arXiv subjects

V. La Parola

Publications and source records attributed to V. La Parola.

At least 55 records · Page 3Linked to original sources

arXiv2012

Timing accuracy of the Swift X-Ray Telescope in WT mode

The X-Ray Telescope (XRT) on board Swift was mainly designed to provide detailed position, timing and spectroscopic information on Gamma-Ray Burst (GRB) afterglows. During the mission lifetime the fraction of observing time allocated to other types of source has been steadily increased. In this paper, we report on the results of the in-flight calibration of the timing capabilities of the XRT in Windowed Timing read-out mode. We use observations of the Crab pulsar to evaluate the accuracy of the pulse period determination by comparing the values obtained by the XRT timing analysis with the values derived from radio monitoring. We also check the absolute time reconstruction measuring the phase position of the main peak in the Crab profile and comparing it both with the value reported in literature and with the result that we obtain from a simultaneous Rossi X-Ray Timing Explorer (RXTE) observation. We find that the accuracy in period determination for the Crab pulsar is of the order of a few picoseconds for the observation with the largest data time span. The absolute time reconstruction, measured using the position of the Crab main peak, shows that the main peak anticipates the phase of the position reported in literature for RXTE by ~270 microseconds on average (~150 microseconds when data are reduced with the attitude file corrected with the UVOT data). The analysis of the simultaneous Swift-XRT and RXTE Proportional Counter Array (PCA) observations confirms that the XRT Crab profile leads the PCA profile by ~200 microseconds. The analysis of XRT Photodiode mode data and BAT event data shows a main peak position in good agreement with the RXTE, suggesting the discrepancy observed in XRT data in Windowed Timing mode is likely due to a systematic offset in the time assignment for this XRT read out mode.

astro-ph.IM↗

arXiv2012

The X-ray variability history of Markarian 3

We aim at constraining the geometry of the reprocessing matter in the nearby prototypical Seyfert 2 Galaxy Markarian 3 by studying the time evolution of spectral components associated to the primary AGN emission and to its Compton-scattering. We have analyzed archival spectroscopic observations of Markarian 3 taken over the last 12 years with the XMM-Newton, Suzaku and Swift observatories, as well as data taken during a monitoring campaign activated by us in 2012. The timescale of the Compton-reflection component variability (originally discovered by ASCA in the mid-'90s) is ~64 days. This upper limit improves by more than a factor of 15 previous estimates of the Compton-reflection variability timescale for this source. When the light curve of the Compton-reflection continuum in the 4-5 keV band is correlated with the 15-150 keV Swift/BAT curve a delay ~1200 days is found. The cross-correlation results are dependent on the model used to fit the spectra, although the detection of the Compton-reflection component variability is independent of the range of models employed to fit the data. Reanalysis of an archival Chandra image of Markarian 3 indicates that the Compton-reflection and the Fe K-alpha emitting regions are extended to the North up to ~300 pc. The combination of these findings suggests that the optically-thick reprocessor in Markarian 3 is clumpy. There is mounting experimental evidence for the structure of the optically-thick gas and dust in the nuclear environment of nearby heavily obscured AGN to be extended and complex. We discuss possible modifications to the standard unification scenarios encompassing this complexity. Markarian 3, exhibiting X-ray absorption and reprocessing on widely different spatial scales, is an ideal laboratory to test these models (abridged).

astro-ph.CO↗

arXiv2012

Evidence for a resonant cyclotron line in IGR J16493-4348 from the Swift-BAT hard X-ray survey

Resonant absorption cyclotron features are a key diagnostic tool to directly measure the strength of the magnetic field of accreting neutron stars. However, typical values for cyclotron features lie in the high-energy part of the spectrum between 20 keV and 50 keV, where detection is often damped by the low statistics from single pointed observations. We show that long-term monitoring campaign performed with Swift-BAT of persistently, but faint, accreting high-mass X-ray binaries is able to reveal in their spectra the presence of cyclotron features. We extracted the average Swift-BAT 15-150 keV spectrum from the 54 months long Swift-BAT survey of the high-mass X-ray source IGR J16493-4348. To constrain the broadband spectrum we used soft X-ray spectra from Swift-XRT and Suzaku pointed observations. We model the spectra using a set of phenomenological models usually adopted to describe the energy spectrum of accreting high-mass X-ray binaries; irrespective of the models we used, we found significant improvements in the spectral fits adding to the models a broad (10 keV width) absorption feature, with best-fitting energy estimate between 30 and 33 keV, that we interpret as evidence for a resonant cyclotron absorption feature. We also discuss instrumental issues related to the use of Swift-BAT for this kind of studies and the statistical method to weight the confidence level of this detection. Correcting for the gravitational redshift of a 1.4 M$_{\sun}$ neutron star, the inferred surface magnetic field is Bsurf 3.7 x 10^{12} Gauss. The spectral parameters of IGR J16493-4348 fit well with empirical correlations observed when the whole sample of high-mass binaries with detected cyclotron features is considered.

astro-ph.HE↗

arXiv2012

A complete sample of bright Swift Gamma-Ray Bursts: X-ray afterglow luminosity and its correlation with the prompt emission

We investigate wheter there is any correlation between the X-ray afterglow luminosity and the prompt emission properties of a carefully selected sub-sample of bright Swift long Gamma-Ray Bursts (GRBs) nearly complete in redshift (~90%). Being free of selection effects (except flux limit), this sample provides the possibility to compare the rest frame physical properties of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray luminosities are computed at four different rest frame times (5 min, 1 hr, 11 hr and 24 hr after trigger) and compared with the prompt emission isotropic energy E_iso, the isotropic peak luminosity L_iso and the rest frame peak energy E_peak. We find that the rest frame afterglow X-ray luminosity do correlate with these prompt emission quantities, but the significance of each correlation decreases over time. This result is in agreement with the idea that the GRB X-ray light curve can be described as the result of a combination of different components whose relative contribution and weight change with time, with the prompt and afterglow emission dominating at early and late time, respectively. In particular, we found evidence that the plateau and the shallow decay phase often observed in GRB X-ray light curves are powered by activity from the central engine. The existence of the L_X-E_iso correlation at late times (t_rf > 11 hr) suggests a similar radiative efficiency among different bursts with on average about 6% of the total kinetic energy powering the prompt emission.

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arXiv2012

Swift follow-up observations of unclassified ASCA sources

We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and plane surveys. Our short pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of ~1E-13 erg cm-2 s-1, which translates into a luminosity of ~1E33 erg s-1 for an assumed distance of D=8 kpc. The brightest source in our sample reaches a maximum 0.3-10 keV luminosity of ~2E36 (D/8kpc)^2 erg s-1 during our observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19 were not detected during our program. Since we are probing the faint end of the ASCA source populations, we expect a large fraction of the non-detections to be due to the Eddington bias. This is strengthened by the fact that we find the observed XRT count rates to be predominantly lower than expected based on the reported ASCA intensities. Investigation of the ASCA properties and long-term X-ray variability leads us to conclude that the non-detections likely include two spurious ASCA detections and three objects that are variable or transient X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies of ~2-4", which significantly improves upon their ASCA uncertainties of ~1'. We use the X-ray spectra and variability to characterise these objects. Most appear to be faint, persistent X-ray emitters that have highly absorbed spectra. Based on their X-ray properties we identify three accreting compact objects: one confirmed (AX J1754.2-2754) and one candidate (AX J1538.3-5541) X-ray binary, and one possible magnetically accreting white dwarf (AX J1620.1-5002). Furthermore, we use the improved positions of XRT-detected sources to search for counterparts in simultaneously obtained Swift/UVOT images and possible associations with cataloged sources at various wavelengths. This reveals three possible main sequence stars amongst our sample.

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arXiv2011

Properties of Supergiant Fast X-ray Transients as observed by Swift

We present the most recent results from our investigation on Supergiant Fast X-ray Transients, a class of High-Mass X-ray Binaries, with a possible counterpart in the gamma-ray energy band. Since 2007 Swift has contributed to this new field by detecting outbursts from these fast transients with the BAT and by following them for days with the XRT. Thus, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further activity is observed at lower fluxes for a remarkably longer time, up to weeks. Furthermore, we have performed several campaigns of intense monitoring with the XRT, assessing the fraction of the time these sources spend in each phase, and their duty cycle of inactivity.

astro-ph.HE↗

arXiv2011

On the nature of the absorber in IRAS 09104+4109: the X-ray and mid-infrared view

