arXiv subjects

A. Tiengo

Publications and source records attributed to A. Tiengo.

At least 37 records · Page 2Linked to original sources

arXiv2020

X-ray and Radio Bursts from the Magnetar 1E1547.0-5408

We report on simultaneous radio and X-ray observations of the radio-emitting magnetar 1E1547.0-5408 on 2009 January 25 and February 3, with the 64-m Parkes radio telescope and the Chandra and XMM-Newton X-ray observatories. The magnetar was observed in a period of intense X-ray bursting activity and enhanced X-ray emission. We report here on the detection of two radio bursts from 1E1547.0-5408, reminiscent of Fast Radio Bursts (FRBs). One of the radio bursts was anticipated by ~1s (about half a rotation period of the pulsar) by a bright SGR-like X-ray burst, resulting in a F_radio/F_X ~ 10^-9. Radio pulsations were not detected during the observation showing the FRB-like radio bursts, while they were detected in the previous radio observation. We also found that the two radio bursts are neither aligned with the latter radio pulsations nor with the peak of the X-ray pulse profile (phase shift of ~0.2). Comparing the luminosity of these FRB-like bursts and those reported from SGR1935+2154, we find that the wide range in radio efficiency and/or luminosity of magnetar bursts in the Galaxy may bridge the gap between "ordinary" pulsar radio bursts and the extragalactic FRB phenomenon.

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arXiv2020

A search for optical and near-infrared counterparts of the compact binary merger GW190814

We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on August 14th, 2019. This signal has the best localisation of any observed gravitational wave (GW) source, with a 90% probability area of 18.5 deg$^2$, and an estimated distance of ~ 240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88% of the probability area down to a limiting magnitude of $w$ = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multi-colour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet's axis is disfavoured by the multi-wavelength dataset, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1 M$_{\odot}$) fast-moving (mean velocity >= 0.3c) wind ejecta for a possible kilonova associated with this merger.

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arXiv2020

Prediction and understanding of soft proton contamination in XMM-Newton: a machine learning approach

One of the major and unfortunately unforeseen sources of background for the current generation of X-ray telescopes are few tens to hundreds of keV (soft) protons concentrated by the mirrors. One such telescope is the European Space Agency's (ESA) X-ray Multi-Mirror Mission (XMM-Newton). Its observing time lost due to background contamination is about 40\%. This loss of observing time affects all the major broad science goals of this observatory, ranging from cosmology to astrophysics of neutron stars and black holes. The soft proton background could dramatically impact future large X-ray missions such as the ESA planned Athena mission (http://www.the-athena-x-ray-observatory.eu/). Physical processes that trigger this background are still poorly understood. We use a Machine Learning (ML) approach to delineate related important parameters and to develop a model to predict the background contamination using 12 years of XMM observations. As predictors we use the location of satellite, solar and geomagnetic activity parameters. We revealed that the contamination is most strongly related to the distance in southern direction, $Z$, (XMM observations were in the southern hemisphere), the solar wind radial velocity and the location on the magnetospheric magnetic field lines. We derived simple empirical models for the first two individual predictors and an ML model which utilizes an ensemble of the predictors (Extra Trees Regressor) and gives better performance. Based on our analysis, future missions should minimize observations during times associated with high solar wind speed and avoid closed magnetic field lines, especially at the dusk flank region in the southern hemisphere.

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arXiv2020

Dust-scattering halo and giant hard X-ray flare from the Supergiant Fast X-ray Transient IGR J16479-4514 investigated with XMM-Newton and INTEGRAL

We report results from the analysis of XMM-Newton and INTEGRAL data of IGR J16479-4514. The unpublished XMM-Newton observation, performed in 2012, occurred during the source eclipse. No point-like X-ray emission was detected from the source, conversely extended X-ray emission was clearly detected up to a size distance compatible with a dust scattering halo produced by the source X-ray emission before being eclipsed by its companion donor star. The diffuse emission of the dust-scattering halo could be observed without any contamination from the central point X-ray source, compared to a previous XMM-Newton observation published in 2008. Our comprehensive analysis of the 2012 unpublished spectrum of the diffuse emission as well as of the 2008 re-analysed spectra extracted from three adjacent time intervals and different extraction regions (optimized for point-like and extended emission) allowed us to clearly disentangle the scattering halo spectrum from the residual point-like emission during the 2008 eclipse. Moreover, the point-like emission detected in 2008 could be separated into two components attributed to the direct emission from the source and to scattering in the stellar wind, respectively. From archival unpublished INTEGRAL data, we identified a very strong (3$\times$10$^{-8}$ erg cm$^{-2}$ s$^{-1}$) and fast (25 minutes duration) flare which was classified as giant hard X-ray flare since the measured peak-luminosity is 7$\times$10$^{37}$ erg s$^{-1}$. Giant X-ray flares from SFXTs are very rare, to date only one has been reported from a different source. We propose a physical scenario to explain the origin in the case of IGR J16479-4514.

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arXiv2020

A Multi-wavelength search for Black Widows and Redbacks counterparts of candidate $γ$-ray millisecond pulsars

The wealth of detections of millisecond pulsars (MSPs) in $γ$-rays by {\em Fermi} has spurred searches for these objects among the several unidentified $γ$-ray sources. Interesting targets are a sub-class of binary MSPs, dubbed "Black Widows" (BWs) and "Redbacks" (RBs), which are in orbit with low-mass non-degenerate companions fully or partially ablated by irradiation from the MSP wind. These systems can be easily missed in radio pulsar surveys owing to the eclipse of the radio signal by the intra-binary plasma from the ablated companion star photosphere, making them better targets for multi-wavelength observations. We used optical and X-ray data from public databases to carry out a systematic investigation of all the unidentified $γ$-ray sources from the Fermi Large Area Telescope (LAT) Third Source Catalog (3FGL), which have been pre-selected as likely MSP candidates according to a machine-learning technique analysis. We tested our procedure by recovering known binary BW/RB identifications and searched for new ones, finding possible candidates. At the same time, we investigated previously proposed BW/RB identifications and we ruled out one of them based upon the updated $γ$-ray source coordinates.

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arXiv2020

The X-ray reactivation of the radio bursting magnetar SGR 1935+2154

A few years after its discovery as a magnetar, SGR J1935+2154 started a new burst-active phase on 2020 April 27, accompanied by a large enhancement of its X-ray persistent emission. Radio single bursts were detected during this activation, strengthening the connection between magnetars and fast radio bursts. We report on the X-ray monitoring of SGR J1935+2154 from ~3 days prior to ~3 weeks after its reactivation, using Swift, NuSTAR, and NICER. We detected X-ray pulsations in the NICER and NuSTAR observations, and constrained the spin period derivative to |Pdot| < 3e-11 s/s (3 sigma c.l.). The pulse profile showed a variable shape switching between single and double-peaked as a function of time and energy. The pulsed fraction decreased from ~34% to ~11% (5-10 keV) over ~10 days. The X-ray spectrum was well fit by an absorbed blackbody model with temperature decreasing from kT ~ 1.6 to 0.45-0.6 keV, plus a non-thermal component (Gamma ~ 1.2) observed up to ~25 keV with NuSTAR. The 0.3-10 keV X-ray luminosity (at 6.6 kpc) increased in less than four days from ~ 6e33 erg/s to about 3e35 erg/s and then decreased again to 2.5e34 erg/s over the following three weeks of the outburst. We also detected several X-ray bursts, with properties typical of short magnetar bursts.

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arXiv2020

INTEGRAL discovery of a burst with associated radio emission from the magnetar SGR 1935+2154

We report on INTEGRAL observations of the soft $γ$-ray repeater SGR 1935+2154 performed between 2020 April 28 and May 3. Several short bursts with fluence of $\sim10^{-7}-10^{-6}$ erg cm$^{-2}$ were detected by the IBIS instrument in the 20-200 keV range. The burst with the hardest spectrum, discovered and localized in real time by the INTEGRAL Burst Alert System, was spatially and temporally coincident with a short and very bright radio burst detected by the CHIME and STARE2 radio telescopes at 400-800 MHz and 1.4 GHz, respectively. Its lightcurve shows three narrow peaks separated by $\sim$29 ms time intervals, superimposed on a broad pulse lasting $\sim$0.6 s. The brightest peak had a delay of 6.5$\pm$1.0 ms with respect to the 1.4 GHz radio pulse (that coincides with the second and brightest component seen at lower frequencies). The burst spectrum, an exponentially cut-off power law with photon index $Γ=0.7_{-0.2}^{+0.4}$ and peak energy $E_p=65\pm5$ keV, is harder than those of the bursts usually observed from this and other magnetars. By the analysis of an expanding dust scattering ring seen in X-rays with the {\it Neil Gehrels Swift Observatory} XRT instrument, we derived a distance of 4.4$_{-1.3}^{+2.8}$ kpc for SGR 1935+2154, independent of its possible association with the supernova remnant G57.2+0.8. At this distance, the burst 20-200 keV fluence of $(6.1\pm 0.3)\times10^{-7}$ erg cm$^{-2}$ corresponds to an isotropic emitted energy of $\sim1.4\times10^{39}$ erg. This is the first burst with a radio counterpart observed from a soft $γ$-ray repeater and it strongly supports models based on magnetars that have been proposed for extragalactic fast radio bursts.

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arXiv2020

The slow heartbeats of an ultra-luminous X-ray source in NGC 3621

We report on the results of X-ray observations of 4XMM J111816.0-324910, a transient ultra-luminous X-ray source located in the galaxy NGC 3621. This system is characterised by a transient nature and marked variability with characteristic time-scale of ~3500 s, differently from other ULXs, which in the vast majority show limited intra-observation variability. Such a behaviour is very reminiscent of the so-called heartbeats sometimes observed in the Galactic black hole binary GRS 1915+105, where the variability time-scale is ~10-1000 s. We study the spectral and timing properties of this object and find that overall, once the differences in the variability time-scales are taken into account, they match quite closely those of both GRS 1915+105, and of a number of objects showing heartbeats in their light-curves, including a confirmed neutron star and a super-massive black hole powering an active galactic nucleus. We investigate the nature of the compact object in 4XMM J111816.0-324910 by searching for typical neutron star signatures and by attempting a mass estimate based on different methods and assumptions. Based on the current available data, we are not able to unambiguously determine the nature of the accreting compact object responsible for the observed phenomenology.

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arXiv2020

A Supernova Candidate at z=0.092 in XMM-Newton Archival Data

During a search for X-ray transients in the XMM-Newton archive within the EXTraS project, we discovered a new X-ray source that is detected only during a ~5 min interval of a ~21 h-long observation performed on 2011 June 21 (EXMM 023135.0-603743, probability of a random Poissonian fluctuation: ~$1.4\times10^{-27}$). With dedicated follow-up observations, we found that its position is consistent with a star-forming galaxy (SFR = 1-2 $M_\odot$ yr$^{-1}$) at redshift $z=0.092\pm0.003$ ($d=435\pm15$ Mpc). At this redshift, the energy released during the transient event was $2.8\times10^{46}$ erg in the 0.3-10 keV energy band (in the source rest frame). The luminosity of the transient, together with its spectral and timing properties, make EXMM 023135.0-603743 a gripping analog to the X-ray transient associated to SN 2008D, which was discovered during a Swift/XRT observation of the nearby ($d=27$ Mpc) supernova-rich galaxy NGC 2770. We interpret the XMM-Newton event as a supernova shock break-out or an early cocoon, and show that our serendipitous discovery is compatible with the rate of core-collapse supernovae derived from optical observations and much higher than that of tidal disruption events.

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arXiv2020

A very young radio-loud magnetar

The magnetar Swift ,J1818.0-1607 was discovered in March 2020 when Swift detected a 9 ms hard X-ray burst and a long-lived outburst. Prompt X-ray observations revealed a spin period of 1.36 s, soon confirmed by the discovery of radio pulsations. We report here on the analysis of the Swift burst and follow-up X-ray and radio observations. The burst average luminosity was $L_{\rm burst} \sim2\times 10^{39}$ erg/s (at 4.8 kpc). Simultaneous observations with XMM-Newton and NuSTAR three days after the burst provided a source spectrum well fit by an absorbed blackbody ($N_{\rm H} = (1.13\pm0.03) \times 10^{23}$ cm$^{-2}$ and $kT = 1.16\pm0.03$ keV) plus a power-law ($Γ=0.0\pm1.3$) in the 1-20 keV band, with a luminosity of $\sim$$8\times10^{34}$ erg/s, dominated by the blackbody emission. From our timing analysis, we derive a dipolar magnetic field $B \sim 7\times10^{14}$ G, spin-down luminosity $\dot{E}_{\rm rot} \sim 1.4\times10^{36}$ erg/s and characteristic age of 240 yr, the shortest currently known. Archival observations led to an upper limit on the quiescent luminosity $<$$5.5\times10^{33}$ erg/s, lower than the value expected from magnetar cooling models at the source characteristic age. A 1 hr radio observation with the Sardinia Radio Telescope taken about 1 week after the X-ray burst detected a number of strong and short radio pulses at 1.5 GHz, in addition to regular pulsed emission; they were emitted at an average rate 0.9 min$^{-1}$ and accounted for $\sim$50% of the total pulsed radio fluence. We conclude that Swift ,J1818.0-1607 is a peculiar magnetar belonging to the small, diverse group of young neutron stars with properties straddling those of rotationally and magnetically powered pulsars. Future observations will make a better estimation of the age possible by measuring the spin-down rate in quiescence.

