VLT observations of the magnetar CXO J164710.2-455216 and the detection of a candidate infrared counterpart

We present deep observations of the field of the magnetar CXOJ164710.2-455216 in the star cluster Westerlund 1, obtained in the near-infrared with the adaptive optics camera NACO@VLT. We detected a possible candidate counterpart at the {\em Chandra} position of the magnetar, of magnitudes $\mathrm{J} = 23.5 \pm 0.2$, $\mathrm{H} = 21.0 \pm 0.1$, and $\mathrm{K}_\mathrm{S} = 20.4 \pm 0.1$. The K$_{\rm S}$-band measurements available for two epochs (2006 and 2013) do not show significant signs of variability but only a marginal indication that the flux varied (at the 2 $\sigma$ level), consistent with the fact that the observations were taken when CXOJ164710.2-455216 was in quiescence. At the same time, we also present colour--magnitude and colour--colour diagrams in the J, H, and K$_{\rm S}$ bands from the 2006 epoch only, the only one with observations in all three bands, showing that the candidate counterpart lies in the main bulk of objects describing a relatively well--defined sequence. Therefore, based on its colours and lack of variability, we cannot yet associate the candidate counterpart to CXOJ164710.2-455216. Future near-infrared observations of the field, following-up a source outburst, would be crucial to confirm the association from the detection of near-infrared variability and colour evolution.Comment: 5 pages, 3 figures, accepted for publication in MNRA

Long term hard X-ray variability of the anomalous X-ray pulsar 1RXS J170849.0-400910 discovered with INTEGRAL

We report on a multi-band high-energy observing campaign aimed at studying the long term spectral variability of the Anomalous X-ray Pulsar (AXP) 1RXS J170849.0-400910, one of the magnetar candidates. We observed 1RXS J170849.0-400910 in Fall 2006 and Spring 2007 simultaneously with Swift/XRT, in the 0.1-10 keV energy range, and with INTEGRAL/IBIS, in the 20-200 keV energy range. Furthermore, we also reanalyzed, using the latest calibration and software, all the publicly available INTEGRAL data since 2002, and the soft X-ray data starting from 1999 taken using BeppoSAX, Chandra, XMM, and Swift/XRT, in order to study the soft and hard X-ray spectral variability of 1RXS J170849.0-400910. We find a long-term variability of the hard X-ray flux, extending the hardness-intensity correlation proposed for this source over 2 orders of magnitude in energy.Comment: 5 pages, 2 figures, accepted for publication in Astronomy & Astrophysics main journa

Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808

We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of $\sim339.97$ Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of $44.3$ minutes and a projected semi-major axis of $\sim17.6$ lt-ms. Based on the mass function we estimate a minimum companion mass of 0.024 M$_{\odot}$, which assumes a neutron star mass of 1.4 M$_{\odot}$ and a maximum inclination angle of $75^{\circ}$ (derived from the lack of eclipses and dips in the light-curve of the source). We find that the companion star's Roche-Lobe could either be filled by a hot ($5\times 10^{6}$ K) pure helium white dwarf with a 0.028 M$_{\odot}$ mass (implying $i\simeq58^{\circ}$) or an old (>5 Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065$-$0.085 M$_{\odot}$ (16 < $i$ < 21). During the outburst the broad-band energy spectra are well described by a superposition of a weak black-body component (kT$\sim$ 0.5 keV) and a hard cutoff power-law with photon index $\Gamma \sim$ 1.7 and cut-off at a temperature kT$_e\sim$ 130 keV. Up to the latest Swift-XRT observation performed on 2016 July 19 the source has been observed in outburst for almost 150 days, which makes MAXI J0911-655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.Comment: 7 pages, 5 figures, accepted for publication in A&

Accurate X-ray position and multiwavelength observations of the isolated neutron star RBS 1774

We report on X-ray, optical, infrared and radio observations of the X-ray dim isolated neutron star (XDINS) 1RXS J214303.7+065419 (also known as RBS 1774). The X-ray observation was performed with the High Resolution Camera on board of the Chandra X-ray Observatory, allowing us to derive the most accurate position for this source (alpha = 21h43m3.38s, delta= +6deg54'17".53; 90% uncertainty of 0."6). Furthermore, we confirmed with a higher spatial accuracy the point-like nature of this X-ray source. Optical and infrared observations were taken in B, V, r', i', J, H and Ks filters using the Keck, VLT, Blanco and Magellan telescopes, while radio observations were obtained from the ATNF Parkes single dish at 2.9GHz and 708MHz. No plausible optical and/or infrared counterpart for RBS 1774 was detected within the refined sub-arsecond Chandra X-ray error circle. Present upper limits to the optical and infrared magnitudes are r'>25.7 and J>22.6 (5 sigma confidence level). Radio observations did not show evidence for radio pulsations down to a luminosity at 1.4 GHz of L < 0.02 mJy kpc^2, the deepest limit up to date for any XDINS, and lower than what expected for the majority of radio pulsars. We can hence conclude that, if RBS 1774 is active as radio pulsar, its non detection is more probably due to a geometrical bias rather than to a luminosity bias. Furthermore, no convincing evidence for RRAT-like radio bursts have been found. Our results on RBS 1774 are discussed and compared with the known properties of other thermally emitting neutron stars and of the radio pulsar population.Comment: 8 pages, 9 figures, accepted for publication on MNRA

Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197

Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission (XMM-Newton) are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant 300 days, but not a power law as predicted in some models of sudden deep crustal heating events. All spectra are well fitted by a blackbody plus a steep power law, a problematic model that is commonly fitted to anomalous X-ray pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.

