A peculiar thermonuclear X-ray burst from the transiently accreting neutron star SAX J1810.8-2609

We report on a thermonuclear (type-I) X-ray burst that was detected from the neutron star low-mass X-ray binary SAX J1810.8-2609 in 2007 with Swift. This event was longer (~20 min) and more energetic (a radiated energy of Eb~6.5E39 erg) than other X-ray bursts observed from this source. A possible explanation for the peculiar properties is that the X-ray burst occurred during the early stage of the outburst when the neutron star was relatively cold, which allows for the accumulation of a thicker layer of fuel. We also report on a new accretion outburst of SAX J1810.8-2609 that was observed with MAXI and Swift in 2012. The outburst had a duration of ~17 days and reached a 2-10 keV peak luminosity of Lx~3E37(D/5.7kpc)^2 erg/s. This is a factor >10 more luminous than the two previous outbursts observed from the source, and classifies it as a bright rather than a faint X-ray transient.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and Opportunities after 80 years", J. van Leeuwen (ed.); 4 pages, 1 figure, 2 tables, contributed tal

The X-ray spectral properties of very-faint persistent neutron star X-ray binaries

AX J1754.2-2754, 1RXS J171824.2-402934 and 1RXH J173523.7-354013 are three persistent neutron star low-mass X-ray binaries that display a 2--10 keV accretion luminosity Lx of only (1-10)x1E34 erg s-1 (i.e., only ~0.005-0.05 % of the Eddington limit). The phenomenology of accreting neutron stars which accrete at such low accretion rates is not yet well known and the reason why they have such low accretion rates is also not clear. Therefore, we have obtained XMM-Newton data of these three sources and here we report our analysis of the high-quality X-ray spectra we have obtained for them. We find that AX J1754.2-2754 has Lx~1E35 erg s-1, while the other two have X-ray luminosities about an order of magnitude lower. However, all sources have a similar, relatively soft, spectrum with a photon index of 2.3-2.5, when the spectrum is fitted with an absorbed power-law model. This model fits the data of AX J1754.2-2754 adequately, but it cannot fit the data obtained for 1RXS J171824.2-402934 and 1RXH J173523.7-354013. For those sources a clear soft thermal component is needed to fit their spectra. This soft component contributes 40% - 50% to the 0.5-10 keV flux of the sources. When including this additional spectral component, the power-law photon indices are significantly lower. It can be excluded that a similar component with similar contributions to the 2-10 keV X-ray flux is present for AX J1754.2-2754, indicating that the soft spectrum of this source is mostly due to the fact that the power-law component itself is not hard. We note that we cannot excluded that weaker soft component is present in the spectrum of this source which only contributes up to ~25% to the 0.5-10 keV X-ray flux. We discuss our results in the context of what is known of accreting neutron stars at very low accretion rate.Comment: 9 pages, 2 tables, 1 figure. Aceppted for publication in MNRA

XMM-Newton and Swift spectroscopy of the newly discovered very-faint X-ray transient IGR J17494-3030

A growing group of low-mass X-ray binaries are found to be accreting at very-faint X-ray luminosities of <1E36 erg/s (2-10 keV). Once such system is the new X-ray transient IGR J17494-3030. We present Swift and XMM-Newton observations obtained during its 2012 discovery outburst. The Swift observations trace the peak of the outburst, which reached a luminosity of ~7 E35 (D/8 kpc)^2 erg/s (2-10 keV). The XMM-Newton data were obtained when the outburst had decayed to an intensity of ~ 8 E34 (D/8 kpc)^2 erg/s. The spectrum can be described by a power-law with an index of ~1.7 and requires an additional soft component with a black-body temperature of ~0.37 keV (contributing ~20% to the total unabsorbed flux in the 0.5-10 keV band). Given the similarities with high-quality spectra of very-faint neutron star low-mass X-ray binaries, we suggest that the compact primary in IGR J17494-3030 is a neutron star. Interestingly, the source intensity decreased rapidly during the ~12 hr XMM-Newton observation, which was accompanied by a decrease in inferred temperature. We interpret the soft spectral component as arising from the neutron star surface due to low-level accretion, and propose that the observed decline in intensity was the result of a decrease in the mass-accretion rate onto the neutron star.Comment: 3 figures, 2 tables, accepted in MNRAS letter, in pres

