Gamma-ray burst (GRB) afterglows have provided important clues to the nature of these massive explosive events, providing direct information on the nearby environment and indirect information on the ...central engine that powers the burst. We report the discovery of two bright x-ray flares in GRB afterglows, including a giant flare comparable in total energy to the burst itself, each peaking minutes after the burst. These strong, rapid x-ray flares imply that the central engines of the bursts have long periods of activity, with strong internal shocks continuing for hundreds of seconds after the gamma-ray emission has ended.
Context. The long-term evolution of ultraluminous X-ray sources (ULX) with their spectral and luminosity variations in time give important clues on the nature of ULX and on the accretion process that ...powers them. Aims. We report here the results of a Swift-XRT six-year monitoring campaign of the closest example of a persistent ULX, M33 X-8, that extends the monitoring of this source in the soft X-rays to 16 years. The luminosity of this source is a few 1039 erg s-1, marking the faint end of the ULX luminosity function. Methods. We analyzed the set of 15 observations collected during the Swift monitoring. We searched for differences in the spectral parameters at different observing epochs, adopting several models commonly used to fit the X-ray spectra of ULX. Results. The source exhibits flux variations of about 30%. No significant spectral variations are observed during the monitoring. The average 0.5−10 keV spectrum can be well described by a thermal model, either in the form of a slim disk, or as a combination of a Comptonized corona and a standard accretion disk.
We study the long-term spectral and timing behaviour of LS I +61°303 in hard X-rays (15–150 keV) using ∼10 years of survey data from the Swift Burst Alert Telescope (BAT) monitor. We focus on the ...detection of long periodicities known to be present in this source in multiple wavelengths. We clearly detect three periods: the shorter one at 26.48 d is compatible with the orbital period of the system; the second, longer, periodicity at 26.93 d, is detected for the first time in X-rays and its value is consistent with an analogous temporal feature recently detected in the radio and in the gamma-ray waveband, and we associate it with a modulation caused by a precessing jet in this system. Finally, we find also evidence of the long-term periodicity at ∼1667 d, that results compatible with a beat frequency of the two close, and shorter, periodicities. We discuss our results in the context of the multiband behaviour of the physical processes of this source.
IGR J16195−4945 is a hard X-ray source discovered by INTEGRAL during the Core Programme observations performed in 2003. We analysed the X-ray emission of this source exploiting the Swift-Burst Alert ...Telescope (BAT) survey data from 2004 December to 2015 March, and all the available Swift-X-ray Telescope (XRT)-pointed observations. The source is detected at a high significance level in the 123-month BAT survey data, with an average 15–150 keV flux of the source of ∼1.6 mCrab. The timing analysis on the BAT data reveals with a significance higher than six standard deviations the presence of a modulated signal with a period of 3.945 d, that we interpret as the orbital period of the binary system. The folded light curve shows a flat profile with a narrow full eclipse lasting ∼3.5 per cent of the orbital period. We requested phase-constrained XRT observations to obtain a more detailed characterization of the eclipse in the soft X-ray range. Adopting reasonable guess values for the mass and radius of the companion star, we derive a semimajor orbital axis of ∼ 31 R⊙, equivalent to ∼1.8 times the radius of the companion star. From these estimates and from the duration of the eclipse, we derive an orbital inclination between 55 and 60 deg. The broad-band time-averaged XRT+BAT spectrum is well modelled with a strongly absorbed flat power law, with absorbing column N
H = 7 × 1022 cm−2 and photon index Γ = 0.5, modified by a high energy exponential cutoff at E
cut = 14 keV.
We present the analysis of the extraordinarily bright gamma-ray burst (GRB) 130427A under the hypothesis that the GRB central engine is an accretion-powered magnetar. In this framework, initially ...proposed to explain GRBs with precursor activity, the prompt emission is produced by accretion of matter on to a newly born magnetar, and the observed power is related to the accretion rate. The emission is eventually halted if the centrifugal forces are able to pause accretion. We show that the X-ray and optical afterglow is well explained as the forward shock emission with a jet break plus a contribution from the spin-down of the magnetar. Our modelling does not require any contribution from the reverse shock, that may still influence the afterglow light curve at radio and mm frequencies, or in the optical at early times. We derive the magnetic field (B ∼ 1016 G) and the spin period (P ∼ 20 ms) of the magnetar and obtain an independent estimate of the minimum luminosity for accretion. This minimum luminosity results well below the prompt emission luminosity of GRB 130427A, providing a strong consistency check for the scenario where the entire prompt emission is the result of continuous accretion on to the magnetar. This is in agreement with the relatively long spin period of the magnetar. GRB 130427A was a well-monitored GRB showing a very standard behaviour and, thus, is a well-suited benchmark to show that an accretion-powered magnetar gives a unique view of the properties of long GRBs.
