Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455

We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues in this system more than 3 years after discovery, at a mean luminosity of 4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the first 70 days. X-ray variability has increased each year, notably with a brief interval of nondetection in 2007, during which the luminosity dropped to below 1e-3 of the mean level. A deep search of data from the intervals of nondetection in 2005 revealed evidence for extremely weak pulsations at an amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen early in the outburst. X-ray burst activity continued through 2008, with bursts typically featuring strong radius expansion. Spectral analysis of the most intense burst detected by RXTE early in the outburst revealed unusual variations in the inferred photospheric radius, as well as significant deviations from a blackbody. We obtained much better fits instead with a comptonisation model.Comment: 8 pages, 3 figures, one table; to appear in the proceedings of the workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands et al. (AIP Conf. Proc.

Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate

We present a study of the X-ray flaring activity of Sgr A* during all the 150 XMM-Newton and Chandra observations pointed at the Milky Way center over the last 15 years. This includes the latest XMM-Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Br-Gamma emitting object called G2. The entire dataset analysed extends from September 1999 through November 2014. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A* has exhibited since their discovery in 2001. We observe that the total bright-or-very bright flare luminosity of Sgr A* increased between 2013-2014 by a factor of 2-3 (~3.5 sigma significance). We also observe an increase (~99.9% significance) from 0.27+-0.04 to 2.5+-1.0 day^-1 of the bright-or-very bright flaring rate of Sgr A*, starting in late summer 2014, which happens to be about six months after G2's peri-center passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.Comment: 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

The X-ray outburst of the Galactic Centre magnetar SGR J1745-2900 during the first 1.5 year

In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an outburst, at only 2.4 arcsec angular distance from the supermassive black hole at the Centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface dipolar magnetic field of ~ 2x10^{14} G, and it is the neutron star closest to a black hole ever observed. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity L_X ~ 5x10^{35} erg s^{-1}. Here we report on the long-term Chandra (25 observations) and XMM-Newton (8 observations) X-ray monitoring campaign of SGR 1745-2900, from the onset of the outburst in April 2013 until September 2014. This unprecedented dataset allows us to refine the timing properties of the source, as well as to study the outburst spectral evolution as a function of time and rotational phase. Our timing analysis confirms the increase in the spin period derivative by a factor of ~2 around June 2013, and reveals that a further increase occurred between 2013 Oct 30 and 2014 Feb 21. We find that the period derivative changed from 6.6x10^{-12} s s^{-1} to 3.3x10^{-11} s s^{-1} in 1.5 yr. On the other hand, this magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. In particular, starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ~200 days of its outburst, and additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.Comment: 16 pages, 12 figures; accepted for publication on MNRA

Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 1745−-2900

We report on multi-frequency, wideband radio observations of the Galactic Center magnetar (SGR 1745$-$2900) with the Green Bank Telescope for $\sim$100 days immediately following its initial X-ray outburst in April 2013. We made multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to examine the magnetar's flux evolution, radio spectrum, and interstellar medium parameters (such as the dispersion measure (DM), the scattering timescale and its index). During two epochs, we have simultaneous observations from the Chandra X-ray Observatory, which permitted the absolute alignment of the radio and X-ray profiles. As with the two other radio magnetars with published alignments, the radio profile lies within the broad peak of the X-ray profile, preceding the X-ray profile maximum by $\sim$0.2 rotations. We also find that the radio spectral index $\gamma$ is significantly negative between $\sim$2 and 9 GHz; during the final $\sim$30 days of our observations $\gamma \sim -1.4$, which is typical of canonical pulsars. The radio flux has not decreased during this outburst, whereas the long-term trends in the other radio magnetars show concomitant fading of the radio and X-ray fluxes. Finally, our wideband measurements of the DMs taken in adjacent frequency bands in tandem are stochastically inconsistent with one another. Based on recent theoretical predictions, we consider the possibility that the dispersion measure is frequency-dependent. Despite having several properties in common with the other radio magnetars, such as $L_{\textrm{X,qui}}/L_{\textrm{rot}} \lesssim 1$, an increase in the radio flux during the X-ray flux decay has not been observed thus far in other systems.Comment: 15 pages, 9 figures, 3 tables; accepted to Ap

Geochemical analysis of bulk marine sediment by Inductively Coupled Plasma–Atomic Emission Spectroscopy on board the JOIDES Resolution

Geochemical analyses on board the JOIDES Resolution have been enhanced with the addition of a Jobin-Yvon Ultrace inductively coupled plasma-atomic emission spectrometer (ICP-AES) as an upgrade from the previous X-ray fluorescence facility. During Leg 199, we sought to both challenge and utilize the capabilities of the ICP-AES in order to provide an extensive bulk-sediment geochemical database during the cruise. These near real-time analyses were then used to help characterize the recovered sedimentary sequences, calculate mass accumulation rates of the different sedimentary components, and assist with cruise and postcruise sampling requests. The general procedures, sample preparation techniques, and basic protocol for ICP-AES analyses on board ship are outlined by Murray et al. (2000) in Ocean Drilling Program Tech Note, 29. We expand on those concepts and offer suggestions for ICP-AES methodology, calibration by standard reference materials, data reduction procedures, and challenges that are specific to the analysis of bulk-sediment samples. During Leg 199, we employed an extensive bulk-sediment analytical program of ~600 samples of varying lithologies, thereby providing several opportunities for refinement of techniques. We also discuss some difficulties and challenges that were faced and suggest how to alleviate such occurrences for sedimentary chemical analyses during future legs