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arXiv2020

Discovery of a 2.8 s pulsar in a 2 d orbit High-Mass X-ray Binary powering the Ultraluminous X-ray source ULX-7 in M51

We discovered 2.8 s pulsations in the X-ray emission of the ultraluminous X-ray source (ULX) M51 ULX-7 within the UNSEeN project, which was designed to hunt for new pulsating ULXs (PULXs) with XMM-Newton. The pulse shape is sinusoidal and large variations of its amplitude were observed even within single exposures (pulsed fraction from less than 5% to 20%). M51 ULX-7 is a variable source, generally observed at an X-ray luminosity between $10^{39}$ and $10^{40}$ erg s$^{-1}$, located in the outskirts of the spiral galaxy M51a at a distance of 8.6 Mpc. According to our analysis, the X-ray pulsar orbits in a 2-d binary with a projected semi-major axis $a_\mathrm{X} \sin i \simeq$ 28 lt-s. For a neutron star (NS) of 1.4 $M_{\odot}$, this implies a lower limit on the companion mass of 8 $M_{\odot}$, placing the system hosting M51 ULX-7 in the high-mass X-ray binary class. The barycentric pulse period decreased by $\simeq$0.4 ms in the 31 d spanned by our May -- June 2018 observations, corresponding to a spin-up rate $\dot{P} \simeq -1.5\times10^{-10}\text{s s}^{-1}$. In an archival 2005 XMM-Newton exposure, we measured a spin period of $\sim$3.3 s, indicating a secular spin-up of $\dot{P}_{\mathrm{sec}}\simeq -10^{-9}\text{ s s}^{-1}$, a value in the range of other known PULXs. Our findings suggest that the system consists of an OB giant and a moderately magnetic (dipole field component in the range $10^{12}$ G $\lesssim B_{\mathrm{dip}}\lesssim 10^{13}$G) accreting NS with weakly beamed emission ($1/12\lesssim b\lesssim1/4$).

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arXiv2020

NuSTAR observation of the Supergiant Fast X-ray Transient IGR J11215-5952 during its 2017 outburst

We report on the results of a NuSTAR observation of the Supergiant Fast X-ray Transient pulsar IGRJ11215-5952 during the peak of its outburst in June 2017. IGRJ11215-5952 is the only SFXT undergoing strictly periodic outbursts, every 165 days. NuSTAR caught several X-ray flares, spanning a dynamic range of 100, and detected X-ray pulsations at 187.0 s, consistent with previous measurements. The spectrum from the whole observation is well described by an absorbed power-law (with a photon index of 1.4) modified, above 7 keV, by a cutoff with an e-folding energy of 24 keV. A weak emission line is present at 6.4 keV, consistent with Kalpha emission from cold iron in the supergiant wind. The time-averaged flux is 1.5E-10 erg/cm2/s (3-78 keV, corrected for the absorption), translating into an average luminosity of about 9E35 erg/s (1-100 keV, assuming a distance of 6.5 kpc). The NuSTAR observation allowed us to perform the most sensitive search for cyclotron resonant scattering features in the hard X-ray spectrum, resulting in no significant detection in any of the different spectral extractions adopted (time-averaged, temporally-selected, spin-phase-resolved and intensity-selected spectra). The pulse profile showed an evolution with both the energy (3-12 keV energy range compared with 12-78 keV band) and the X-ray flux: a double peaked profile was evident at higher fluxes (and in both energy bands), while a single peaked, sinusoidal profile was present at the lowest intensity state achieved within the NuSTAR observations (in both energy bands). The intensity-selected analysis allowed us to observe an anti-correlation of the pulsed fraction with the X-ray luminosity. The pulse profile evolution can be explained by X-ray photon scattering in the accreting matter above magnetic poles of a neutron star at the quasi-spherical settling accretion stage.

