arXiv subjects

J. A. Kennea

Publications and source records attributed to J. A. Kennea.

At least 109 records · Page 6Linked to original sources

arXiv2011

Swift Follow-up Observations of MAXI Discovered Galactic Transients

We describe the results of the first year of a program to localize new Galactic Transient sources discovered by MAXI with NASA's Swift mission. Swift is ideally suited for follow-up of MAXI discovered transients as its X-ray Telescope (XRT) field of view (~0.2 degrees radius) is closely matched to the typical MAXI error circle. The XRT is capable of localizing new sources to an accuracy of up to 1.5 arc-seconds radius (90% confidence), and the Swift Optical/UV Telescope also provides optical imaging of any optical counterpart of the X-ray source. If no optical counterpart is found with Swift (usually due to absorption), the XRT position is good enough to allow for ground based IR telescopes to positively identify the optical counterpart. Although localization and identification of MAXI transients is the main aim of the program, these are often followed up by long term monitoring of the source. We present here results from 2 of these monitoring programs: the black-hole candidate MAXI J1659-152, and the Be/X-ray binary candidate MAXI J1409-619.

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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.

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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.

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arXiv2010

Two years of monitoring Supergiant Fast X-ray Transients with Swift

We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon indices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming the continued activity of these sources. This demonstrates that true quiescence is a rare state, and that these transients accrete matter throughout their life at different rates. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)

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arXiv2010

GRB 090926A and Bright Late-time Fermi LAT GRB Afterglows

GRB 090926A was detected by both the GBM and LAT instruments on-board the Fermi Gamma-Ray Space Telescope. Swift follow-up observations began ~13 hours after the initial trigger. The optical afterglow was detected for nearly 23 days post trigger, placing it in the long lived category. The afterglow is of particular interest due to its brightness at late times, as well as the presence of optical flares at T0+10^5 s and later, which may indicate late-time central engine activity. The LAT has detected a total of 16 GRBs; 9 of these bursts, including GRB 090926A, also have been observed by Swift. Of the 9 Swift observed LAT bursts, 6 were detected by UVOT, with 5 of the bursts having bright, long-lived optical afterglows. In comparison, Swift has been operating for 5 years and has detected nearly 500 bursts, but has only seen ~30% of bursts with optical afterglows that live longer than 10^5 s. We have calculated the predicted gamma-ray fluence, as would have been seen by the BAT on-board Swift, of the LAT bursts to determine whether this high percentage of long-lived optical afterglows is unique, when compared to BAT-triggered bursts. We find that, with the exception of the short burst GRB 090510A, the predicted BAT fluences indicate the LAT bursts are more energetic than 88% of all Swift bursts, and also have brighter than average X-ray and optical afterglows.

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arXiv2010

Supergiant fast X-ray transients: the Swift monitoring program

For the first time, Swift is giving us the opportunity to study supergiant fast X-ray transients (SFXTs) throughout all phases of their life: outbursts, intermediate level, and quiescence. We present our intense monitoring of four SFXTs, observed 2-3 times per week since October 2007. We find that, unexpectedly, SFXTs spend most of their time in an intermediate level of accretion ($L_{X}\sim 10^{33-34} $ erg s$^{-1}$), characterized by rich flaring activity. We present an overview of our investigation on SFXTs with Swift, the key results of our Project. We highlight the unique contribution Swift is giving to this field, both in terms of outburst observations and through a systematic monitoring.

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arXiv2010

The Swift SFXT monitoring campaign: the IGR J16479-4514 outburst in 2009

IGR J16479-4514 is a member of the Supergiant Fast X-ray transient (SFXT) class. We present the light curves of its latest outburst, which occurred on January 29, 2009. During this outburst, IGR J16479-4514 showed prolonged activity lasting several days. The presence of eclipses was successfully tested.