Context. The hard X-ray transient source IGR J11215-5952 was discovered in April 2005 with INTEGRAL and is a confirmed member of the new class of high mass X-ray binaries, the supergiant fast X-ray ...transients (SFXTs). Archival INTEGRAL data and RXTE observations have shown that the outbursts occur with a periodicity of similar to 330 days. Thus, IGR J11215-5952 is the first SFXT displaying periodic outbursts, possibly related to the orbital period. Aims. We performed a target of opportunity observation with Swift with the main aim of monitoring the source behaviour around the time of the fifth outburst, expected on 2007 Feb. 9. Methods. The source field was observed with Swift twice a day (2 ks/day) starting from 2007 Feb. 4 until the fifth outburst, and then for similar to 5 ks a day afterwards, during a monitoring campaign that lasted 23 days for a total on-source exposure of similar to 73 ks. This is the most complete monitoring campaign of an outburst from an SFXT. Results. The spectrum during the brightest flares is described well by an absorbed power law with a photon index of 1 and N_{\rm H} \sim1\times10 super(22) cm super(-2). A 1-10 keV peak luminosity of similar to 10 super(36) erg s super(-1) was derived (assuming 6.2 kpc, the distance of the optical counterpart). Conclusions. These Swift observations are a unique data-set for an outburst of an SFXT, thanks to the combination of sensitivity and time coverage, and they allowed a study of IGR J11215-5952 from outburst onset to almost quiescence. We find that the accretion phase lasts longer than previously thought on the basis of lower-sensitivity instruments observing only the brightest flares. The observed phenomenology is consistent with a smoothly increasing flux triggered at the periastron passage in a wide eccentric orbit with many flares superimposed, possibly due to episodic or inhomogeneous accretion.
The Neil Gehrels Swift Observatory carried out prompt searches for gravitational-wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift ...performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW170814 and the epochal GW170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2 and how these are being used to improve the Swift follow-up of GW events. In particular, we simulate a population of gamma-ray burst afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncataloged X-ray sources. We find that 60%-70% of afterglows whose jets are oriented toward Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming that their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.
We have analysed the Swift data relevant to the high-mass X-ray binary Swift J1816.7–1613. The timing analysis of the Burst Alert Telescope survey data unveiled a modulation at a period of P
...0 = 118.5 ± 0.8 d, which we interpret as the orbital period of the X-ray binary system. The modulation is the result of a sequence of bright flares, lasting ∼30 d, separated by long quiescence intervals. This behaviour is suggestive of a Be binary system, where periodic or quasi-periodic outbursts are the consequence of an enhancement of the accretion flow from the companion star at the periastron passage. The position of Swift J1816.7–1613 on the Corbet diagram strengthens this hypothesis. The broad-band 0.2–150 keV spectrum is well modelled with a strongly absorbed power law with a flat photon index Γ ∼ 0.2 and a cut-off at ∼10 keV.
Supergiant fast X-ray transients (SFXTs) are a new class of high-mass X- ray binaries (HMXBs) discovered thanks to the monitoring of the Galactic plane performed with the INTEGRAL satellite in the ...last 5 years. These sources display short outbursts (significantly shorter than typical Be/X-ray binaries) with a peak luminosity of a few 10 super(36) erg simage. The quiescent level, measured only in a few sources, is around 10 super(32) erg simage. The X-ray spectral properties are reminiscent of those of accreting pulsars; thus, it is likely that all the members of the new class are indeed HMXBs hosting a neutron star, although only two SFXTs have a measured pulse period, IGR J11215-5952 (image187 s) and IGR J18410-0535 (image4.7 s). Several competing mechanisms have been proposed to explain the shortness of these outbursts, mostly involving the structure of the wind from the supergiant companion. To characterize the properties of these sources on timescales of months (e.g., the quiescent level and the outburst recurrence), we are performing a monitoring campaign with Swift of four SFXTs (IGR J16479-4514, XTE J1739-302, IGR J17544-2619, and AX J1841.0-0536/IGR J18410- 0535). We report on the first 4 months of Swift observations, which started on 2007 October 26. We detect low-level X-ray activity in all four SFXTs, which demonstrates that these transient sources accrete matter even outside their outbursts. This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The light-curve variability is also evident on larger timescales of days, weeks, and months, with a dynamic range of more than 1 order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 10 super(33)-10 super(34) erg simage. We detected pulsations from the pulsar AX J1841.0-0536/IGR J18410- 0535, with a period of image s. This monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (image100-1000 times lower) than during the bright outbursts, and that the 'true quiescence,' characterized by a soft spectrum and a luminosity of a few 10 super(32) erg simage, observed in the past in only a couple of members of this class, is probably a very rare state.
We present observations of GRB 060124, the first event for which both the prompt and the afterglow emission could be observed simultaneously and in their entirety by the three Swift instruments. ...Indeed, Swift-BAT triggered on a precursor similar to 570 s before the main burst peak, and this allowed Swift to repoint the narrow field instruments to the burst position similar to 350 s before the main burst occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt emission in a harder gamma-ray band (up to 2 MeV). Thanks to these exceptional circumstances, the temporal and spectral properties of the prompt emission can be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission (0.2-10 keV) clearly tracks the gamma-ray burst, the optical component follows a different pattern, likely indicating a different origin, possibly the onset of external shocks. The prompt GRB spectrum shows significant spectral evolution, with both the peak energy and the spectral index varying. As observed in several long GRBs, significant lags are measured between the hard- and low-energy components, showing that this behaviour extends over 3 decades in energy. The GRB peaks are also much broader at soft energies. This is related to the temporal evolution of the spectrum, and can be accounted for by assuming that the electron spectral index softened with time. The burst energy ( E_{\rm iso} \sim 5 \times 10 super(53) erg) and average peak energy ( E_{\rm p} \sim 300 keV) make GRB 060124 consistent with the Amati relation. The X-ray afterglow is characterized by a decay which presents a break at t_{\rm b} \sim 10 one fourth s.
PDF