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arXiv2020

EXTraS discovery of an X-ray superflare from an L dwarf

We present the first detection of an X-ray flare from an ultracool dwarf of spectral class L. The event was identified in the EXTraS database of XMM-Newton variable sources, and its optical counterpart, J0331-27, was found through a cross-match with the Dark Energy Survey Year 3 release. Next to an earlier four-photon detection of Kelu-1, J0331-27 is only the second L dwarf detected in X-rays, and much more distant than other ultracool dwarfs with X-ray detections (photometric distance of 240 pc). From an optical spectrum with the VIMOS instrument at the VLT, we determine the spectral type of J0331-27 to be L1. The X-ray flare has an energy of E_X,F ~ 2x10^33 erg, placing it in the regime of superflares. No quiescent emission is detected, and from 2.5 Msec of XMM data we derive an upper limit of L_X,qui < 10^27 erg/s. The flare peak luminosity L_X,peak = 6.3x10^29 erg/s, flare duration tau_decay ~ 2400 s, and plasma temperature (~16 MK) are similar to values observed in X-ray flares of M dwarfs. This shows that strong magnetic reconnection events and the ensuing plasma heating are still present even in objects with photospheres as cool as ~2100 K. However, the absence of any other flares above the detection threshold of E_X,F ~2.5x10^32 erg in a total of ~2.5 Ms of X-ray data yields a flare energy number distribution inconsistent with the canonical power law dN/dE ~ E^-2, suggesting that magnetic energy release in J0331-27 -- and possibly in all L dwarfs -- takes place predominantly in the form of giant flares.

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arXiv2020

The Ultraluminous X-ray sources population of the galaxy NGC 7456

Ultraluminous X-ray sources (ULXs) are a class of accreting compact objects with X-ray luminosities above 1e39 erg/s. The ULX population counts several hundreds objects but only a minor fraction is well studied. Here we present a detailed analysis of all ULXs hosted in the galaxy NGC 7456. It was observed in X-rays only once in the past (in 2005) by XMM-Newton, but the observation was short and strongly affected by high background. In 2018, we obtained a new, deeper (~90 ks) XMM-Newton observation that allowed us to perform a detailed characterization of the ULXs hosted in the galaxy. ULX-1 and ULX-2, the two brightest objects (Lx~(6-10)e39 erg/s), have spectra that can be described by a two-thermal component model as often found in ULXs. ULX-1 shows also one order of magnitude in flux variability on short-term timescales (hundreds to thousand ks). The other sources (ULX-3 and ULX-4) show flux changes of at least an order of magnitude, and these objects may be candidate transient ULXs although longer X-ray monitoring or further studies are required to ascribe them to the ULX population. In addition, we found a previously undetected source that might be a new candidate ULX (labelled as ULX-5) with a luminosity of ~1e39 erg/s and hard power-law spectral shape, whose nature is still unclear and for which a background Active Galactic Nucleus cannot be excluded. We discuss the properties of all the ULXs in NGC 7456 within the framework of super-Eddington accretion onto stellar mass compact objects. Although no pulsations were detected, we cannot exclude that the sources host neutron stars.

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arXiv2019

The afterglow and kilonova of the short GRB 160821B

GRB 160821B is a short duration gamma-ray burst (GRB) detected and localized by the Neil Gehrels Swift Observatory in the outskirts of a spiral galaxy at z=0.1613, at a projected physical offset of 16 kpc from the galaxy's center. We present X-ray, optical/nIR and radio observations of its counterpart and model them with two distinct components of emission: a standard afterglow, arising from the interaction of the relativistic jet with the surrounding medium, and a kilonova, powered by the radioactive decay of the sub-relativistic ejecta. Broadband modeling of the afterglow data reveals a weak reverse shock propagating backward into the jet, and a likely jet-break at 3.5 d. This is consistent with a structured jet seen slightly off-axis while expanding into a low-density medium. Analysis of the kilonova properties suggests a rapid evolution toward red colors, similar to AT2017gfo, and a low nIR luminosity, possibly due to the presence of a long-lived neutron star. The global properties of the environment, the inferred low mass (M_ej < 0.006 Msun) and velocities (v > 0.05 c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.