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

Spectral line shape of resonant four-wave mixing induced by broad-bandwidth lasers

We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate Γ, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of √2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4Γ, whereas in the limit of dominant homogeneous broadening, the predicted width is 2Γ. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, "modeless", laser resonant with the Q1 (6) line of the A2 Σ - X2 Π(0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory

X-ray and radio observations of the magnetar Swift J1834.9-0846 and its dust-scattering halo

We present a long-term study of the 2011 outburst of the magnetar Swift J1834.9-0846 carried out using new Chandra observations, as well as all the available Swift, RXTE, and XMM-Newton data. The last observation was performed on 2011 November 12, about 100 days after the onset of the bursting activity that had led to the discovery of the source on 2011 August 07. This long time span enabled us to refine the rotational ephemeris and observe a downturn in the decay of the X-ray flux. Assuming a broken power law for the long-term light curve, the break was at ~46 d after the outburst onset, when the decay index changed from alpha ~ 0.4 to ~4.5. The flux decreased by a factor ~2 in the first ~50 d and then by a factor ~40 until November 2011 (overall, by a factor ~70 in ~100 d). At the same time, the spectrum, which was well described by an absorbed blackbody all along the outburst, softened, the temperature dropping from ~1 to ~0.6 keV. Diffuse X-ray emission extending up to 20" from the source was clearly detected in all Chandra observations. Its spatial and spectral properties, as well as its time evolution, are consistent with a dust-scattering halo due to a single cloud located at a distance of $\approx$200 pc from Swift J1834.9-0846, which should be in turn located at a distance of ~5 kpc. Considering the time delay of the scattered photons, the same dust cloud might also be responsible for the more extended emission detected in XMM-Newton data taken in September 2011. We searched for the radio signature of Swift J1834.9-0846 at radio frequencies using the Green Bank Radio Telescope and in archival data collected at Parkes from 1998 to 2003. No evidence for radio emission was found, down to a flux density of 0.05 mJy (at 2 GHz) during the outburst and ~0.2-0.3 mJy (at 1.4 GHz) in the older data.Comment: 11 pages, 9 figures and 4 tables, accepted for publication in MNRA

A new low magnetic field magnetar: the 2011 outburst of Swift J1822.3-1606

We report on the long term X-ray monitoring with Swift, RXTE, Suzaku, Chandra and XMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3-1606 (SGR 1822-1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time-span from July 2011, until end of April 2012). We also report on archival ROSAT observations which witnessed the source during its likely quiescent state, and on upper limits on Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with the Green Bank Telescope (GBT) and the Gran Telescopio Canarias (GTC), respectively. Our X-ray timing analysis finds the source rotating with a period of P=8.43772016(2) s and a period derivative \dot{P}=8.3(2)x10^{-14} s s^{-1} , which entails an inferred dipolar surface magnetic field of B~2.7x10^{13} G at the equator. This measurement makes Swift J1822.3-1606 the second lowest magnetic field magnetar (after SGR 0418+5729; Rea et al. 2010). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3-1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of B_{p}~1.5x10^{14} G and B_{tor}~7x10^{14} G, respectively, and if its current age is ~550 kyr.Comment: 14 pages, 9 figures; new observations added; ApJ in pres

VLT/NACO observations of the High-Magnetic field radio pulsar PSR J1119-6127

Recent radio observations have unveiled the existence of a number of radio pulsars with spin-down derived magnetic fields in the magnetar range. However, their observational properties appears to be more similar to classical radio pulsars than to magnetars. To shed light on this puzzle we first have to determine whether the spin-down derived magnetic field values for these radio pulsars are indeed representative of the actual neutron star magnetic field or if they are polluted, e.g. by the effects of a torque from a fallback disk. To investigate this possibility, we have performed deep IR observations of one of these high magnetic field radio pulsars (PSR J1119-6127) with the ESO VLT to search for IR emission which can be associated with a disk. No IR emission is detected from the pulsar position down to J=24, H=23, Ks=22. By comparing our flux upper limits with the predictions of fallback disk models, we have found that we can only exclude the presence of a disk with accretion rate dot M >3x10^16 g/s. This lower limit cannot rule out the presence of a substantial disk torque on the pulsar, which would then lead to overestimate the value of the magnetic field inferred from P and dot P.Comment: 8 pages, 4 figures, A&A, in pres