arXiv subjects

S. Dall'Osso

Publications and source records attributed to S. Dall'Osso.

18 recordsLinked to original sources

arXiv2023

A semi-coherent generalization of the 5-vector method to search for continuous gravitational waves

The emission of continuous gravitational waves (CWs), with duration much longer than the typical data taking runs, is expected from several sources, notably spinning neutron stars, asymmetric with respect to their rotation axis and more exotic sources, like ultra-light scalar boson clouds formed around Kerr black holes and sub-solar mass primordial binary black holes. Unless the signal time evolution is well predicted and its relevant parameters accurately known, the search for CWs is typically based on semi-coherent methods, where the full data set is divided in shorter chunks of given duration, which are properly processed, and then incoherently combined. In this paper we present a semi-coherent method, in which the so-called \textit{5-vector} statistics is computed for the various data segments and then summed after the removal of the Earth Doppler modulation and signal intrinsic spin-down. The method can work with segment duration of several days, thanks to a double stage procedure in which an initial rough correction of the Doppler and spin-down is followed by a refined step in which the residual variations are removed. This method can be efficiently applied for directed searches, where the source position is known to a good level of accuracy, and in the candidate follow-up stage of wide-parameter space searches.

gr-qc↗

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.

astro-ph.HE↗

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 ($1/12\lesssim b\lesssim1/4$).

astro-ph.HE↗

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.

astro-ph.HE↗

arXiv2018

On the magnetar origin of the GRBs presenting X-ray afterglow plateaus

The X-ray afterglow plateau emission observed in many Gamma-ray Bursts (GRBs) has been interpreted as either being fueled by fallback onto a newly formed black hole, or by the spin-down luminosity of an ultra-magnetized millisecond neutron star. If the latter model is assumed, GRB X-ray afterglow light curves can be analytically reproduced. We fit a sample of GRB X-ray plateaus, interestingly yielding a distribution in the magnetic field versus spin period (B-P) diagram consistent with $B\propto P^{7/6}$. This is expected from the well-established physics of the spin-up line minimum period for Galactic millisecond pulsars. The normalisation of the relation we obtain perfectly matches spin-up line predictions for the expected masses ($\sim 1 M_{\odot}$) and radii ($\sim 10 {\rm ~km}$) of newly born magnetars, and mass accretion rates consistent with GRB expectations of $10^{-4} M_{\odot}/{\rm s} <\dot{M}< 10^{-1} M_{\odot}/{\rm s}$. Short GRBs with extended emission (SEE) appear towards the high period end of the distribution, while the long GRBs (LGRBs) towards the short period end. This result is consistent with spin-up limit expectations where the total accreted mass determines the position of the neutron star in the B-P diagram. The P-B distribution for LGRBs and SEE are statistically different, further supporting the idea that the fundamental plane relation \citep{dainotti16c,Dainotti2017} is a powerful discriminant among those populations. Our conclusions are robust against suppositions regarding the GRB collimation angle and magnetar breaking index, which shifts the resulting magnetar properties parallel to the spin-up line, and strongly support a magnetar origin for GRBs presenting X-ray plateaus.

astro-ph.HE↗

arXiv2016

X-rays from the location of the Bactrian Transient ASASSN-15lh

We present the detection of persistent soft X-ray radiation with L_x ~ 10^41-10^42 erg/s at the location of the extremely luminous, double-humped transient ASASSN-15lh as revealed by Chandra and Swift. We interpret this finding in the context of observations from our multiwavelength campaign, which revealed the presence of weak narrow nebular emission features from the host-galaxy nucleus and clear differences with respect to superluminous supernova optical spectra. Significant UV flux variability on short time-scales detected at the time of the re-brightening disfavors the shock interaction scenario as the source of energy powering the long-lived UV emission, while deep radio limits exclude the presence of relativistic jets propagating into a low-density environment. We propose a model where the extreme luminosity and double-peaked temporal structure of ASASSN-15lh is powered by a central source of ionizing radiation that produces a sudden change of the ejecta opacity at later times. As a result, UV radiation can more easily escape, producing the second bump in the light-curve. We discuss different interpretations for the intrinsic nature of the ionizing source. We conclude that, IF the X-ray source is physically associated with the optical-UV transient, ASASSN-15lh most likely represents the tidal disruption of a main-sequence star by the most massive spinning black hole detected to date. In this case, ASASSN-15lh and similar events discovered in the future would constitute the most direct probes of very massive, dormant, spinning, supermassive black holes in galaxies. Future monitoring of the X-rays may allow us to distinguish between the supernova and TDE hypothesis.

astro-ph.HE↗

arXiv2013

Swift X-ray and ultraviolet observations of the shortest orbital period double-degenerate system RX J0806.3+1527 (HM Cnc)

The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 per cent modulation on a period of 321.5 s (5.4 min). This period reflects the orbital motion of a close binary consisting of two interacting white dwarfs. Here we present a series of simultaneous X-ray (0.2-10 keV) and near-ultraviolet (2600 angstrom and 1928 angstrom) observations that were carried out with the Swift satellite. In the near-ultraviolet, the counterpart of RX J0806.3+1527 was detected at flux densities consistent with a blackbody with temperature 27E+3 K. We found that the emission at 2600 angstrom is modulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28 cycles and is coincident within 0.05 cycles with the optical. This phase-shift measurement confirms that the X-ray hot spot (located on the primary white dwarf) is at about 80-100 degrees from the direction that connects the two white dwarfs. Albeit at lower significance, the 321.5-s signature is present also in the 1928-angstrom data; at this wavelength, however, the pulse peak is better aligned with that observed at X-rays. We use the constraints on the source luminosity and the geometry of the emitting regions to discuss the merits and limits of the main models for RX J0806.3+1527.

astro-ph.HE↗

arXiv2012

Magnetic field decay in neutron stars: from Soft Gamma Repeaters to "weak field magnetars"

The recent discovery of the "weak field, old magnetar", the soft gamma repeater SGR 0418+5729, whose dipole magnetic field is less than 7.5 \times 10^{12} G, has raised perplexing questions: How can the neutron star produce SGR-like bursts with such a low magnetic field? What powers the observed X-ray emission when neither the rotational energy nor the magnetic dipole energy are sufficient? These observations, that suggest either a much larger energy reservoir or a much younger true age (or both), have renewed the interest in the evolutionary sequence of magnetars. We examine, here, a phenomenological model for the magnetic field decay: B_dip} \propto (B_dip)^{1+a} and compare its predictions with the observed period, P,the period derivative, \dot{P}, and the X-ray luminosity, L_X, of magnetar candidates. We find a strong evidence for a dipole field decay on a timescale of \sim 10^3 yr for the strongest (\sim 10^{15} G) field objects, with a decay index within the range 1 \leq a < 2 and more likely within 1.5\lesssim a \lesssim 1.8. The decaying field implies a younger age than what is implied by the spinown age. Surprisingly, even with the younger age, the energy released in the dipole field decay is insufficient to power the X-ray emission, suggesting the existence of a stronger internal field, B_int. Examining several models for the internal magnetic field decay we find that it must have a very large (> 10^{16} G) initial value. Our findings suggest two clear distinct evolutionary tracks -- the SGR/AXP branch and the transient branch, with a possible third branch involving high-field radio pulsars that age into low luminosity X-ray dim isolated neutron stars.

astro-ph.HE↗

arXiv2010

LOFT - a Large Observatory For x-ray Timing

The high time resolution observations of the X-ray sky hold the key to a number of diagnostics of fundamental physics, some of which are unaccessible to other types of investigations, such as those based on imaging and spectroscopy. Revealing strong gravitational field effects, measuring the mass and spin of black holes and the equation of state of ultradense matter are among the goals of such observations. At present prospects for future, non-focused X-ray timing experiments following the exciting age of RXTE/PCA are uncertain. Technological limitations are unavoidably faced in the conception and development of experiments with effective area of several square meters, as needed in order to meet the scientific requirements. We are developing large-area monolithic Silicon Drift Detectors offering high time and energy resolution at room temperature, which require modest resources and operation complexity (e.g., read-out) per unit area. Based on the properties of the detector and read-out electronics that we measured in the lab, we developed a realistic concept for a very large effective area mission devoted to X-ray timing in the 2-30 keV energy range. We show that effective areas in the range of 10-15 square meters are within reach, by using a conventional spacecraft platform and launcher of the small-medium class.

