and the optical afterglow luminosity at one day after the GRB in the z=1 system. A comparative study of the optical luminosities of GRB afterglows with echelle spectra (which show a high number of foreground absorbing systems) and those without reveals no indication that the former are statistically significantly more luminous. (abridged)
TL;DR: In this paper, the authors compared the data on Hete J1900.1-2455 with other similar objects and made an attempt at deriving constraints on the physical processes responsible for a spectral formation.
Abstract: Aims: Hete J1900.1-2455 is the seventh known X-ray transient accreting millisecond pulsar and has been in outburst for more than one year. We compared the data on Hete J1900.1-2455 with other similar objects and made an attempt at deriving constraints on the physical processes responsible for a spectral formation. Methods: The broad-band spectrum of the persistent emission in the 2-300 keV energy band and the timing properties were studied using simultaneous INTEGRAL and the publicly available RXTE data obtained in October 2005. The properties of the X-ray bursts observed from Hete J1900.1-2455 were also investigated. Results: The spectrum is well described by a two-component model consisting of a blackbody-like soft X-ray emission at 0.8 keV temperature and a thermal Comptonized spectrum with the electron temperature of 30 keV and Thomson optical depth tau_T ~ 2 for the slab geometry. The source is detected by INTEGRAL up to 200 keV at the luminosity of 5E36 erg/s (assuming a distance of 5 kpc) in the 0.1-200 keV energy band. We have also detected one type I X-ray burst which shows the photospheric radius expansion. The burst occurred at an inferred persistent emission level of ~ 3-4% of the Eddington luminosity. Using the data for all X-ray bursts observed to date from Hete J1900.1-2455, the burst recurrence time is estimated to be about 2 days. No pulsations have been detected either in the RXTE or in the INTEGRAL data which puts interesting constraints on theories of the magnetic field evolution in neutron star low-mass X-ray binaries.
TL;DR: In this article, an empirical spectral modeling of the high-energy emission of the anomalous X-ray pulsar 4U 0142+614 based on simultaneous Swift and INTEGRAL observations from X- to γ-ray energies is presented.
Abstract: We present an empirical spectral modeling of the high-energy emission of the anomalous X-ray pulsar 4U 0142+614 based on simultaneous Swift and INTEGRAL observations from X- to γ-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components that produced a good fit while requiring the lowest number of free parameters. Only three models were found to satisfactorily fit the 0.5-250 keV spectrum of 4U 0142+614: (1) a ~0.4 keV blackbody and two power laws, (2) a resonant cyclotron scattering model plus a power law, and (3) two log-parabolic functions. We found that only the latter two models do not overpredict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the γ-ray range. A possible interpretation of the two log parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed.
TL;DR: In this article, X-ray and IR observations of 1RXS J170849-400910 were reported, showing that the AXP flux is rising again, and the spectrum hardening.
Abstract: We report here on X-ray and IR observations of the Anomalous X-ray Pulsar (AXP) 1RXS J170849-400910. First, we report on new XMM-Newton, Swift-XRT and Chandra observations of this AXP, which confirm the intensity–hardness correlation observed in the long term X-ray monitoring of this source. These new X-ray observations show that the AXP flux is rising again, and the spectrum hardening. If the increase of the source intensity is indeed connected with the glitches and a possible bursting activity, we expect this source to enter in a bursting active phase around 2006–2007. Second, we report on deep IR observations of 1RXS J170849-400910, taken with the VLT-NACO adaptive optics, showing that there are many weak sources consistent with the AXP position. Neither star A or B, as previously proposed by different authors, might yet be conclusively recognised as the IR counterpart of 1RXS J170849-400910. Third, using Monte Carlo simulations, we re-address the calculation of the significance of the absorption line found in a phase-resolved spectrum of this source, and interpreted as a resonant scattering cyclotron feature.
TL;DR: In this paper, the authors detected two large spin glitches ( ∆ν ν of 1.2 and 2.1 × 10 −6 ) that occurred in January and June 2005.