The nature of very faint X-ray binaries; hints from light curves

Very faint X-ray binaries (VFXBs), defined as having peak luminosities Lx of 10^34-10^36 erg/s, have been uncovered in significant numbers, but remain poorly understood. We analyse three published outburst light curves of two transient VFXBs using the exponential and linear decay formalism of King and Ritter (1998). The decay timescales and brink luminosities suggest orbital periods of order 1 hour. We review various estimates of VFXB properties, and compare these with suggested explanations of the nature of VFXBs. We suggest that: 1) VFXB outbursts showing linear decays might be explained as partial drainings of the disc of "normal" X-ray transients, and many VFXB outbursts may belong to this category; 2) VFXB outbursts showing exponential decays are best explained by old, short-period systems involving mass transfer from a low-mass white dwarf or brown dwarf; 3) persistent (or quasi-persistent) VFXBs, which maintain an Lx of 10^34-10^35 erg/s for years, may be explained by magnetospheric choking of the accretion flow in a propeller effect, permitting a small portion of the flow to accrete onto the neutron star's surface. We thus predict that (quasi-)persistent VFXBs may also be transitional millisecond pulsars, turning on as millisecond radio pulsars when their Lx drops below 10^32 erg/s.Comment: 12 pages, 4 figures. MNRAS, in pres

The X-ray flaring properties of Sgr A* during six years of monitoring with Swift

Starting in 2006, Swift has been targeting a region of ~21'X21' around Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short, quasi-daily observations offer an unique view of the long-term X-ray behavior of the supermassive black hole. We report on the data obtained between 2006 February and 2011 October, which encompasses 715 observations with a total accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the number of such bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may have been softer than the other five, which would indicate that flares of similar intensity can have different spectral properties. The Swift campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray flares to be ~0.1-0.2 per day, which is in line with previous estimates. This analysis of the occurrence rate and properties of the X-ray flares seen with Swift offers an important calibration point to asses whether the flaring behavior of Sgr A* changes as a result of its interaction with the gas cloud that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap

The Swift X-ray monitoring campaign of the center of the Milky Way

In 2006 February, shortly after its launch, Swift began monitoring the center of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures performed every 1-4 days. Between 2006 and 2014, over 1200 observations have been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth of information about the long-term X-ray behavior of the supermassive black hole Sgr A*, and numerous transient X-ray binaries that are located within the 25'x25' region covered by the campaign. In this review we highlight the discoveries made during these first nine years, which includes 1) the detection of seven bright X-ray flares from Sgr A*, 2) the discovery of the magnetar SGR J1745-29, 3) the first systematic analysis of the outburst light curves and energetics of the peculiar class of very-faint X-ray binaries, 4) the discovery of three new transient X-ray sources, 5) exposing low-level accretion in otherwise bright X-ray binaries, and 6) the identification of a candidate X-ray binary/millisecond radio pulsar transitional object. We also reflect on future science to be done by continuing this Swift's legacy campaign of the Galactic center, which includes high-cadence monitoring of how the interaction between the gaseous object `G2' and Sgr A* plays out in the future.Comment: 13 pages, 6 figures, 4 tables. Invited review to appear in Elsevier's Journal of High Energy Astrophysics dedicated issue "Swift: 10 years of discovery

The Galactic center X-ray transients AX J1745.6-2901 and GRS 1741-2853

AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass X-ray binaries that are located within ~10' from the Galactic center. Multi-year monitoring observations with the Swift/XRT has exposed several accretion outbursts from these objects. We report on their updated X-ray light curves and renewed activity that occurred in 2010-2013.Comment: 2 pages, 1 figure, 1 table. To appear in conference proceedings of IAU symposium 303 "The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus

Levels of trace elements in blood in healthy aging subjects

The effects of age and sex on the serum levels of trace elements were determined after an overnight fast in SO ambulatory, disease-free adults who had undergone rigorous health screening. Significant age and sex differences were found for Mn. Blood levels of Cu and Zn showed both age and sex differences. No age or sex differences were found for Pb, Cd, Cr, Ni, Se, and Al