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arXiv2019

Detailed X-ray spectroscopy of the magnetar 1E 2259+586

Magnetic field geometry is expected to play a fundamental role in magnetar activity. The discovery of a phase-variable absorption feature in the X-ray spectrum of SGR 0418+5729, interpreted as cyclotron resonant scattering, suggests the presence of very strong non-dipolar components in the magnetic fields of magnetars. We performed a deep XMM-Newton observation of pulsar 1E 2259+586, to search for spectral features due to intense local magnetic fields. In the phase-averaged X-ray spectrum, we found evidence for a broad absorption feature at very low energy (0.7 keV). If the feature is intrinsic to the source, it might be due to resonant scattering/absorption by protons close to star surface. The line energy implies a magnetic field of ~ 10^14 G, roughly similar to the spin-down measure, ~ 6x10^13 G. Examination of the X-ray phase-energy diagram shows evidence for a further absorption feature, the energy of which strongly depends on the rotational phase (E >~ 1 keV ). Unlike similar features detected in other magnetar sources, notably SGR 0418+5729, it is too shallow and limited to a small phase interval to be modeled with a narrow phase-variable cyclotron absorption line. A detailed phase-resolved spectral analysis reveals significant phase-dependent variability in the continuum, especially above 2 keV. We conclude that all the variability with phase in 1E 2259+586 can be attributed to changes in the continuum properties which appear consistent with the predictions of the Resonant Compton Scattering model.

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arXiv2019

Long X-ray flares from the central source in RCW 103

We observed the slowly revolving pulsar 1E 161348-5055 (1E 1613, spin period of 6.67 h) in the supernova remnant RCW 103 twice with XMM-Newton and once with the Very Large Telescope (VLT). The VLT observation was performed on 2016 June 30, about a week after the detection of a large outburst from 1E 1613. At the position of 1E 1613, we found a near-infrared source with K_S = 20.68 +/- 0.12 mag that was not detected (K_S > 21.2 mag) in data collected with the same instruments in 2006, during X-ray quiescence. Its position and behavior are consistent with a counterpart in the literature that was discovered with the Hubble Space Telescope in the following weeks in adjacent near-IR bands. The XMM-Newton pointings were carried out on 2016 August 19 and on 2018 February 14. While the collected spectra are similar in shape between each other and to what is observed in quiescence (a blackbody with kT~0.5 keV plus a second, harder component, either another hotter blackbody with kT ~ 1.2 keV or a power law with photon index ~3), the two pointings caught 1E 1613 at different luminosity throughout its decay pattern: about 4.8E34 erg/s in 2016 and 1.2E34 erg/s in 2018 (0.5-10 keV, for the double-blackbody model and for 3.3 kpc), which is still almost about ten times brighter than the quiescent level. The pulse profile displayed dramatic changes, apparently evolving from the complex multi-peak morphology observed in high-luminosity states to the more sinusoidal form characteristic of latency. The inspection of the X-ray light curves revealed two flares with unusual properties in the 2016 observation: they are long (~1 ks to be compared with 0.1-1 s of typical magnetar bursts) and faint (~1E34 erg/s, with respect to 1E38 erg/s or more in magnetars). Their spectra are comparatively soft and resemble the hotter thermal component of the persistent emission.

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arXiv2019

The multi-outburst activity of the magnetar in Westerlund I

After two major outbursts in 2006 and 2011, on 2017 May 16 the magnetar CXOU J164710.2-455216, hosted within the massive star cluster Westerlund I, emitted a short (20 ms) burst, which marked the onset of a new active phase. We started a long-term monitoring campaign with Swift (45 observations), Chandra (5 observations) and NuSTAR (4 observations) from the activation until 2018 April. During the campaign, Swift BAT registered the occurrence of multiple bursts, accompanied by two other enhancements of the X-ray persistent flux. The long time span covered by our observations allowed us to study the spectral as well as the timing evolution of the source. After 11 months since the 2017 May outburst onset, the observed flux was about 15 times higher than its historical minimum level and a factor of 3 higher than the level reached after the 2006 outburst. This suggests that the crust has not fully relaxed to the quiescent level, or that the source quiescent level has changed following the multiple outburst activities in the past 10 years or so. This is another case of multiple outbursts from the same source on a yearly time scale, a somehow recently discovered behaviour in magnetars.

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