We present a long (~76 ks) Chandra observation of IRAS 09104+4109, a hyper-luminous galaxy, optically classified as a Type 2 AGN hosted in a cD galaxy in a cluster at z=0.442. We also report on the results obtained by fitting its broad-band spectral energy distribution. The Compton-thick nature of this source (which has been often referred to as an "archetype" of Compton-thick Type 2 quasars) was formerly claimed on the basis of its marginal detection in the PDS instrument onboard BeppoSAX, being then disputed using XMM-Newton data. Both Chandra analysis and optical/mid-IR spectral fitting are consistent with the presence of heavy (~1-5 10^{23} cm^{-2}) but not extreme (Compton-thick) obscuration. However, using the mid-IR and the [OIII] emission as proxies of the nuclear hard X-ray luminosity suggests the presence of heavier obscuration. The 54-month Swift BAT map shows excess hard X-ray emission likely related to a nearby (z=0.009) Type 2 AGN, close enough to IRAS 09104+4109 to significantly enhance and contaminate its emission in the early BeppoSAX PDS data.

astro-ph.HE↗

arXiv2011

Optimizing the Search for High-z GRBs: The JANUS X-ray Coded Aperture Telescope

We discuss the optimization of gamma-ray burst (GRB) detectors with a goal of maximizing the detected number of bright high-redshift GRBs, in the context of design studies conducted for the X-ray transient detector on the JANUS mission. We conclude that the optimal energy band for detection of high-z GRBs is below about 30 keV. We considered both lobster-eye and coded aperture designs operating in this energy band. Within the available mass and power constraints, we found that the coded aperture mask was preferred for the detection of high-z bursts with bright enough afterglows to probe galaxies in the era of the Cosmic Dawn. This initial conclusion was confirmed through detailed mission simulations that found that the selected design (an X-ray Coded Aperture Telescope) would detect four times as many bright, high-z GRBs as the lobster-eye design we considered. The JANUS XCAT instrument will detect 48 GRBs with z > 5 and fluence Sx > 3 {\times} 10-7 erg cm-2 in a two year mission.

astro-ph.IM↗

arXiv2011

The populations of hard X- and γ-ray sources: a correlation study and new possible identifications

We present the results of our analysis devoted to the research of sources emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue (1FGL) of γ-ray sources and the second Palermo BAT catalogue (2PBC) of hard X-ray sources, establishing a correspondence between sources when their error boxes overlap. We also extract the significance value in the BAT 15-150 keV map, obtained using a dedicated software for the reduction of BAT data, in the direction of the 1FGL sources and take into account those above the significance threshold σ = 3. We obtain a sample of common sources emitting in both the hard X- and the γ-ray energy bands and evaluate its content in galactic and extragalactic objects. We assess the fraction of unidentified sources and describe in greater detail the properties of two of them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification as blazars after the analysis of their broad-band spectral energy distribution. We discuss the blazar content of the collected 1FGL-2PBC sources: we build its redshift distibution and compare it with that of the whole blazar population as reported in the second edition of the BZCAT blazar catalogue.

astro-ph.HE↗

arXiv2011

The Swift Supergiant Fast X-ray Transients Project: recent results

We present an overview of our Supergiant Fast X-ray Transients (SFXT) project, that started in 2007, by highlighting the unique observational contribution Swift is giving to this exciting new field. By means of outburst detection with Swift/BAT and follow-up with Swift/XRT, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further significant activity is observed at lower fluxes for a considerably longer (weeks) time. After intense monitoring with Swift/XRT, we now have a firm estimate of the time SFXTs spend in each phase. The 4 SFXTs we monitored for 1-2 years spend between 3 and 5 % of the time in bright outbursts. The most most probable flux level at which a random observation will find these sources, when detected, is F(2-10 keV) ~ 1-2E-11 erg cm^{-2} s^{-1} (unabsorbed), corresponding to luminosities of a few 10^{33} to a few 10^{34} erg s^{-1}. Finally, the duty-cycle of inactivity ranges between 19 and 55 %.

astro-ph.HE↗

arXiv2011

The Swift-BAT survey reveals the orbital period of three high-mass X-ray binaries