The Prelude to and Aftermath of the Giant Flare of 2004 December 27: Persistent and Pulsed X-ray Properties of SGR 1806-20 from 1993 to 2005

On 2004 December 27, a highly-energetic giant flare was recorded from the magnetar candidate SGR 1806-20. In the months preceding this flare, the persistent X-ray emission from this object began to undergo significant changes. Here, we report on the evolution of key spectral and temporal parameters prior to and following this giant flare. Using the Rossi X-ray Timing Explorer, we track the pulse frequency of SGR 1806-20 and find that the spin-down rate of this SGR varied erratically in the months before and after the flare. Contrary to the giant flare in SGR 1900+14, we find no evidence for a discrete jump in spin frequency at the time of the December 27th flare (|dnu/nu| < 5 X 10^-6). In the months surrounding the flare, we find a strong correlation between pulsed flux and torque consistent with the model for magnetar magnetosphere electrodynamics proposed by Thompson, Lyutikov & Kulkarni (2002). As with the flare in SGR 1900+14, the pulse morphology of SGR 1806-20 changes drastically following the flare. Using the Chandra X-ray Observatory and other publicly available imaging X-ray detector observations, we construct a spectral history of SGR 1806-20 from 1993 to 2005. The usual magnetar persistent emission spectral model of a power-law plus a blackbody provides an excellent fit to the data. We confirm the earlier finding by Mereghetti et al. (2005) of increasing spectral hardness of SGR 1806-20 between 1993 and 2004. Contrary to the direct correlation between torque and spectral hardness proposed by Mereghetti et al., we find evidence for a sudden torque change that triggered a gradual hardening of the energy spectrum on a timescale of years. Interestingly, the spectral hardness, spin-down rate, pulsed, and phase-averaged of SGR 1806-20 all peak months before the flare epoch.Comment: 37 pages, 8 figures, 8 tables. Accepted for publication in ApJ. To appear in the Oct 20 2006 editio

Sedimentation and subsidence history of the Lomonosov Ridge

During the first scientific ocean drilling expedition to the Arctic Ocean (Arctic Coring Expedition [ACEX]; Integrated Ocean Drilling Program Expedition 302), four sites were drilled and cored atop the central part of the Lomonosov Ridge in the Arctic Ocean at ~88°N, 140°E (see Fig. F18 in the "Sites M0001–M0004" chapter). The ridge was rifted from the Eurasian continental margin at ~57 Ma (Fig. F1) (Jokat et al., 1992, 1995). Since the rifting event and the concurrent tilting and erosion of this sliver of the outer continental margin, the Lomonosov Ridge subsided while hemipelagic and pelagic sediments were deposited above the angular rifting unconformity (see Fig. F7A in the "Sites M0001–M0004" chapter).The sections recovered from the four sites drilled during Expedition 302 can be correlated using their seismic signature, physical properties (porosity, magnetic susceptibility, resistivity, and P-wave velocity), chemostratigraphy (ammonia content of pore waters), lithostratigraphy, and biostratigraphy. The lithostratigraphy of the composite section combined with biostratigraphy provides an insight into the complex history of deposition, erosion, and preservation of the biogenic fraction. Eventually, the ridge subsided to its present water depth as it drifted from the Eurasian margin. In this chapter, we compare a simple model of subsidence history with the sedimentary record recovered from atop the ridge

Long-term spectral and timing properties of the soft gamma-ray repeater SGR 1833-0832 and detection of extended X-ray emission around the radio pulsar PSR B1830-08

SGR 1833-0832 was discovered on 2010 March 19 thanks to the Swift detection of a short hard X-ray burst and follow-up X-ray observations. Since then, it was repeatedly observed with Swift, Rossi X-ray Timing Explorer, and XMM-Newton. Using these data, which span about 225 days, we studied the long-term spectral and timing characteristics of SGR 1833-0832. We found evidence for diffuse emission surrounding SGR 1833-0832, which is most likely a halo produced by the scattering of the point source X-ray radiation by dust along the line of sight, and we show that the source X-ray spectrum is well described by an absorbed blackbody, with temperature kT=1.2 keV and absorbing column nH=(10.4+/-0.2)E22 cm^-2, while different or more complex models are disfavoured. The source persistent X-ray emission remained fairly constant at about 3.7E-12 erg/cm^2/s for the first 20 days after the onset of the bursting episode, then it faded by a factor 40 in the subsequent 140 days, following a power-law trend with index alpha=-0.5. We obtained a phase-coherent timing solution with the longest baseline (225 days) to date for this source which, besides period P=7.5654084(4) s and period derivative dP/dt=3.5(3)E-12 s/s, includes higher order period derivatives. We also report on our search of the counterpart to the SGR at radio frequencies using the Australia Telescope Compact Array and the Parkes radio telescope. No evidence for radio emission was found, down to flux densities of 0.9 mJy (at 1.5 GHz) and 0.09 mJy (at 1.4 GHz) for the continuum and pulsed emissions, respectively, consistently with other observations at different epochs.Comment: 12 pages, 7 colour figures and 3 tables, accepted for publication in MNRAS. Figure 6 in reduced quality and abstract abridged for astro-ph submissio