Abstract: Aims. Previous studies of the X-ray flux and spectral properties of 1RXS J170849−400910 showed a possible correlation with the spin glitches that occurred in 1999 and 2001. However, due to the sparseness of spectral measurements and the paucity of detected glitches, no firm conclusion could be drawn. Methods. We retrieved and analysed archival Rossi-XTE pointings of 1RXS J170849−400910 covering the time interval between January 2003 and June 2006 and carried out a detailed timing analysis with phase fitting techniques. Results. We detected two large glitches ( ∆ν ν of 1.2 and 2.1 × 10 −6 ) that occurred in January and June 2005. The occurrence times of these glitches are in agreement with the predictions made in our previous studies. This finding strongly suggests a connection between the flux, spectral and timing properties of 1RXS J170849−400910.
TL;DR: In this article, the optical and NIR photometry of the field of IGR-J00291+5934 during quiescence was performed to look for the presence of a variable counterpart.
Abstract: Aims. The recent (December 2004) discovery of the sixth accretion-powered millisecond X-ray pulsar IGR J00291+5934 provides a very good chance to deepen our knowledge of such systems. Although these systems are well studied at high energies, poor informations are available for their optical/NIR counterparts during quiescence. Up to now, only for SAX J1808.4-3658, the first discovered system of this type, we have a secure multiband detection of its optical counterpart in quiescence. Among the seven known system IGR J00291+5934 is the one that resembles SAX J1808.4-3658 more closely. Methods. With the Italian 3.6 m TNG telescope, we have performed deep optical and NIR photometry of the field of IGR J00291+5934 during quiescence in order to look for the presence of a variable counterpart. Results. We present here the first multiband ( VRIJH ) detection of the optical and NIR counterpart of IGR J00291+5934 in quiescence as well as a deep upper limit in the K -band. We obtain an optical light curve that shows variability consistent with a sinusoidal modulation at the known 2.46 h orbital period and present evidence for a strongly irradiated companion.
INAF1
TL;DR: Fast quasi-periodic oscillations (QPOs; frequencies of ~20-1840 Hz) have recently been discovered in the ringing tail of giant flares from soft gamma repeaters (SGRs) when the luminosity is of order 1041-1041.5 ergs s-1 as mentioned in this paper.
Abstract: Fast quasi-periodic oscillations (QPOs; frequencies of ~20-1840 Hz) have recently been discovered in the ringing tail of giant flares from soft gamma repeaters (SGRs) when the luminosity is of order 1041-1041.5 ergs s-1. These oscillations persisted for many tens of seconds, remained coherent for up to hundreds of cycles, and were observed over a wide range of rotational phases of the neutron stars believed to host SGRs. Therefore, these QPOs must have originated from a compact, virtually nonexpanding region inside the star's magnetosphere, emitting with a very moderate degree of beaming (if at all). The fastest QPOs imply a luminosity variation of ΔL/Δt 6 × 1043 ergs s-2, the largest luminosity variation ever observed from a compact source. It exceeds by over an order of magnitude the usual Cavallo-Fabian-Rees (CFR) luminosity variability limit for a matter-to-radiation conversion efficiency of 100%. We show that such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field 1015 G at the star surface, providing independent evidence for the superstrong magnetic fields of magnetars.
TL;DR: Fast quasi-periodic oscillations (QPOs, frequencies of 20 - 1840$ Hz) have been recently discovered in the ringing tail of giant flares from Soft Gamma Repeaters (SGRs), when the luminosity was of order $10.41-10.5}$ erg/s.