A growing number of previously hidden Galactic X-ray sources are now detected with recent surveys performed by the Integral and Swift satellites. Most of these new sources eluded past surveys due to their large local X-ray extinction and consequent low soft X-ray flux. The Swift-BAT performs daily monitoring of the sky in an energy band (15-150 keV) which is only marginally affected by X-ray extinction, thus allowing for the search of long periodicities in the light curve and identification of the nature of the X-ray sources. We performed a period search using the folding technique in the Swift-BAT light curves of three Integral sources: IGR J05007-7047, IGR J13186-6257 and IGR J17354-3255. Their periodograms show significant peaks at 30.77$\pm$0.01 d, 19.994$\pm$0.01 d and 8.448$\pm$0.002 d, respectively. We estimate the significance of these features from the chi squared distribution of all the trials, finding a probability less than 1.5$\times10^{-4}$ that the detections occurred due to chance. We complement our analysis with the study of their broadband X-ray emission. We identify the periodicities with the orbital periods of the sources. The periods are typical for the wind accretors X-ray binaries and we support this identification showing that also their energy spectra are compatible with an X-ray spectral emission characteristic of high-mass X-ray binaries. The spectrum of IGR J05007-704 that resides in the Large Magellanic Cloud, does not show any intrinsic local absorption, whereas the spectra of the Galactic sources IGR J17354-3255 and IGR J13186-6257 may be affected by a local absorber. The folded light curve for IGR J13186-6257 suggests a possible Be companion star.

astro-ph.HE↗

arXiv2011

A two-year monitoring campaign of Supergiant Fast X-ray Transients with Swift

Swift is the only observatory which, due to its unique fast-slewing capability and broad-band energy coverage, can detect outbursts from Supergiant Fast X-ray Transients (SFXTs) from the very beginning and study their evolution panchromatically. Thanks to its flexible observing scheduling, which makes monitoring cost-effective, Swift has also performed a campaign that covers all phases of the lives of SFXTs with a high sensitivity in the soft X-ray regime, where most SFXTs had not been observed before. Our continued effort at monitorning SFXTs with 2-3 observations per week (1-2 ks) with the Swift X-Ray Telescope (XRT) over their entire visibility period has just finished its second year. We report on our findings on the long-term properties of SFXTs, their duty cycle, and the new outbursts caught by Swift during the second year.

astro-ph.HE↗

arXiv2011

Supergiant Fast X-ray Transients with Swift: catching outbursts and monitoring programs

Swift is shedding new light on the phenomenon of Supergiant Fast X-ray Transients (SFXTs), a recently discovered class of High-Mass X-ray Binaries, whose optical counterparts are O or B supergiants, and whose X-ray outbursts are about 10000 times brighter than their quiescent state. Thanks to its unique automatic fast-slewing and broad-band energy coverage, Swift is the only observatory which can detect outbursts from SFXTs from the very beginning and observe their evolution panchromatically. Taking advantage of Swift's scheduling flexibility, we have been able to regularly monitor a small sample of SFXTs with 2-3 observations per week (1-2 ks) for two years with the X-Ray Telescope (XRT). Our campaigns cover all phases of their lives, across 4 orders of magnitude in flux. We report on the most recent outburst of AX J1841.0-0536 caught by Swift which we followed in the X-rays for several days, and on our findings on the long-term properties of SFXTs and their duty cycle.

astro-ph.HE↗

arXiv2010

Confirmation of the Supergiant Fast X-ray Transient nature of AX J1841.0-0536 from Swift outburst observations

Swift observed an outburst from the supergiant fast X-ray transients (SFXT) AX J1841.0-0536 on 2010 June 5, and followed it with XRT for 11 days. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of ~1600. Our observations allow us to analyse the simultaneous broad-band (0.3-100 keV) spectrum of this source, for the first time down to 0.3 keV, can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high absorption (N_H~2x10^22 cm-2), a flat power law (Gamma~0.2), and a high energy cutoff. All of these properties resemble those of the prototype of the class, IGR J17544-2619, which underwent an outburst on 2010 March 4, whose observations we also discuss. We show how well AX J1841.0-0536 fits in the SFXT class, based on its observed properties during the 2010 outburst, its large dynamical range in X-ray luminosity, the similarity of the light curve (length and shape) to those of the other SFXTs observed by Swift, and the X-ray broad-band spectral properties.