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arXiv2010

New results with Swift on Supergiant Fast X-ray Transients

We report here on the most recent results obtained on a new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients. Since October 2007, we have been performing a monitoring campaign with Swift of four SFXTs (IGRJ17544-2916, XTEJ1739-302, IGRJ16479-4514 and the X-ray pulsar AXJ1841.0-0536) for about 1-2 ks, 2-3 times per week, allowing us to derive the previously unknown long term properties of this new class of sources (their duty cycles, spectral properties in outbursts and out-of-outbursts, temporal behaviour). We also report here on additional Swift observations of two SFXTs which are not part of the monitoring: IGRJ18483-0311 (observed with Swift/XRT during a whole orbital cycle) and SAXJ1818.6-1703 (observed for the first time simultaneously in the energy range 0.3-100 keV during a bright flare).

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arXiv2010

Swift observations of the SFXT SAX J1818.6-1703 in outburst

We present the Swift observations of the supergiant fast X-ray transient (SFXT) SAX J1818.6-1703 collected during the most recent outburst, which occurred on May 6 2009. In particular, we present broad-band spectroscopic and timing analysis as well as a Swift/XRT light curve that spans more than two weeks of observations. The broad-band spectral models and length of the outburst resemble those of the prototype of the SFXT class, XTE J1739-302, further confirming SAX J1818.6-1703 as a member of this class.

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arXiv2009

Swift monitoring of the new accreting millisecond X-ray pulsar IGRJ17511-3057 in outburst

A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring programme. To study the evolution of the source X-ray flux and spectral properties during the outburst, we requested a Swift monitoring of IGRJ17511-3057. In this paper we report on the results of the first two weeks of monitoring the source. The persistent emission of IGR J17511-3057 during the outburst is modeled well with an absorbed blackbody (kT~0.9 keV) and a power-law component (photon index~1-2), similar to what has been observed from other previously known millisecond pulsars. Swift also detected three type-I Xray bursts from this source. By assuming that the peak luminosity of these bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we derived an upper limit to the source distance of ~10 kpc. The theoretical, expected recurrence time of the bursts according to the helium burst hypothesis is 0.2-0.9 days, in agreement with the observations.

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arXiv2009

Swift/XRT monitoring of the Supergiant Fast X-ray Transient IGR J18483-0311 for an entire orbital period

IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray Transients. This system is one of the two members of this class, together with IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are known. We report on the first complete monitoring of the X-ray activity along an entire orbital period of a Supergiant Fast X-ray Transient. These Swift observations, lasting 28d, cover more than one entire orbital phase consecutively. They are a unique data-set, which allows us to constrain the different mechanisms proposed to explain the nature of this new class of X-ray transients. We applied the new clumpy wind model for blue supergiants developed by Ducci et al. (2009), to the observed X-ray light curve. Assuming an eccentricity of e=0.4, the X-ray emission from this source can be explained in terms of the accretion from a spherically symmetric clumpy wind, composed of clumps with different masses, ranging from 10^{18}g to 5x 10^{21}g.

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arXiv2009

Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution

GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ROTSE and a host of other telescopes and was detected in the radio by the VLA. The redshift of z= 3.355 +/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasithermal model shifting from ~60 keV to ~3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this originated from the afterglow component, not the pulsed/flaring prompt emission. Broadband optical and X-ray coverage of the afterglow at the start of the final X-ray decay (~8 ks) reveals a spectral break between the optical and X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there is no evidence for a jet break in the afterglow up to six days following the burst.

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arXiv2009

The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAXJ1818.6-1703 in outburst

The Supergiant Fast X-ray Transient (SFXT) SAXJ1818.6-1703 underwent an outburst on 2009 May 6 and was observed with Swift. We report on these observations which, for the first time, allow us to study the broad-band spectrum from soft to hard X-rays of this source. No X-ray spectral information was available on this source before the Swift monitoring. The spectrum can be deconvolved well with models usually adopted to describe the emission from High Mass X-ray Binary X-ray pulsars, and is characterized by a very high absorption, a flat power law (photon index ~0.1-0.5) and a cutoff at about 7-12 keV. Alternatively, the SAXJ1818.6-1703 emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kTe=5-7 keV, a hot seed photon temperature, kT0, of 1.3-1.4 keV, and an optical depth for the Comptonizing plasma of about 10. The 1-100 keV luminosity at the peak of the flare is 3E36 erg/s (assuming the optical counterpart distance of 2.5 kpc). These properties of SAXJ1818.6-1703 resemble those of the prototype of the SFXT class, XTEJ1739-302. The monitoring with Swift/XRT reveals an outburst duration of about 5 days, similarly to other members of the class of SFXTs, confirming SAXJ1818.6-1703 as a member of this class.

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arXiv2009

Monitoring Supergiant Fast X-ray Transients with Swift. Results from the first year

Swift has allowed the possibility to give Supergiant Fast X-ray Transients (SFXTs), the new class of High Mass X-ray Binaries discovered by INTEGRAL, non serendipitous attention throughout all phases of their life. We present our results based on the first year of intense Swift monitoring of four SFXTs, IGR J16479-4514, XTE J1739-302, IGR J17544-2619 and AX J1841.0-0536. We obtain the first assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. The duty-cycle of inactivity is 17, 28, 39, 55% (5% uncertainty), for IGR J16479-4514, AX J1841.0-0536, XTE J1739-302, and IGR J17544-2619, respectively, so that true quiescence is a rare state. This demonstrates that these transients accrete matter throughout their life at different rates. AX J1841.0-0536 is the only source which has not undergone a bright outburst during our campaign. Although individual sources behave somewhat differently, common X-ray characteristics of this class are emerging such as outburst lengths well in excess of hours, with a multiple peaked structure. A high dynamic range (including bright outbursts) of 4 orders of magnitude has been observed. We performed out-of-outburst intensity-based spectroscopy. Spectral fits with an absorbed blackbody always result in blackbody radii of a few hundred meters, consistent with being emitted from a small portion of the neutron star surface, very likely the neutron star polar caps. We also present the UVOT data of these sources. (Abridged)

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arXiv2009

Swift Observations of Hard X-ray Emitting White Dwarfs in Symbiotic Stars

The X-ray emission from most accreting white dwarfs (WDs) in symbiotic binary stars is quite soft. Several symbiotic WDs, however, produce strong X-ray emission at energies greater than ~20 keV. The Swift BAT instrument has detected hard X-ray emission from 4 such accreting WDs in symbiotic stars: RT Cru, T CrB, CD -57 3057, and CH Cyg. In one case (RT Cru), Swift detected X-rays out to greater than 50 keV at a > 5 sigma confidence level. Combining data from the XRT and BAT detectors, we find that the 0.3-150 keV spectra of RT Cru, T CrB, and CD -57 3057 are well described by emission from a single-temperature, optically thin thermal plasma, plus an unresolved 6.4-6.9 keV Fe line complex. The X-ray spectrum of CH Cyg contains an additional bright soft component. For all 4 systems, the spectra suffer high levels of absorption from material that both fully and partially covers the source of hard X-rays. The XRT data did not show any of the rapid, periodic variations that one would expect if the X-ray emission were due to accretion onto a rotating, highly magnetized WD. The X-rays were thus more likely from the accretion-disk boundary layer around a massive, non-magnetic WD in each binary. The X-ray emission from RT Cru varied on timescales of a few days. This variability is consistent with being due to changes in the absorber that partially covers the source, suggesting localized absorption from a clumpy medium moving into the line of sight. The X-ray emission from CD -57 3057 and T CrB also varied during the 9 months of Swift observations, in a manner that was also consistent with variable absorption.