Abstract: Fast Quasi-Periodic Oscillations (QPOs, frequencies of $\sim 20 - 1840$ Hz) have been recently discovered in the ringing tail of giant flares from Soft Gamma Repeaters (SGRs), when the luminosity was of order $10^{41}-10^{41.5}$ erg/s. These oscillations persisted for many tens of seconds, remained coherent for up to hundreds of cycles and were observed over a wide range of rotational phases of the neutron stars believed to host SGRs. Therefore these QPOs must have originated from a compact, virtually non-expanding region inside the star's magnetosphere, emitting with a very moderate degree of beaming (if at all). The fastest QPOs imply a luminosity variation of $\Delta L/\Delta t \simeq 6 \times 10^{43}$ erg s$^{-2}$, the largest luminosity variation ever observed from a compact source. It exceeds by over an order of magnitude the usual Cavallo-Fabian-Rees (CFR) luminosity variability limit for a matter-to-radiation conversion efficiency of 100%. We show that such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field $\gtrsim 10^{15}$ G at the star surface, providing independent evidence for the superstrong magnetic fields of magnetars.
INAF1
TL;DR: In this paper, an empirical spectral modelling of the high energy emission of the anomalous X-ray pulsar 4U 0142+614, based on simultaneous Swift and INTEGRAL observations from X to gamma-ray energies, was presented.
Abstract: We present an empirical spectral modelling of the high energy emission of the anomalous X-ray pulsar 4U 0142+614, based on simultaneous Swift and INTEGRAL observations from X to gamma-ray energies. We adopted models contained in the XSPEC analysis package, as well as models based on recent theoretical studies, and restricted ourselves to those combinations of up to three components which produce a good fit while requiring the lowest number of free parameters. Only three models were found to fit satisfactorily the 0.5-250keV spectrum of 4U 0142+614: i) a ~0.4keV blackbody and two power-laws, ii) a resonant cyclotron scattering model plus a power-law and iii) two log-parabolic functions. We found that only the latter two models do not over-predict the infrared/optical emission observed simultaneously from this AXP, and only the log-parabolic functions can naturally account for the upper limits set by COMPTEL in the gamma-ray range. A possible interpretation of the two log-parabolae in terms of inverse Compton scattering of soft X-ray photons by very energetic particles is discussed.
TL;DR: In this paper, a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main Gamma Ray Burst (GRB) event is presented.
Abstract: We present a detailed study of the spectral and temporal properties of the X-ray and optical emission of GRB050713a up to 0.5 day after the main Gamma Ray Burst (GRB) event. The X-ray light curve exhibits large amplitude variations with several rebrightenings superposed on the underlying three-segment broken power-law that is often seen in Swift GRBs. Our time-resolved spectral analysis supports the interpretation of a long-lived central engine, with rebrightenings consistent with energy injection in refreshed shocks as slower shells generated in the central engine prompt phase catch up with the afterglow shock at later times. Our sparsely sampled light curve of the optical afterglow can be fitted with a single power-law without large flares. The optical decay index appears flatter than the X-ray one, especially at later times.
INAF1
TL;DR: In this article, it was shown that a newborn millisecond-spinning magnetar would radiate for a few days a strong gravitational wave signal the frequency of which (∼0.5-2 kHz range) decreases in time.
Abstract: Two classes of high-energy sources, the Soft Gamma Repeaters and the Anomalous X-ray Pulsars are believed to contain slowly spinning “magnetars,” i.e. neutron stars the emission of which derives from the release of energy from their extremely strong magnetic fields (>1015 G). The enormous energy liberated in the 2004 December 27 giant flare from SGR 1806-20 (∼5×1046 erg), together with the likely recurrence time of such events, points to an internal magnetic field strength of ≥1016 G. Such strong fields are expected to be generated by a coherent α−Ω dynamo in the early seconds after the Neutron Star (NS) formation, if its spin period is of a few milliseconds at most. A substantial deformation of the NS is caused by such fields and, provided the deformation axis is offset from the spin axis, a newborn millisecond-spinning magnetar would thus radiate for a few days a strong gravitational wave signal the frequency of which (∼0.5–2 kHz range) decreases in time. This signal could be detected with Advanced LIGO-class detectors up to the distance of the Virgo cluster, where ≥1 yr−1 magnetars are expected to form. Recent X-ray observations revealed that SNRs around magnetar candidates do not appear to have received a larger energy input than in standard SNRs (see Vink and Kuiper, Mon. Not. Roy. Astron. Soc. 319, L14 (2006)). This is at variance with what would be expected if the spin energy of the young, millisecond NS were radiated away as electromagnetic radiation and/or relativistic particle winds. In fact, such energy would be transferred quickly and efficiently to the expanding gas shell. This may thus suggest that magnetars did not form with the expected very fast initial spin. We show here that these findings can be reconciled with the idea of magnetars being formed with fast spins, if most of their initial spin energy is radiated through GWs. In particular, we find that this occurs for essentially the same parameter range that would make such objects detectable by Advanced LIGO-class detectors up to the Virgo Cluster. If our argument holds for at least a fraction of newly formed magnetars, then these objects constitute a promising new class of gravitational wave emitters.
