Swift monitoring of supergiant fast X-ray transients: The out-of-outburst behaviour and the flares from IGR J17544-2916 and XTE J1739-302

Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-ray Binaries (HMXBs) associated with OB supergiant companions and displaying transient X-ray activity. This behaviour is quite surprising since HMXBs hosting supergiants were known to be persistent sources, until the INTEGRAL discoveries obtained by means of the monitoring of the Galactic plane. We have been performing a monitoring campaign with Swift of four SFXTs with the main aim of characterizing both the long-term behaviour of these transients and the properties during bright outbursts. Here we discuss the properties of the X-ray emission observed outside the outbursts as well as the flares observed from two SFXTs: IGR J17544-2916 and XTE J1739-302. Contrarily to what previously thought, Swift allowed us to discover that SFXTs spend most of the time in accretion at a low level, even outside the bright outbursts, with an accretion luminosity of 1033-1034 erg s-1, and that the quiescent level ∼1032 erg s-1, is a much rarer state

Two years of monitoring supergiant fast X-ray transients with Swift

We present results based on 2 yr of intense Swift monitoring of three supergiant fast X-ray transients (SFXTs), IGR J16479−4514, XTE J1739−302 and IGR J17544−2619, which we started in 2007 October. Our out-of-outburst intensity-based X-ray (0.3–10 keV) spectroscopy yields absorbed power laws characterized by hard photon indices (Γ∼ 1 –2). The broad-band (0.3–150 keV) spectra of these sources, obtained while they were undergoing new outbursts observed during the second year of monitoring, can be fitted well with models typically used to describe the X-ray emission from accreting neutron stars in high-mass X-ray binaries. We obtain an assessment of how long each source spends in each state using a systematic monitoring with a sensitive instrument. By considering our monitoring as a casual sampling of the X-ray light curves, we can infer that the time these sources spend in bright outbursts is between 3 and 5 per cent of the total. The most probable X-ray flux for these sources is ∼(1 –2) × 10−11 erg cm−2 s−1 (2–10 keV, unabsorbed), corresponding to luminosities of the order of a few 1033 to a few 1034 erg s−1 (two orders of magnitude lower than the bright outbursts). In particular, the duty-cycle of inactivity is ∼19, 39 and 55 per cent (∼5 per cent uncertainty) for IGR J16479−4514, XTE J1739−302 and IGR J17544−2619, respectively. We present a complete list of BAT onboard detections, which further confirm 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. Variability in the X-ray flux is observed at all time-scales and intensity ranges we can probe. Superimposed on the day-to-day variability is intraday flaring, which involves flux variations up to one order of magnitude that can occur down to time-scales as short as ∼1 ks, and which can be naturally explained by the accretion of single clumps composing the donor wind with masses Mcl∼ (0.3 –2) × 1019 g. Thanks to the Swift observations, the general picture we obtain is that, despite individual differences, common X-ray characteristics of this class are now well defined, such as outburst lengths well in excess of hours, with a multiple peaked structure, and a high dynamic range (including bright outbursts), up to approximately four orders of magnitude

Swift observations of GRB 070110: An extraordinary X-ray afterglow powered by the central engine

We present a detailed analysis of Swift multiwavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2 days after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at ~2 × 10^4 s after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point toward a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning-down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock

Swift observations of GRB 070110: An extraordinary X-ray afterglow powered by the central engine

We present a detailed analysis of Swift multiwavelength observations of GRB 070110 and its remarkable afterglow. The early X-ray light curve, interpreted as the tail of the prompt emission, displays a spectral evolution already seen in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2 days after the burst, which is not consistent with standard afterglow models. The most intriguing feature is a very steep decay in the X-ray flux at ~2 × 10^4 s after the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve rules out an external shock as the origin of the plateau in this burst and implies long-lasting activity of the central engine. The temporal and spectral properties of the plateau phase point toward a continuous central engine emission rather than the episodic emission of X-ray flares. We suggest that the observed X-ray plateau is powered by a spinning-down central engine, possibly a millisecond pulsar, which dissipates energy at an internal radius before depositing energy into the external shock

Swift/UVOT follow-up of gravitational wave alerts in the O3 era

In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the gravitational wave (GW) alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and analysis of candidate optical/UV sources. For the O3 period, we also provide a statistical overview and report on serendipitous sources discovered by Swift/UVOT. Swift followed 18 GW candidate alerts, with UVOT observing a total of 424 deg2. We found 27 sources that changed in magnitude at the 3σ level compared with archival u- or g-band catalogued values. Swift/UVOT also followed up a further 13 sources reported by other facilities during the O3 period. Using catalogue information, we divided these 40 sources into five initial classifications: 11 candidate active galactic nuclei (AGNs)/quasars, three cataclysmic variables (CVs), nine supernovae, 11 unidentified sources that had archival photometry, and six uncatalogued sources for which no archival photometry was available. We have no strong evidence to identify any of these transients as counterparts to the GW events. The 17 unclassified sources are likely a mix of AGN and a class of fast-evolving transient, and one source may be a CV.<br

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four where finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 × 10−12 erg cm−2 s−1 over 0.3–10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg2), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here, we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, as well as the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q}\lt 1.4\times {10}^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave E < 4.1 × 1051 erg (assuming typical GRB parameters)

A short gamma-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225.

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z approximately 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10'') and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from--and the localization of--the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs