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The $P\dot P$ diagram is a cornerstone of pulsar research. It is used in multiple ways for classifying the population, understanding evolutionary tracks, identifying issues in our theoretical reach, and more. However, we have been looking at the same plot for more than five decades. A fresh appraisal may be healthy. Is the $P\dot P$-diagram the most useful or complete way to visualize the pulsars we know? Here we pose a fresh look at the information we have on the pulsar population. First, we use principal components analysis over magnitudes depending on the intrinsic pulsar's timing properties (proxies to relevant physical pulsar features), to analyze whether the information contained by the pulsar's period and period derivative is enough to describe the variety of the pulsar population. Even when the variables of interest depend on $P$ and $\dot P$, we show that $P\dot P$ are not principal components. Thus, any distance ranking or visualization based only on $P$ and $\dot P$ is potentially misleading. Next, we define and compute a properly normalized distance to measure pulsar nearness, calculate the minimum spanning tree of the population, and discuss possible applications. The pulsar tree hosts information about pulsar similarities that go beyond $P$ and $\dot P$, and are thus naturally difficult to read from the $P\dot P$-diagram. We use this work to introduce the pulsar tree website http://www.pulsartree.ice.csic.es containing visualization tools and data to allow users to gather information in terms of MST and distance ranking., Comment: In press in MNRAS. The pulsar tree website is at http://www.pulsartree.ice.csic.es

Accreting millisecond X-ray pulsars (AMXPs) show burst oscillations during thermonuclear explosions of the accreted plasma which are markedly different from those observed in non-pulsating low mass X-ray binaries. The AMXP XTE J1814-338 is known for having burst oscillations that are phase locked (constant phase difference) and coincident with the accretion powered pulsations during all its thermonuclear bursts but the last one. In this work we use a coherent timing analysis to investigate this phenomenon in more detail and with higher time resolution than was done in the past. We confirm that the burst oscillation phases are, on average, phase locked to the accretion powered pulsations. However, they also display moderate (<~ 0.1 cycles) drifts during each individual burst, showing a repeating pattern that is consistently observed according to the thermonuclear burst phase (rise, peak, tail). Despite the existence of these drifting patterns, the burst oscillation phases somehow are able to average out at almost the exact position of the accretion powered pulsations. We provide a kinematic description of the phenomenon and review the existing models in the literature. The phenomenon remains without a clear explanation, but we can place important constraints on the thermonuclear burst mechanism. In particular, the observations imply that the ignition point of the thermonuclear burst occurs close to the foot of the accretion column. We speculate that the burning fluid expands in a backward tilted accretion column trapped by the magnetic field, while at the same time the burning flame covers the surface., Comment: Accepted for publication by MNRAS

3FGL J1544.6-1125 is a candidate transitional millisecond pulsar (tMSP). Similar to the well-established tMSPs - PSR J1023+0038, IGR J18245-2452, and XSS J12270-4859 -- 3FGL J1544.6-1125 shows $\gamma$-ray emission and discrete X-ray "low" and "high" modes during its low-luminosity accretion-disk state. Coordinated radio/X-ray observations of PSR J1023+0038 in its current low-luminosity accretion-disk state showed rapidly variable radio continuum emission-possibly originating from a compact, self-absorbed jet, the "propellering" of accretion material, and/or pulsar moding. 3FGL J1544.6-1125 is currently the only other (candidate) tMSP system in this state, and can be studied to see whether tMSPs are typically radio-loud compared to other neutron star binaries. In this work, we present a quasi-simultaneous Very Large Array and Swift radio/X-ray campaign on 3FGL J1544.6-1125. We detect 10 GHz radio emission varying in flux density from $47.7 \pm 6.0$ $\mu$Jy down to $\sim$15 $\mu$Jy (3$\sigma$ upper limit) at four epochs spanning three weeks. At the brightest epoch, the radio luminosity is $L_{5 GHz}$ $= (2.17 \pm 0.17) \times 10^{27}$ erg s$^{-1}$ for a quasi-simultaneous X-ray luminosity $L_{2-10 keV}$ $= (4.32 \pm 0.23) \times 10^{33}$ erg s$^{-1}$ (for an assumed distance of 3.8 kpc). These luminosities are close to those of PSR J1023+0038, and the results strengthen the case that 3FGL J1544.6-1125 is a tMSP showing similar phenomenology to PSR J1023+0038., Comment: Accepted for publication in the Astrophysical Journal

PSR J1023+0038 is a rapidly-spinning neutron star with a low-mass-binary companion that switches between a radio pulsar and low-luminosity disk state. In 2013, it transitioned to its current disk state accompanied by brightening at all observed wavelengths. In this state, PSR J1023+0038 now shows optical and X-ray pulsations and abrupt X-ray luminosity switches between discrete 'low' and 'high' modes. Continuum radio emission, denoting an outflow, is also present and brightens during the X-ray low modes. Here, we present a simultaneous optical, ultraviolet (UV) and X-ray campaign comprising Kepler ($400-800$ nm), Hubble Space Telescope ($180-280$ nm), XMM-Newton ($0.3-10$ keV) and NuSTAR ($3 - 79$ keV). We demonstrate that low and high luminosity modes in the UV band are strictly simultaneous with the X-ray modes and change the UV brightness by a factor of $\sim25$\% on top of a much brighter persistent UV component. We find strong evidence for UV pulsations (pulse fraction of $0.82\pm0.19$\%) in the high-mode, with a similar waveform as the X-ray pulsations making it the first known UV millisecond pulsar. Lastly, we find that the optical mode changes occur synchronously with the UV/X-ray mode changes, but optical modes are inverted compared to the higher frequencies. There appear to be two broad-band emission components: one from radio to near-infrared/optical that is brighter when the second component from optical to hard X-rays is dimmer (and vice-versa). We suggest that these components trace switches between accretion into the neutron star magnetosphere (high-energy high-mode) versus ejection of material (low-energy high-mode). Lastly, we propose that optical/UV/X-ray pulsations can arise from a shocked accretion flow channeled by the neutron star's magnetic field., Comment: 18 pages, 10 figures

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science., Comment: 50 pages, Probe class mission concept study report submitted to NASA for Astro2020 Decadal Survey

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

We present coordinated Chandra X-ray Observatory and Karl G. Jansky Very Large Array observations of the transitional millisecond pulsar PSR J1023+0038 in its low-luminosity accreting state. The unprecedented five hours of strictly simultaneous X-ray and radio continuum coverage for the first time unambiguously show a highly reproducible, anti-correlated variability pattern. The characteristic switches from the X-ray high mode into a low mode are always accompanied by a radio brightening with duration that closely matches the X-ray low mode interval. This behavior cannot be explained by a canonical inflow/outflow accretion model where the radiated emission and the jet luminosity are powered by, and positively correlated with, the available accretion energy. We interpret this phenomenology as alternating episodes of low-level accretion onto the neutron star during the X-ray high mode that are interrupted by rapid ejections of plasma by the active rotation-powered pulsar, possibly initiated by a reconfiguration of the pulsar magnetosphere, that cause a transition to a less luminous X-ray mode. The observed anti-correlation between radio and X-ray luminosity has an additional consequence: transitional MSPs can make excursions into a region of the radio/X-ray luminosity plane previously thought to be occupied solely by black hole X-ray binary sources. This complicates the use of this luminosity relation to identify candidate black holes, suggesting the need for additional discriminants when attempting to establish the true nature of the accretor., Comment: 11 pages, 6 figures; submitted to the Astrophysical Journal

The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ~10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our XMM-Newton one. We found that the source luminosity was significantly higher and, although the spectrum could be fitted with a black-body model the temperature was higher and the emitting radius smaller. Several weeks later the system started a sequence of type-I accretion outbursts. In between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. We discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase., Comment: 8 pages, 4 figures, 2 tables, accepted for publication in MNRAS

The pulsar J1203+0038 rotates with a frequency $\nu\approx 592$ Hz and has been observed to transition between a radio state, during which it is visible as a millisecond radio pulsar, and and a Low Mass X-ray Binary state, during which accretion powered X-ray pulsations are visible. Timing during the two phases reveals that during the LMXB phase the neutron star is spinning down at a rate of $\dot{\nu}\approx -3 \times 10^{-15}$ Hz/s, which is approximately 27\% faster than the rate measured during the radio phase, $\dot{\nu}\approx -2.4 \times 10^{-15}$ Hz/s, and at odds with the predictions of accretion models. In this letter we suggest that the increase in spin-down rate is compatible with gravitational wave emission, and in particular to the creation of a `mountain' during the accretion phase. We show that asymmetries in pycno-nuclear reaction rates in the crust can lead to a large enough mass quadrupole to explain the observed spin-down rate, which so far has no other self-consistent explanation, and that radio timing at the onset of the next millisecond radio pulsar phase can test this scenario. Another possibility is that an unstable $r$-mode with amplitude $\alpha\approx 5\times10^{-8}$ may be present in the system., Comment: 5 pages, submitted to PRL

The accreting millisecond X-ray pulsar (AMXP) SAX J1808.4-3658, shows a peculiar orbital evolution that proceeds at a much faster pace than predicted by conservative binary evolution models. It is important to identify the underlying mechanism responsible for this behavior because it can help to understand how this system evolves. It has also been suggested that, when in quiescence, SAX J1808.4-3658 turns on as a radio pulsar, a circumstance that might provide a link between AMXPs and black-widow radio pulsars. In this work we report the results of a deep radio pulsation search at 2 GHz using the Green Bank Telescope in August 2014 and an X-ray monitoring of the 2015 outburst with Chandra, Swift, and INTEGRAL. In particular, we present the X-ray timing analysis of a 30-ks Chandra observation executed during the 2015 outburst. We detect no radio pulsations, and place the strongest limit to date on the pulsed radio flux density of any AMXP. We also find that the orbit of SAX J1808.4-3658 continues evolving at a fast pace and we compare it to the bhevior of other accreting and non-accreting binaries. We discuss two scenarios: either the neutron star has a large moment of inertia (I>1.7x10^45 g cm^2) and is ablating the donor (by using its spin-down power) thus generating mass-loss with an efficiency of 40% or the donor star is undergoing quasi-cyclic variations due to a varying mass-quadrupole induced by either a strong (1 kG) field or by some unidentified mechanism probably linked to irradiation., Comment: Submitted to ApJ

Transitional millisecond pulsars (tMSPs) switch, on roughly multi-year timescales, between rotation-powered radio millisecond pulsar (RMSP) and accretion-powered low-mass X-ray binary (LMXB) states. The tMSPs have raised several questions related to the nature of accretion flow in their LMXB state and the mechanism that causes the state switch. The discovery of coherent X-ray pulsations from PSR J1023+0038 (while in the LMXB state) provides us with the first opportunity to perform timing observations and to compare the neutron star's spin variation during this state to the measured spin-down in the RMSP state. Whereas the X-ray pulsations in the LMXB state likely indicate that some material is accreting onto the neutron star's magnetic polar caps, radio continuum observations indicate the presence of an outflow. The fraction of the inflowing material being ejected is not clear, but it may be much larger than that reaching the neutron star's surface. Timing observations can measure the total torque on the neutron star. We have phase-connected nine XMM-Newton observations of PSR J1023+0038 over the last 2.5 years of the LMXB state to establish a precise measurement of spin evolution. We find that the average spin-down rate as an LMXB is 26.8+/-0.4% faster than the rate (-2.39x10^-15 Hz s-1) determined during the RMSP state. This shows that negative angular momentum contributions (dipolar magnetic braking and outflow) exceed positive ones (accreted material), and suggests that the pulsar wind continues to operate at a largely unmodified level. We discuss implications of this tight observational constraint in the context of possible accretion models., Comment: 19 pages, 7 Figures, Accepted for publication in the Astrophysical Journal

One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. The flux we observe from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star's rotation, giving rise to a pulsation. Information about mass and radius is encoded into the pulse profile via relativistic effects, and tight constraints on mass and radius can be obtained. The second technique involves characterising the spin distribution of accreting neutron stars. The most rapidly rotating stars provide a very clean constraint, since the mass-shedding limit is a function of mass and radius. However the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasi-periodic oscillations in X-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state. We illustrate how these complementary X-ray timing techniques can be used to constrain the dense matter equation of state, and discuss the results that might be expected from a 10m$^2$ instrument. We also discuss how the results from such a facility would compare to other astronomical investigations of neutron star properties. [Modified for arXiv], Comment: To appear in Reviews of Modern Physics as a Colloquium, 23 pages, 9 figures

We study the 205.9 Hz pulsations of the accreting millisecond X-ray pulsar NGC 6440 X-2 across all outbursts observed with the Rossi X-ray Timing Explorer over a period of 800 days. We find the pulsations are highly sinusoidal with a fundamental amplitude of 5%-15% rms and a second harmonic that is only occasionally detected with amplitudes of <2% rms. By connecting the orbital phase across multiple outbursts, we obtain an accurate orbital ephemeris for this source and constrain its 57 min orbital period to sub-ms precision. We do not detect an orbital period derivative to an upper limit of $ | \dot{P} | \leq 8 \times 10^{-11}$ s/s. We investigate the possibility of coherently connecting the pulse phase across all observed outbursts, but find that due to the poorly constrained systematic uncertainties introduced by a flux-dependent bias in the pulse phase, multiple statistically acceptable phase-connected timing solutions exist., Comment: 6 pages, 2 figures, 3 tables. Accepted for publication in ApJ

Rapidly rotating Neutron Stars (NSs) in Low Mass X-ray Binaries (LMXBs) are thought to be interesting sources of Gravitational Waves (GWs) for current and next generation ground based detectors, such as Advanced LIGO and the Einstein Telescope. The main reason is that many of the NS in these systems appear to be spinning well below their Keplerian breakup frequency, and it has been suggested that torques associated with GW emission may be setting the observed spin period. This assumption has been used extensively in the literature to assess the strength of the likely gravitational wave signal. There is now, however, a significant amount of theoretical and observation work that suggests that this may not be the case, and that GW emission is unlikely to be setting the spin equilibrium period in many systems. In this paper we take a different starting point and predict the GW signal strength for two physical mechanisms that are likely to be at work in LMXBs: crustal mountains due to thermal asymmetries and magnetically confined mountains. We find that thermal crustal mountains in transient LMXBs are unlikely to lead to detectable GW emission, while persistent systems are good candidates for detection by Advanced LIGO and by the Einstein Telescope. Detection prospects are pessimistic for the magnetic mountain case, unless the NS has a buried magnetic field of $B\approx 10^{12}$ G, well above the typically inferred exterior dipole fields of these objects. Nevertheless, if a system were to be detected by a GW observatory, cyclotron resonant scattering features in the X-ray emission could be used to distinguish between the two different scenarios., Comment: submitted to MNRAS

With an average density higher than the nuclear density, neutron stars (NS) provide a unique test-ground for nuclear physics, quantum chromodynamics (QCD), and nuclear superfluidity. Determination of the fundamental interactions that govern matter under such extreme conditions is one of the major unsolved problems of modern physics, and -- since it is impossible to replicate these conditions on Earth -- a major scientific motivation for SKA. The most stringent observational constraints come from measurements of NS bulk properties: each model for the microscopic behaviour of matter predicts a specific density-pressure relation (its `Equation of state', EOS). This generates a unique mass-radius relation which predicts a characteristic radius for a large range of masses and a maximum mass above which NS collapse to black holes. It also uniquely predicts other bulk quantities, like maximum spin frequency and moment of inertia. The SKA, in Phase 1 and particularly in Phase 2 will, thanks to the exquisite timing precision enabled by its raw sensitivity, and surveys that dramatically increase the number of sources: 1) Provide many more precise NS mass measurements (high mass NS measurements are particularly important for ruling out EOS models); 2) Allow the measurement of the NS moment of inertia in highly relativistic binaries such as the Double Pulsar; 3) Greatly increase the number of fast-spinning NS, with the potential discovery of spin frequencies above those allowed by some EOS models; 4) Improve our knowledge of new classes of binary pulsars such as black widows and redbacks (which may be massive as a class) through sensitive broad-band radio observations; and 5) Improve our understanding of dense matter superfluidity and the state of matter in the interior through the study of rotational glitches, provided that an ad-hoc campaign is developed., Comment: 22 pages, 8 figures, to be published in: "Advancing Astrophysics with the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)043

Non pulsating neutron stars in low mass X-ray binaries largely outnumber those that show pulsations. The lack of detectable pulses represents a big open problem for two important reasons. The first is that the structure of the accretion flow in the region closest to the neutron star is not well understood and it is therefore unclear what is the mechanism that prevents the pulse formation. The second is that the detection of pulsations would immediately reveal the spin of the neutron star. Aql X-1 is a special source among low mass X-ray binaries because it has showed the unique property of pulsating for only ~150 seconds out of a total observing time of more than 1.5 million seconds. However, the existing upper limits on the pulsed fraction leave open two alternatives. Either Aql X-1 has very weak pulses which have been undetected, or it has genuinely pulsed only for a tiny amount of the observed time. Understanding which of the two scenarios is the correct one is fundamental to increase our knowledge about the pulse formation process and understand the chances we have to detect weak pulses in other LMXBs. In this paper we perform a semi-coherent search on the entire X-ray data available for Aql X-1. We find no evidence for (new) weak pulsations with the most stringent upper limits being of the order of 0.3% in the 7-25 keV energy band., Comment: 15 pages, 7 figures

The PSR J1023+0038 binary system hosts a neutron star and a low-mass, main-sequence-like star. It switches on year timescales between states as an eclipsing radio millisecond pulsar and a low-mass X-ray binary. We present a multi-wavelength observational campaign of PSR J1023+0038 in its most recent low-mass X-ray binary state. Two long XMM-Newton observations reveal that the system spends ~70% of the time in a $\approx$$3\times10^{33}$ erg/s X-ray luminosity mode, which, as shown in Archibald et al. (2014), exhibits coherent X-ray pulsations. This emission is interspersed with frequent lower flux mode intervals with $\approx$$5\times 10^{32}$ erg/s and sporadic flares reaching up to $\approx$$10^{34}$ erg/s, with neither mode showing significant X-ray pulsations. The switches between the three flux modes occur on timescales of order 10 s. In the UV and optical, we observe occasional intense flares coincident with those observed in X-rays. Our radio timing observations reveal no pulsations at the pulsar period during any of the three X-ray modes, presumably due to complete quenching of the radio emission mechanism by the accretion flow. Radio imaging detects highly variable, flat-spectrum continuum emission from PSR J1023+0038, consistent with an origin in a weak jet-like outflow. Our concurrent X-ray and radio continuum data sets do not exhibit any correlated behavior. The observational evidence we present bears qualitative resemblance to the behavior predicted by some existing propeller and trapped disk accretion models although none can fully account for all aspects of the rich phenomenology of this system., Comment: 24 pages, 15 figures; accepted for publication in the Astrophysical Journal

Accreting millisecond X-ray pulsars are an important subset of low-mass X-ray binaries in which coherent X-ray pulsations can be observed during occasional, bright outbursts (X-ray luminosity $L_X\sim 10^{36}$ erg s$^{-1}$). These pulsations show that matter is being channeled onto the neutron star's magnetic poles. However, such sources spend most of their time in a low-luminosity, quiescent state ($L_X\lesssim 10^{34}$ erg s$^{-1}$), where the nature of the accretion flow onto the neutron star (if any) is not well understood. Here we report that the millisecond pulsar/low-mass X-ray binary transition object PSR J1023+0038 intermittently shows coherent X-ray pulsations at luminosities nearly 100 times fainter than observed in any other accreting millisecond X-ray pulsar. We conclude that in spite of its low luminosity PSR J1023+0038 experiences episodes of channeled accretion, a discovery that challenges existing models for accretion onto magnetized neutron stars., Comment: 9 pages, 7 figures, 1 table; accepted in ApJ

Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed as an interesting source of gravitational waves. In this chapter we present estimates of the gravitational wave emission for various scenarios, given the (electromagnetically) observed characteristics of these systems. First of all we focus on the r-mode instability and show that a 'minimal' neutron star model (which does not incorporate exotica in the core, dynamically important magnetic fields or superfluid degrees of freedom), is not consistent with observations. We then present estimates of both thermally induced and magnetically sustained mountains in the crust. In general magnetic mountains are likely to be detectable only if the buried magnetic field of the star is of the order of $B\approx 10^{12}$ G. In the thermal mountain case we find that gravitational wave emission from persistent systems may be detected by ground based interferometers. Finally we re-asses the idea that gravitational wave emission may be balancing the accretion torque in these systems, and show that in most cases the disc/magnetosphere interaction can account for the observed spin periods., Comment: To appear in 'Gravitational Waves Astrophysics: 3rd Session of the Sant Cugat Forum on Astrophysics, 2014', Editor: Carlos F. Sopuerta

We report NuSTAR observations of the millisecond pulsar - low mass X-ray binary (LMXB) transition system PSR J1023+0038 from June and October 2013, before and after the formation of an accretion disk around the neutron star. Between June 10-12, a few days to two weeks before the radio disappearance of the pulsar, the 3-79 keV X-ray spectrum was well fit by a simple power law with a photon index of Gamma=1.17 +/-0.08 (at 90% confidence) with a 3-79 keV luminosity of 7.4+/-0.4 x 10^32 erg/s. Significant orbital modulation was observed with a modulation fraction of 36+/-10%. During the October 19-21 observation, the spectrum is described by a softer power law (Gamma=1.66+/-0.06) with an average luminosity of 5.8+/-0.2 x 10^33 erg/s and a peak luminosity of ~1.2 x 10^34 erg/s observed during a flare. No significant orbital modulation was detected. The spectral observations are consistent with previous and current multi-wavelength observations and show the hard X-ray power law extending to 79 keV without a spectral break. Sharp edged, flat bottomed `dips' are observed with widths between 30-1000 s and ingress and egress time-scales of 30-60 s. No change in hardness ratio was observed during the dips. Consecutive dip separations are log-normal in distribution with a typical separation of approximately 400 s. These dips are distinct from dipping activity observed in LMXBs. We compare and contrast these dips to observations of dips and state changes in the similar transition systems PSR J1824-2452I and XSS J1227.0-4859 and discuss possible interpretations based on the transitions in the inner disk., Comment: 13 pages, 17 figures, accepted for publication in ApJ

We present XMM-Newton and Chandra observations of the low-mass X-ray binary XSS J12270--4859, which experienced a dramatic decline in optical/X-ray brightness at the end of 2012, indicative of the disappearance of its accretion disk. In this new state, the system exhibits previously absent orbital-phase-dependent, large-amplitude X-ray modulations with a decline in flux at superior conjunction. The X-ray emission remains predominantly non-thermal but with an order of magnitude lower mean luminosity and significantly harder spectrum relative to the previous high flux state. This phenomenology is identical to the behavior of the radio millisecond pulsar binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray emission is produced in an intra-binary shock driven by the pulsar wind. This further demonstrates that XSS J12270-4859 no longer has an accretion disk and has transformed to a full-fledged eclipsing "redback" system that hosts an active rotation-powered millisecond pulsar. There is no evidence for diffuse X-ray emission associated with the binary that may arise due to outflows or a wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely to be associated, and is probably a previously uncatalogued galaxy cluster., Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical Journal

The X-ray transient IGR J18245-2452 in the globular cluster M28 contains the first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March-April 25d-long outburst as observed by Swift, which had a peak bolometric luminosity of ~6% of the Eddington limit (L$_{E}$), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245-2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (L$_X$, 0.5-10 keV). The Swift spectrum softens during the outburst decay (photon index $\Gamma$ from 1.3 above L$_X$/L$_{E}$=10$^{-2}$ to ~2.5 at L$_X$/L$_{E}$=10$^{-4}$), similar to other NS and black hole (BH) transients. At even lower luminosities, deep Chandra observations reveal hard ($\Gamma$=1-1.5), purely non-thermal and highly variable X-ray emission in quiescence. We therefore find evidence for a spectral transition at L$_X$/L$_{E}$~10$^{-4}$, where the X-ray spectral softening observed during the outburst decline turns into hardening as the source goes to quiescence. Furthermore, we find a striking variability pattern in the 2008 Chandra light curves: rapid switches between a high-L$_X$ "active" state (L$_X\simeq$3.9x10$^{33}$ erg/s) and a low-L$_X$ "passive" state (L$_X\simeq$5.6x10$^{32}$ erg/s), with no detectable spectral change. We put our results in the context of low luminosity accretion flows around compact objects and X-ray emission from millisecond radio pulsars. Finally, we discuss possible origins for the observed mode switches in quiescence, and explore a scenario where they are caused by fast transitions between the magnetospheric accretion and pulsar wind shock emission regimes., Comment: Accepted for publication in MNRAS. 12 pages, 6 figures, 2 tables

We discuss a scenario for the very faint X-ray transients as X-ray binaries fed by winds from detached M-dwarf donors in binary stars within the "period gap" -- the range of periods where donor stars have become fully convective, and shrunken so that they no longer fill their Roche lobes, but have not yet re-attached due to the systems shrinking through gravitational radiation. This wind-fed detached binary scenario can reproduce the two key properties of the very faint X-ray transients -- their faintness, which defines them, and their relatively low duty cycle outbursts which require that they have low mean mass transfer rates. We discuss feasible observational tests of the scenario., Comment: 7 pages, no figures, accepted to MNRAS

In this Letter we re-examine the idea that gravitational waves are required as a braking mechanism to explain the observed maximum spin-frequency of neutron stars. We show that for millisecond X-ray pulsars, the existence of spin equilibrium as set by the disk/magnetosphere interaction is sufficient to explain the observations. We show as well that no clear correlation exists between the neutron star magnetic field B and the X-ray outburst luminosity Lx when considering an enlarged sample size of millisecond X-ray pulsars., Comment: 5 pages, 2 figures, Submitted to ApJ Letters

Accreting millisecond pulsars show significant variability of their pulse profiles, especially at low accretion rates. On the other hand, their X-ray spectra are remarkably similar with not much variability over the course of the outbursts. For the first time, we have discovered that during the 2008 outburst of SAX J1808.4-3658 a major pulse profile change was accompanied by a dramatic variation of the disc luminosity at almost constant total luminosity. We argue that this phenomenon is related to a change in the coupling between the neutron star magnetic field and the accretion disc. The varying size of the pulsar magnetosphere can influence the accretion curtain geometry and affect the shape and the size of the hotspots. Using this physical picture, we develop a self-consistent model that successfully describes simultaneously the pulse profile variation as well as the spectral transition. Our findings are particularly important for testing the theories of accretion onto magnetized neutron stars, better understanding of the accretion geometry as well as the physics of disc-magnetosphere coupling. The identification that varying hotspot size can lead to pulse profile changes has profound implications for determination of the neutron star masses and radii., Comment: 12 pages, 5 figures and 3 tables; accepted to MNRAS

In this letter we present a new analysis of the torques acting on the accreting millisecond X-ray pulsars SAX J1808.4-365 and XTE J1814-338, and show how our results can be used to constrain theoretical models of the spin evolution. In particular we find upper limits on any spin-up/down phase of XTE J1814-338 of $|\dot{\nu}|\lesssim\,1.5\times\,10^{-14}$ Hz/s at 95% confidence level. We examine the possibility that a gravitational wave torque may be acting in these systems and suggest that a more likely scenario is that both systems are close to spin equilibrium, as set by the disc/magnetosphere interaction., Comment: Submitted to ApJ Letters

The amount of data available for NGC 1313 X-2 make it a cornerstone for the study of ultraluminous X-ray sources (ULXs). We modelled the optical and X-ray data of this ULX with a binary evolution code that takes into account X-ray irradiation. We restricted the candidate binary system to be either a 50-100 Msun black hole (BH) accreting from a 12-15 Msun main sequence star or a 20 Msun BH with a 12-15 Msun giant donor. If the orbital period of the system is ~6 days, a 20 Msun BH model becomes unlikely and we are left with the only possibility that the compact accretor in NGC1313 X-2 is a massive BH of 50-100 Msun. We briefly discuss these results within the framework of an alternative scenario for the formation of ULXs, in which a portion of them may contain BHs of >~30-40 Msun formed from very massive stars in a low metallicity environment., Comment: 4 pages, 2 figures. Accepted for publication in the Astronomische Nachrichten (Astronomical Notes), to appear in the proceedings of the conference "Ultra-Luminous X-ray sources and Middle Weight Black Holes" (Madrid, May 24-26, 2010)

We perform phase-resolved spectroscopy of the accreting millisecond pulsar, SAX J1808.4-3658, during the slow-decay phase of the 2002 outburst. Simple phenomenological fits to RXTE PCA data reveal a pulsation in the iron line at the spin frequency of the neutron star. However, fitting more complex spectral models reveals a degeneracy between iron-line pulsations and changes in the underlying hotspot blackbody temperature with phase. By comparing with the variations in reflection continuum, which are much weaker than the iron line variations, we infer that the iron-line is not pulsed. The observed spectral variations can be explained by variations in blackbody temperature associated with rotational Doppler shifts at the neutron star surface. By allowing blackbody temperature to vary in this way, we also find a larger phase-shift between the pulsations in the Comptonised and blackbody components than has been seen in previous work. The phase-shift between the pulsation in the blackbody temperature and normalisation is consistent with a simple model where the Doppler shift is maximised at the limb of the neutron star, ~90 degrees prior to maximisation of the hot-spot projected area., Comment: 8 pages, 10 figures, 2 tables. Accepted by MNRAS

Measuring the spin of Accreting Neutron Stars is important because it can provide constraints on the Equation of State of ultra-dense matter. Particularly crucial to our physical understanding is the discovery of sub-millisecond pulsars, because this will immediately rule out many proposed models for the ground state of dense matter. So far, it has been impossible to accomplish this because, for still unknown reasons, only a small amount of Accreting Neutron Stars exhibit coherent pulsations. An intriguing explanation for the lack of pulsations is that they form only on neutron stars accreting with a very low average mass accretion rate. I have searched pulsations in the faintest persistent X-ray source known to date and I found no evidence for pulsations. The implications for accretion theory are very stringent, clearly showing that our understanding of the pulse formation process is not complete. I discuss which sources are optimal to continue the search of sub-ms pulsars and which are the new constraints that theoretical models need to explain to provide a complete description of these systems, Comment: 10 pages, 2 figures, to appear in Proceedings of Science: "High Time Resolution Astrophysics IV - The Era of Extremely Large Telescopes - HTRA-IV" -- One typo in Table 1 corrected

Accreting Millisecond X-ray Pulsars like IGR J00291+5934 are important because it is possible to test theories of pulsar formation and evolution. They give also the possibility to constrain gravitational wave emission theories and the equation of state of ultra dense matter. Particularly crucial to our understanding is the measurement of the long term spin evolution of the accreting neutron star. An open question is whether these accreting pulsars are spinning up during an outburst and spinning down in quiescence as predicted by the recycling scenario. Until now it has been very difficult to measure torques, due to the presence of fluctuations in the pulse phases that compromise their measurements with standard coherent timing techniques. By applying a new method, I am now able to measure a spin up during an outburst and a spin down during quiescence. I ascribe the spin up (Fdot=5.1(3)x10^{-13}\Hz/s) to accretion torques and the spin down (Fdot=-3.0(8)x10^{-15} Hz/s) to magneto dipole torques, as those observed in radio pulsars. Both values nicely fit in the recycling scenario and I infer the existence of a magnetic field for the pulsar of B~2x10^{8} G. No evidence for an enhanced spin down due to gravitational wave emission is found. The accretion torques are smaller than previously reported and there is strong evidence for an ordered process that is present in all outbursts that might be connected with a motion of the hot spot on the neutron star surface., Comment: 12 Pages, 5 Figures, 7 Tables. Accepted for publication in The Astrophysical Journal

We present a theoretical study on the nature of the ultra-luminous X-ray source NGC 1313 X-2. We evolved a set of binaries with high mass donor stars orbiting a 20 M_Sun or a 50-100 M_Sun black hole. Using constraints from optical observations we restricted the candidate binary system for NGC 1313 X-2 to be either a 50-100 M_Sun black hole accreting from a 12-15 M_Sun main sequence star or a ~20 M_Sun black hole with a 12-15 M_Sun giant donor. If the modulation of ~6.12 days recently identified as the orbital period of the system is confirmed, a ~20 M_Sun black hole model becomes unlikely and we are left with the only possibility that the compact accretor in NGC 1313 X-2 is a massive black hole of ~50-100 M_Sun., Comment: 6 pages, 2 figures, Accepted by MNRAS Letters