TL;DR: In this article, the authors present timing and spectral analyses of two XMM-Newton observations of 4U 2129+47, carried out in May and June, 2005, and find evidence for a delay between two mid-eclipse epochs measured ∼22 days apart.
Abstract: Aims. 4U 2129+47 was discovered in the early 80’s and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 h binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached. Methods. Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005. Results. We find evidence for a delay between two mid-eclipse epochs measured ∼22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47.
TL;DR: In this article, Dall'Osso et al. discuss the Unipolar Inductor Model (UIM) coupled to GW emission and compare it to observed properties of the two candidate ultrashort period binaries RX J0806+15 and RX J1914+24.
Abstract: We further discuss the Unipolar Inductor Model (UIM) coupled to GW emission (Dall'Osso et al. 2005) and compare it to observed properties of the two candidate ultrashort period binaries RX J0806+15 and RX J1914+24 . We consider the measured orbital periods, period derivatives and inferred X-ray luminosities of these two sources and find constraints on system parameters in order for the model to account for them. We find that these properties point to the two sources being in different regimes of the UIM, with the requirement of low magnetic moment primaries (∼ 10 30 G cm 3 ) for both. Given this weak magnetization, RX J0806+15 has a sufficientlylow luminosity that it can be interpreted as having a primary spin almost synchronous to and just slightly slower than the orbital motion. Its measured orbital spin-up is only slightly affected by spin-orbit coupling and is mostly due to GW emission. RX J1914+24, on the other hand, is too bright in X-rays and has too slow an orbital spin-up for the same regime to apply. We suggest that this binary system may be emitting GWs at a significantly higher rate than implied by its measured u !o ≃ 6 ×10 −17 rad s −2 . The latter is explained, in this framework, by the primary spin being slightly faster than the orbital motion (� ≤ 1.1). In this case, the associated spin-orbit coupling transfers to the orbit a significant amount of angular momentum, thus partially balancing that lost to GW emission. All expectations can be tested in the near future to confirm the viability of the model.
TL;DR: In this article, a selection procedure that identifies high-redshift (up to 6.3 ) candidates based only upon the promptly available information provided by Swift is presented and discussed, which relies upon Galactic extinction, GRB duration, and the absence of an optical counterpart in the UVOT telescope onboard Swift.
Abstract: A good fraction of GRBs detected by Swift have a high redshift (up to $z=6.3$, so far). Their study allows us to investigate, among other things, cosmic star formation in the early Universe (possibly up to the re-ionization era) and the chemical enrichment of the high-redshift gas. Here we present and discuss a selection procedure that identifies high-redshift ($z\gtrsim 5$) candidates based only upon the promptly-available information provided by Swift. This method relies upon Galactic extinction, GRB duration, and the absence of an optical counterpart in the UVOT telescope onboard Swift. This tool may provide an extremely effective way to locate high-redshift astrophysical objects and to follow them in the optical/NIR band in near real time.
TL;DR: In this article, the authors present timing and spectral analyses of two XMM-Newton observations of 4U 2129+47, carried out in May and June, 2005, and find evidence for a delay between two mid-eclipse epochs measured ~22 days apart.
Abstract: 4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star.
The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached.
Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005. We find evidence for a delay between two mid-eclipse epochs measured ~22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47.