astro-ph.IM↗

arXiv2010

The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E 1547.0-5408

We report on the detailed study of the 2008 October outburst from the anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October 3. The Swift/X-ray Telescope (XRT) started observing the source after less than 100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10 keV range) was >50 times higher than its quiescent level. Swift monitored the outbursting activity of 1E 1547.0-5408 on a daily basis for approximately three weeks. This strategy allowed us to find a phase-coherent solution for the source pulsations after the burst, which, besides period and period derivative, requires a positive Period second derivative term (spin-down increase). The time evolution of the pulse shape is complex and variable, with the pulsed fraction increasing from 20% to 50% within the Swift observational window. The XRT spectra can be fitted well by means of a single component, either a power-law (PL) or a blackbody (BB). During the very initial phases of the outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4 keV) which steepens to about 4 (or kT about 0.8 keV) within one day from the BAT trigger, though the two components are likely present simultaneously during the first day spectra. An INTEGRAL observation carried out five days after the trigger provided an upper limit of about 2E-11 erg/cm^2/s to the emission of 1E 1547.0-5408 in the 18-60 keV band.

astro-ph.HE↗

arXiv2010

VLT Phase Resolved Optical Spectroscopy of the Ultra-Compact Binary HM Cnc

A 321.5 s modulation was discovered in 1999 in the X-ray light curve of HM Cnc. In 2001 and 2002, optical photometric and spectroscopic observations revealed that HM Cnc is a very blue object with no intrinsic absorptions but broad (FWHM 1500 km s^-1) low equivalent width emission lines (EW 1-6A), which were first identified with the HeII Pickering series. The combination of X-ray and optical observations pictures HM Cnc as a double degenerate binary hosting two white dwarfs, and possibly being the shortest orbital period binary discovered so far. The present work is aimed at studying the orbital motion of the two components by following the variations of the shape, centroid and intensity of the emission lines through the orbit. In February 2007, we carried out the first phase resolved optical spectroscopic study with the VLT/FORS2 in the High Time Resolution (HIT) mode, yielding five phase bins in the 321 s modulation. Despite the low SNR, the data show that the intensity of the three most prominent emission lines, already detected in 2001, varies with the phase. These lines are detected at phases 0.2-0.6 where the optical emission peaks, and marginally detected or not detected at all elsewhere. Moreover, the FWHM of the emission lines in the phase resolved spectra is smaller, by almost a factor 2, than that in the the phase-averaged 2001 spectrum. Our results are consistent with both the pulsed optical component and emission lines originating in the same region which we identify with the irradiated surface of the secondary. Moreover, regardless of the origin of the un-pulsed optical continuum, we note that the EWs of the emission lines might be up to -15 / -25A, larger than thought before; these values are more similar to those detected in cataclysmic variables. All the findings further confirm that the 321s modulation observed in HM Cnc is the orbital period of the system, the shortest known to date.

astro-ph.GA↗

arXiv2009

From outburst to quiescence: the decay of the transient Anomalous X-ray Pulsar XTE J1810-197

XTE J1810-197 is the first transient Anomalous X-ray Pulsar ever discovered. Its highly variable X-ray flux allowed us to study the timing and spectral emission properties of a magnetar candidate over a flux range of about two orders of magnitude. We analyzed nine XMM-Newton observations of XTE J1810-197 collected over a four years baseline (September 2003 - September 2007). EPIC PN and MOS data were reduced and used for detailed timing and spectral analysis. Pulse phase spectroscopic studies were also carried out for observations with sufficiently high signal to noise. We find that: (i) a three blackbodies model reproduces the spectral properties of the source over the entire outburst statistically better than the two blackbodies model previously used in the literature, (ii) the coldest blackbody is consistent with the thermal emission from the whole surface, and has temperature and radius similar to those inferred from ROSAT observations before the outburst onset, (iii) there is a spectral feature around 1.1 keV during six consecutive observations (since March 2005); if due to proton resonant cyclotron scattering, it would imply a magnetic field of around 2E14 G. This is in a very good agreement with the value from the spin period measurements.