We present a timing analysis of the 2009 outburst of the accreting millisecond X-ray pulsar Swift J1756.9-2508, and a re-analysis of the 2007 outburst. The source shows a short recurrence time of only ~2 years between outbursts. Thanks to the approximately 2 year long baseline of data, we can constrain the magnetic field of the neutron star to be 0.4x10^8 G < B < 9x10^8 G, which is within the range of typical accreting millisecond pulsars. The 2009 timing analysis allows us to put constraints on the accretion torque: the spin frequency derivative within the outburst has an upper limit of $|\dot{\nu}| < 3x10^-13 Hz/s at the 95% confidence level. A study of pulse profiles and their evolution during the outburst is analyzed, suggesting a systematic change of shape that depends on the outburst phase., Comment: 7 pages, 4 figures, submitted to MNRAS

We present the identification of the most likely near-infrared/optical counterparts of five low-luminosity X-ray pulsators (AX J1700.1-4157, AX 1740.1-2847, AX J1749.2-2725, AX J1820.5-1434 and AX J1832.3-0840) which have long pulse periods (> 150 s). The X-ray properties of these systems suggest that they are likely members of persistent high mass X-ray binaries or intermediate polars. Using our Chandra observations, we detected the most likely counterparts of three sources (excluding AX J1820.5-1434 and AX J1832.3-0840) in their ESO - NTT near-infrared observations, and a possible counterpart for AX J1820.5-1434 and AX J1832.3-0840 in the 2MASS and DSS observations respectively. We also performed the X-ray timing and spectral analysis for all the sources using our XMM-Newton observations, which further helped us to constrain the nature of these systems. Our multiwavelength observations suggest that AX J1749.2-2725 and AX J1820.5-1434 most likely harbor accreting neutron stars while AX J1700.1-4157, AX J1740.1-2847 and AX J1832.3-0840 could be intermediate polars., Comment: 11 pages, 5 figures, Submitted to MNRAS

We report the discovery of burst oscillations from the intermittent accretion-powered millisecond pulsar (AMP) HETE J1900.1-2455, with a frequency approximately 1 Hz below the known spin frequency. The burst oscillation properties are far more similar to those of the non-AMPs and Aql X-1 (an intermittent AMP with a far lower duty cycle), than those of the AMPs SAX J1808.4-3658 and XTE J1814-338. We discuss the implications for models of the burst oscillation and intermittency mechanisms., Comment: 4 pages, 4 figures, submitted to ApJ Letters

XMM-Newton observed the accreting millisecond pulsar SAX J1808.4-3658 during its 2008 outburst. We present timing and spectral analyses of this observation, in particular the first pulse profile study below 2 keV, and the high-resolution spectral analysis of this source during the outburst. Combined spectral and pulse profile analyses suggest the presence of a strong unpulsed source below 2 keV that strongly reduces the pulsed fraction and a hard pulsed component that generates markedly double peaked profiles at higher energies. We also studied the high-resolution grating spectrum of SAX J1808.4-3658, and found several absorption edges and Oxygen absorption lines with whom we infer, in a model independent way, the interstellar column densities of several elements in the direction of SAX J1808.4-3658., Comment: MNRAS Letters, accepted

We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar XTE J1807-294. We find an upper limit for the spin frequency derivative of 5E-14 Hz/s. The sinusoidal fractional amplitudes of the pulsations are the highest observed among the accreting millisecond pulsars and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion over the surface of the neutron star both in longitude and latitude. An anti-correlation between residuals and X-ray flux suggests an influence of accretion rate on pulse phase, and casts doubts on the use of standard timing techniques to measure spin frequencies and torques on the neutron star., Comment: Submitted to ApJ

The measurement of the spin frequency in accreting millisecond X-ray pulsars (AMXPs) is strongly affected by the presence of an unmodeled component in the pulse arrival times called 'timing noise'. We show that it is possible to attribute much of this timing noise to a pulse phase offset that varies in correlation with X-ray flux, such that noise in flux translates into timing noise. This could explain many of the pulse frequency variations previously interpreted in terms of true spin up or spin down, and would bias measured spin frequencies. Spin frequencies improved under this hypothesis are reported for six AMXPs. The effect would most easily be accounted for by an accretion rate dependent hot spot location., Comment: Submitted to ApJ Letters

The primary concern of this contribution is that accreting millisecond pulsars (AMXPs) show a much larger amount of information than is commonly believed. The three questions to be addressed are: 1. Is the apparent spin torque observed in AMXPs real ? 2. Why do we see correlations and anti-correlations between fractional amplitudes and timing residuals in some AMXPs ? 3. Why the timing residuals, the lightcurve and the 1Hz QPO in SAX J1808.4$-$3658 are related ?, Comment: To be published in the proceedings of the workshop 'A Decade of Accreting Millisecond X-ray pulsars' (Amsterdam 14-18 April 2008; Eds. Wijnands et al.)