TL;DR: In this paper, a systematic analysis of all the BeppoSAX data of SGR1900+14 was presented, showing that the source was brighter than usual on two occasions: ∼20 days after the August 1998 giant flare and during the 105 s long X-ray afterglow following the April 2001 intermediate flare.
Abstract: We present a systematic analysis of all the BeppoSAX data of SGR1900+14. The observations spanning five years show that the source was brighter than usual on two occasions: ∼20 days after the August 1998 giant flare and during the 105 s long X-ray afterglow following the April 2001 intermediate flare. In the latter case, we explore the possibility of describing the observed short term spectral evolution only with a change of the temperature of the blackbody component. In the only BeppoSAX observation performed before the giant flare, the spectrum of the SGR1900+14 persistent emission was significantly harder and detected also above 10 keV with the PDS instrument. In the last BeppoSAX observation (April 2002) the flux was at least a factor 1.2 below the historical level, suggesting that the source was entering a quiescent period.
TL;DR: In this article, the authors reported on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes.
Abstract: . We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. . This work is aimed at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. . In order to perform photometry of objects as faint as Ks~20, we have used data taken with the largest telescopes, equipped with the most advanced IR detectors and in most of the cases with Adaptive Optics devices. The latter are critical to achieve the sharp spatial accuracy required to pinpoint faint objects in crowded fields. . We confirm with high confidence the identification of the IR counterpart to XTE J1810-197, and its IR variability. For 1E 1841-045 and SGR 1900+14 we propose two candidate IR counterparts based on the detection of IR variability. For 1RXS J1708-4009 we show that none of the potential counterparts within the source X-ray error circle can be yet convincingly associated with this AXP. . The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease of its post-outburst X-ray emission. Instead, the IR variability appears more similar to the one observed in radio band, although simultaneous IR and radio observations are crucial to draw any conclusion in this respect. For 1E 1841-045 and SGR 1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence.
TL;DR: In this article, the authors performed deep searches for radio pulsations from four anomalous X-ray pulsars (AXPs) to investigate their physical nature in comparison with the rotation powered pulsars.
Abstract: We have performed deep searches for radio pulsations from four southern anomalous X-ray pulsars (AXPs) to investigate their physical nature in comparison with the rotation powered pulsars. The data were acquired using the Parkes radio telescope with the 1.4 GHz multibeam receiver. No pulsed emission with periodicity matching the X-ray ephemeris have been found in the observed targets down to a limit of ∼0.1 mJy. A blind search has also been performed on all the 13 beams of the multibeam receiver (the central beam being pointed on the target AXP), leading to the serendipitous discovery of two new radio pulsars and to the further detection of 18 pulsars. Also a search for single dispersed pulses has been performed in the aim to detect signals similar to those of the recently discovered rotating radio transients.
TL;DR: In this article, two new XMM-Newton observations of the anomalous X-ray Pulsar (AXP) 4U 0142+614 were performed in March and July 2004, collecting the most accurate spectrum for this source to date.
Abstract: We report on two new XMM-Newton observations of the Anomalous X-ray Pulsar (AXP) 4U 0142+614 performed in March and July 2004, collecting the most accurate spectrum for this source to date. Furthermore, we analyse two short archival observations performed in February 2002 and January 2003 (the latter already reported by Gohler et al. 2005) in order to study the long term behaviour of this AXP. 4U 0142+614 appears to be relatively steady in flux between 2002 and 2004, and the phase-averaged spectrum does not show any significant variability between the four epochs. We derive the deepest upper limits to date on the presence of lines in the 4U 0142+614 spectrum as a function of energy: equivalent width in the 1-3 keV energy range < 4 eV and < 8 eV for narrow and broad lines, respectively. A remarkable energy dependence in both the pulse profile and the pulsed fraction is detected, and consequently pulse-phase spectroscopy shows spectral variability as a function of phase. By making use of XMM-Newton and INTEGRAL data, we successfully model the 1-250 keV spectrum of 4U 0142+614 with three models presented in Rea et al. (2007a), namely the canonical absorbed blackbody plus two power-laws, a resonant cyclotron scattering model plus one power-law and two log-parabolic functions.