astro-ph.HE↗

arXiv2008

Early evolution of newly born magnetars with a strong toroidal field

We present a state-of-the-art scenario for newly born magnetars as strong sources of Gravitational Waves (GWs)in the early days after formation. We address several aspects of the astrophysics of rapidly rotating, ultramagnetized neutron stars (NSs), including early cooling before transition to superfluidity, the effects of the magnetic field on the equilibrium shape of NSs, the internal dynamical state of a fully degenerate, oblique rotator and the strength of the electromagnetic torque on the newly born NS. We show that our scenario is consistent with recent studies of SNRs surrounding AXPs and SGRs in the Galaxy that constrain the electromagnetic energy input from the central NS to be <= 10^51 erg. We further show that if this condition is met, then the GW signal from such sources is potentially detectable with the forthcoming generation of GW detectors up to Virgo cluster distances where an event rate <= 1/yr can be estimated. Finally, we point out that the decay of an internal magnetic field in the 10^16 G range couples strongly to the NS cooling at very early stages, thus significantly slowing down both processes: the field can remain this strong for at least 10^3 yrs, during which the core temperature stays higher than several times 10^8 K.

astro-ph↗

arXiv2008

A Swift gaze into the 2006 March 29th burst forest of SGR 1900+14

We report on the intense burst ``forest'' recorded on 2006 March 29 which lasted for ~30s. More than 40 bursts were detected both by BAT and by XRT, seven of which are rare intermediate flares (IFs): several times 10^{42} ergs were released. The BAT data were used to carry out time-resolved spectroscopy in the 14-100keV range down to 8ms timescales. This unique dataset allowed us to test the magnetar model predictions such as the magnetically trapped fireball and the twisted magnetosphere over an unprecedented range of fluxes and with large statistics (in terms of both photons and IFs). We confirmed that a two blackbody component fits adequately the time-resolved and integrated spectra of IFs. However, Comptonization models give comparable good reduced chi^2. Moreover, we found: i) a change of behavior, around ~10^{41} erg/s, above which the softer blackbody shows a sort of saturation while the harder one still grows to a few times 10^{41} erg/s; ii) a rather sharp correlation between temperature and radii of the blackbodies (R^2 prop kT^{-3}), which holds for the most luminous parts of the flares (approximately for L_{tot} > 10^{41} erg/s). Within the magnetar model, the majority of these findings can be accounted for in terms of thermalised emission from the E-mode and O-mode photospheres. Interestingly, the maximum observed luminosity coming from a region of ~15km matches the magnetic Eddington luminosity at the same radius, for a surface dipole field of ~8 x 10^{14} G (virtually equal to the one deduced from the spindown of SGR 1900+14).

astro-ph↗

arXiv2007

The Post-Burst Awakening of the Anomalous X-ray Pulsar in Westerlund 1

On September 21, 2006, an intense (~10^39 erg/s) and short (20 ms) burst was detected by Swift BAT at a position consistent with that of the candidate Anomalous X-ray Pulsar, CXOU J164710.2-455216, discovered by Chandra in 2005. Swift follow-up observations began about 13 hours after the event and found the source at a 1-10keV flux level of about 4.5 x 10^-11 erg/s/cm^2, i.e. ~300 times brighter than measured 5 days earlier by XMM. We report the results obtained from Swift BAT observations of the burst and subsequent Swift XRT observations carried out during the first four months after the burst. These data are complemented with those from two XMM observations (carried out just before and after the BAT event) and four archival Chandra observations carried out between 2005 and 2007. We find a phase coherent solution for the source pulsations after the burst. The evolution of the pulse phase comprises an exponential component decaying with timescale of 1.4d which we interpret as the recovery stage following a large glitch (Delta nu / nu about 6 x 10^-5). We also detect a quadratic component corresponding to a spin-down rate of Pdot ~ 9 x 10^-13, implying a magnetic field strength of 10^14 Gauss. During the first Swift XRT observation taken 0.6 days after the burst, the spectrum showed a kT = 0.65keV blackbody (R_BB = 1.5km) plus a Gamma=2.3 power-law accounting for about 60% of the 1-10 keV observed flux. Analysis of Chandra archival data, taken during 2005 when the source was in quiescence, reveal that the modulation in quiescence is 100% pulsed at energies above 4 keV and consistent with the (unusually small-sized) blackbody component being occulted by the neutron star as it rotates.

astro-ph↗

arXiv2006

Unipolar Inductor Model coupled to GW emission: energy budget and model application to RX J0806+15 and RX J1914+24

We further discuss the Unipolar Inductor Model (UIM) coupled to GW emission (Dall'Osso et al. 2005) and compare it to observed properties of the two candidate ultrashort period binaries RX J0806+15 and RX J1914+24 . We consider the measured orbital periods, period derivatives and inferred X-ray luminosities of these two sources and find constraints on system parameters in order for the model to account for them. We find that these properties point to the two sources being in different regimes of the UIM, with the requirement of low magnetic moment primaries ($\sim 10^{30} G cm^3$) for both. Given this weak magnetization, RX J0806+15 has a sufficiently low luminosity that it can be interpreted as having a primary spin almost synchronous to and just slightly slower than the orbital motion. Its measured orbital spin-up is only slightly affected by spin-orbit coupling and is mostly due to GW emission. RX J1914+24, on the other hand, is too bright in X-rays and has too slow an orbital spin-up for the same regime to apply. We suggest that this binary system may be emitting GWs at a significantly higher rate than implied by its measured $\dotω_o = 6 \times 10^{-17} rad s^{-2}$. The latter is explained, in this framework, by the primary spin being slightly faster than the orbital motion ($α<or= 1.1$). In this case, the associated spin-orbit coupling transfers to the orbit a significant amount of angular momentum, thus partially balancing that lost to GW emission. All expectations can be tested in the near future to confirm the viability of the model.

astro-ph↗

arXiv2004

Taking the pulse of the shortest orbital period binary system RX J0806.3+1527

RX J0806.3+1527 is thought to be a 321s orbital period (the shortest known) double white dwarf binary system. According to the double degenerate binary (DDB) scenario this source is expected to be one of the strongest gravitational wave (GW) emitter candidates. In the last years RX J0806.3+1527 has been studied in great details, through multiwavelength observational campaigns and from the point of view of data analysis result interpretations. We present here the timing results obtained thanks to a 3.5-year long optical monitoring campaign carried out by the Very Large Telescope (VLT) and the Telescopio Nazionale Galileo (TNG) which allowed us to detect and study the orbital period derivative (spin-up at a rate of about 10^-3 s/yr) of the 321s modulation, to detect the linear polarisation (at a level of about 2%), and to study the broad band energy spectrum. The VLT/TNG observational strategy we used allowed us to rely upon a P-Pdot coherent solution which we finally extended backward to the 1994 ROSAT observation of RX J0806.3+1527.

astro-ph↗

arXiv2003

The glitches of the Anomalous X-ray Pulsar 1RXS J170849.0--400910

We report on a timing analysis of archival observations of the Anomalous X-ray Pulsar 1RXSJ170849.0-400910 made with the RXTE Proportional Counter Array. We detect a new large glitch (Delta nu / nu ~ 3 x 10^-6) which occurred between 2001 March 27 and 2001 May 6, with an associated large increase in the spin-down rate (Delta nudot/nudot ~ 0.3). The short time (1.5 yrs) elapsed from the previously detected glitch and the large amplitude of the new spin-up place this source among the most frequent glitchers, with large average glitch amplitudes, similar to those of the Vela pulsar. The source shows different recoveries after the glitches: in the first one it is well described by a long term linear trend similar to those seen in Vela-like glitches; in the second case the recovery is considerably faster and is better described by an exponential plus a fractional change in the long-term spin-down rate of the order of 1%. No recovery of the latter is detected but additional observations are necessary to confirm this result. Observed glitch properties are compared to those of radio pulsar glitches; current models are discussed in light of our results. It appears that glitches may represent yet another peculiarity of AXPs. Starquake-based models appear to be prefered on qualitative grounds. Alternative models can be applied to individual glitches but fail in explaining both. Thus the two events may as well arise from two different mechanisms.

astro-ph↗