astro-ph.HE↗

arXiv2010

Suzaku and SWIFT-BAT observations of a newly discovered Compton-thick AGN

Obscured AGN are fundamental to understand the history of Super Massive Black Hole growth and their influence on galaxy formation. However, the Compton-thick AGN (NH>1e24 cm^-2) population is basically unconstrained, with less than few dozen confirmed Compton-thick AGN found and studied so far. A way to select heavily obscured AGN is to compare the X-ray emission below 10 keV (which is strongly depressed) with the emission from other bands less affected by the absorption, i.e. the IR band. To this end, we have cross-correlated the 2XMM catalogue with the IRAS Point Source catalogue and, by using the X-ray to infrared flux ratio and X-ray colors, we selected a well defined sample of Compton-thick AGN candidates at z<0.1. The aim of this work is to confirm the nature and to study one of these local Compton-thick AGN candidates, the nearby (z=0.029) Seyfert 2 galaxy IRAS 04507+0358, by constraining the amount of intrinsic absorption (NH) and thus the intrinsic luminosity. To this end we obtained deep (100 ks) Suzaku observations (AO4 call) and performed a joint fit with SWIFT-BAT data. We analyzed XMM-Newton, Suzaku and SWIFT-BAT data and we present here the results of this broad-band (0.4-100 keV) spectral analysis. We found that the broad-band X-ray emission of IRAS 04507+0358 requires a large amount of absorption (larger than 1e24 cm^-2) to be well reproduced, thus confirming the Compton-thick nature of this source. In particular, the most probable scenario is that of a mildly (NH (1.3-1.5)x1e24 cm^-2, L(2-10 keV) (5-7)x1e43 erg s^-1) Compton-thick AGN.

astro-ph.CO↗

arXiv2010

Galaxy Clusters in the Swift/BAT era II: 10 more Clusters detected above 15 keV

We report on the discovery of 10 additional galaxy clusters detected in the ongoing Swift/BAT all-sky survey. Among the newly BAT-discovered clusters there are: Bullet, Abell 85, Norma, and PKS 0745-19. Norma is the only cluster, among those presented here, which is resolved by BAT. For all the clusters we perform a detailed spectral analysis using XMM-Newton and Swift/BAT data to investigate the presence of a hard (non-thermal) X-ray excess. We find that in most cases the clusters' emission in the 0.3-200keV band can be explained by a multi-temperature thermal model confirming our previous results. For two clusters (Bullet and Abell 3667) we find evidence for the presence of a hard X-ray excess. In the case of the Bullet cluster, our analysis confirms the presence of a non-thermal, power-law like, component with a 20-100 keV flux of 3.4 \times 10-12 erg cm-2 s-1 as detected in previous studies. For Abell 3667 the excess emission can be successfully modeled as a hot component (kT=~13keV). We thus conclude that the hard X-ray emission from galaxy clusters (except the Bullet) has most likely thermal origin.

astro-ph.CO↗

arXiv2010

The Swift view of Supergiant Fast X-ray Transients

We report here on the recent results of a monitoring campaign we have been carrying out with Swift/XRT on a sample of four Supergiant Fast X-ray Transients. The main goal of this large programme (with a net Swift/XRT exposure of 540 ks, updated to 2009, August, 31) is to address several main open issues related to this new class of High Mass X-ray Binaries hosting OB supergiant stars as companions. Here we summarize the most important results obtained between October 2007 and August 2009.

astro-ph.HE↗

arXiv2010

The Palermo Swift-BAT hard X-ray catalogue III. Results after 54 months of sky survey

We present the Second Palermo Swift-BAT hard X-ray catalogue obtained by analysing data acquired in the first 54 months of the Swift mission. Using our software dedicated to the analysis of data from coded mask telescopes, we analysed the BAT survey data in three energy bands (15-30 keV, 15-70 keV, 15-150 keV), obtaining a list of 1256 detections above a significance threshold of 4.8 standard deviations. The identification of the source counterparts is pursued using two strategies: the analysis of field observations of soft X-ray instruments and cross-correlation of our catalogue with source databases.The survey covers 50% of the sky to a 15--150 keV flux limit of 1.0 x 10^-11 erg s^-1 cm^-2 and 9.2 x 10^-12 erg s^-1 cm^-2 for |b|< 10 degrees and |b|> 10 degrees, respectively. The Second Palermo Swift-BAT hard X-ray catalogue includes 1079 (86%) hard X-ray sources with an associated counterpart (26 with a double association and 2 with a triple association) and 177 BAT excesses (14%) that still lack a counterpart. The distribution of the BAT sources among the different object classes consists of 19% Galactic sources, 57% extragalactic sources, and 10% sources with a counterpart at softer energies whose nature has not yet been determined. About half of the BAT associated sources lack a counterpart in the ROSAT catalogues. This suggests that either moderate or strong absorption may be preventing their detection in the ROSAT energy band. The comparison of our BAT catalogue with the Fermi Large Area Telescope First Source Catalogue identifies 59 BAT/Fermi correspondences: 48 blazars, 3 Seyfert galaxies, 1 interacting galaxy, 3 high mass X-ray binaries, and 4 pulsars/supernova remnants. This small number of correspondences indicates that different populations make the sky shine in these two different energy bands.

astro-ph.HE↗