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arXiv2009

Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs

We present a homogeneous X-ray analysis of all 318 Gamma Ray Bursts detected by the X-ray Telescope on the Swift satellite up to 2008 July 23; this represents the largest sample of X-ray GRB data published to date. In Sections 2--3 we detail the methods which the Swift-XRT team has developed to produce the enhanced positions, light curves, hardness ratios and spectra presented in this paper. Software using these methods continues to create such products for all new GRBs observed by the Swift-XRT. We also detail web-based tools allowing users to create these products for any object observed by the XRT, not just GRBs. In Sections 4--6 we present the results of our analysis of GRBs, including probability distribution functions of the temporal and spectral properties of the sample. We demonstrate evidence for a consistent underlying behaviour which can produce a range of light curve morphologies, and attempt to interpret this behaviour in the framework of external forward shock emission. We find several difficulties, in particular that reconciliation of our data with the forward shock model requires energy injection to continue for days to weeks.

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arXiv2009

Supergiant Fast X-ray Transients in outburst: new Swift observations of XTEJ1739-302, IGRJ17544-2619, and IGRJ08408-4503

We report on new X-ray outbursts observed with Swift from three Supergiant Fast X-ray Transients (SFXTs): XTEJ1739-302, IGRJ17544-2619 and IGRJ08408-4503. The former two outbursts were caught during the monitoring campaign we have been performing with the Swift satellite since October 2007: XTEJ1739-302 underwent a new outburst on 2008, August 13, IGRJ17544-2619 on 2008, September 4, while IGRJ08408-4503 on 2008, September 21. While XTEJ1739-302 and IGRJ08408-4503 bright emission triggered the Swift/Burst Alert Telescope, IGRJ17544-2619 did not, thus we could perform a spectral investigation only of the spectrum below 10 keV. The broad band spectra from XTEJ1739-302 and IGRJ08408-4503 were compatible with the X-ray spectral shape displayed during the previous flares. A variable absorbing column density during the flare was observed in XTEJ1739-302 for the first time. The broad band spectrum of IGRJ08408-4503 requires the presence of two distinct photon populations, a cold one (0.3 keV) most likely from a thermal halo around the neutron star and a hotter one (1.4-1.8 keV) from the accreting column. The outburst from XTEJ1739-302 could be monitored with a very good sampling, thus revealing a shape which can be explained with a second wind component in this SFXT, in analogy to what we have suggested in the periodic SFXT IGRJ11215-5952. The outburst recurrence timescale in IGRJ17544-2619 during our monitoring campaign with Swift suggests a long orbital period of ~150 days (in an highly eccentric orbit), compatible with what previously observed with INTEGRAL.

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arXiv2009

Swift and Supergiant Fast X-ray Transients: a novel monitoring approach

We describe our monitoring strategy which best exploits the sensitivity and flexibility of Swift to study the long-term behaviour of Supergiant Fast X-ray Transients (SFXTs). We present observations of the recent outbursts from two objects of this class. IGR J16479-4514, underwent an outburst on 2008 March 19, reaching a peak luminosity of about 6E37 erg/s (0.5-100keV; at a distance of 4.9 kpc). We obtained a simultaneous broad-band spectrum (0.3-100 keV), the first for the SFXT class, which is fit with a heavily absorbed (column density 5E22 cm^-2) hard power-law with a high energy cut-off at about 7keV. This spectrum shows properties similar to the ones of accreting pulsars, although no X-ray pulsations were found. IGR J11215-5952, one of the only two periodic SFXT known to date, was observed with Swift several times, first with an intense 23-day long monitoring campaign around the 2007 February 9 outburst; then with a 26-day long monitoring around the unexpected July 24 outburst; finally with a deep exposure during the 2008 June 16 outburst. We present the whole dataset, which also includes observations which allowed us to firmly establish the outburst period at P~165 days. Thanks to our combined observations common characteristics to this class of objects are emerging, i.e., outburst lengths well in excess of hours, often with a multiple peaked structure, dynamic range ~3 orders of magnitude, and periodicities are starting to be found.

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