We present results from our Chandra and XMM-Newton observations of two low-luminosity X-ray pulsators SAX J1324.4-6200 and SAX J1452.8-5949 which have spin-periods of 172 s and 437 s respectively. The XMM-Newton spectra for both sources can be fitted well with a simple power-law model of photon index ~ 1.0. A black-body model can equally well fit the spectra with a temperature of ~ 2 keV for both sources. During our XMM-Newton observations, SAX J1324.4-6200 is detected with coherent X-ray pulsations at a period of $172.86 \pm 0.02$ s while no pulsations with a pulse fraction greater than 15% (at 98% confidence level) are detected in SAX J1452.8--5949. The spin period of SAX J1324.4-6200 is found to be increasing on a time-scale of $\dot{P}$ = $(6.34 \pm 0.08) \times 10^{-9}$ s s$^{-1}$ which would suggest that the accretor is a neutron star and not a white dwarf. Using sub-arcsec spatial resolution of the Chandra telescope, possible counterparts are seen for both sources in the near-infrared images obtained with the SOFI instrument on the New Technology Telescope. The X-ray and near-infrared properties of SAX J1324.4-6200 suggest it to be either a persistent high mass accreting X-ray pulsar or a symbiotic X-ray binary pulsar at a distance $\le$ 9 kpc. We identify the infrared counterpart of SAX J1452.8--5949 to be a late-type main sequence star at a distance $\le$ 10 kpc, thus ruling out SAX J1452.8--5949 to be a high mass X-ray binary. However with the present X-ray and near-infrared observations, we cannot make any further conclusive conclusion about the nature of SAX J1452.8-5949., Comment: 9 pages, 6 figures, 5 tables (Submitted to MNRAS)

X-ray timing of the accretion-powered pulsations during the 2003 outburst of the accreting millisecond pulsar XTE J1814-338 has revealed variation in the pulse time of arrival residuals. These can be interpreted in several ways, including spin-down and wandering of the fuel impact point around the magnetic pole. In this Letter we show that the burst oscillations of this source are coherent with the persistent pulsations, to the level where they track all of the observed fluctuations. Only one burst, which occurs at the lowest accretion rates, shows a significant phase offset. We discuss what might lead to such rigid phase-locking between the modulations in the accretion and thermonuclear burst emission, and consider the implications for spin variation and the burst oscillation mechanism. Wandering of the fuel impact hot spot around a fixed magnetic pole seems the most likely cause for the accretion-powered pulse phase variations. This means that the burst asymmetry is coupled to the hot spot, not the magnetic pole. If premature ignition at this point (due to higher local temperatures) triggers a burning front that stalls before spreading over the entire surface, the resulting localized nuclear hot spot may explain the unusual burst and burst oscillation properties of this source., Comment: Improvements throughout, accepted for publication in ApJ Letters

We present a phase-coherent timing analysis of the intermittent accreting millisecond pulsar SAX J1748.9-2021. A new timing solution for the pulsar spin period and the Keplerian binary orbital parameters was achieved by phase connecting all episodes of intermittent pulsations visible during the 2001 outburst. We investigate the pulse profile shapes, their energy dependence and the possible influence of Type I X-ray bursts on the time of arrival and fractional amplitude of the pulsations. We find that the timing solution of SAX J1748.9-2021 shows an erratic behavior when selecting different subsets of data, that is related to substantial timing noise in the timing post-fit residuals. The pulse profiles are very sinusoidal and their fractional amplitude increases linearly with energy and no second harmonic is detected. The reason why this pulsar is intermittent is still unknown but we can rule out a one-to-one correspondence between Type I X-ray bursts and the appearance of the pulsations., Comment: 22 pages, 7 figures, ApJ accepted. Two figures removed, one typo in the Timing parameters corrected (Table 2)

We present evolutionary tracks of binary systems with high mass companion stars and stellar-through-intermediate mass BHs. Using Eggleton's stellar evolution code, we compute the luminosity produced by accretion from the donor during its entire evolution. We compute also the evolution of the optical spectrum of the binary system taking the disc contribution and irradiation effects into account. The calculations presented here can be used to constrain the properties of the donor stars in Ultraluminous X-ray Sources by comparing their position on the HR or color-magnitude diagrams with the evolutionary tracks of massive BH binaries. This approach may actually provide interesting clues also on the properties of the binary system itself, including the BH mass. We found that, on the basis of their position on the color-magnitude diagram, some of the candidate counterparts considered can be ruled out and more stringent constraints can be applied to the donor masses., Comment: 11 pages, 7 figures. Submitted to MNRAS

We present a 7 yr timing study of the 2.5 ms X-ray pulsar SAX J1808.4-3658, an X-ray transient with a recurrence time of ~2 yr, using data from the Rossi X-ray Timing Explorer covering 4 transient outbursts (1998-2005). We verify that the 401 Hz pulsation traces the spin frequency fundamental and not a harmonic. Substantial pulse shape variability, both stochastic and systematic, was observed during each outburst. Analysis of the systematic pulse shape changes suggests that, as an outburst dims, the X-ray "hot spot" on the pulsar surface drifts longitudinally and a second hot spot may appear. The overall pulse shape variability limits the ability to measure spin frequency evolution within a given X-ray outburst (and calls previous nudot measurements of this source into question), with typical upper limits of |nudot| < 2.5x10^{-14} Hz/s (2 sigma). However, combining data from all the outbursts shows with high (6 sigma) significance that the pulsar is undergoing long-term spin down at a rate nudot = (-5.6+/-2.0)x10^{-16} Hz/s, with most of the spin evolution occurring during X-ray quiescence. We discuss the possible contributions of magnetic propeller torques, magnetic dipole radiation, and gravitational radiation to the measured spin down, setting an upper limit of B < 1.5x10^8 G for the pulsar's surface dipole magnetic field and and Q/I < 5x10^{-9} for the fractional mass quadrupole moment. We also measured an orbital period derivative of Pdot = (3.5+/-0.2)x10^{-12} s/s. This surprising large Pdot is reminiscent of the large and quasi-cyclic orbital period variation observed in the so-called "black widow" millisecond radio pulsars, supporting speculation that SAX J1808.4-3658 may turn on as a radio pulsar during quiescence. In an appendix we derive an improved (0.15 arcsec) source position from optical data., Comment: 22 pages, 10 figures; accepted for publication in ApJ