Discovery of X-ray pulsations in the Be/X-ray binary LS992/ RX J0812.4-3114

We report on the discovery of X-ray pulsations from the Be/X-ray system LS 992/RX J0812.4-3114 during an RXTE observation. From a timing analysis of the source we obtained a barycentric pulse period of 31.8851 \pm 0.0004 s. The pulse profile is highly structured and departs from a pure sinusoidal shape. It shows a sharp dip that may indicate absorption by the accretion flow. The energy spectrum from 3-30 keV can be fitted by a power-law model with an exponential cut-off in accordance with other X-ray pulsars. The X-ray luminosity is estimated to be $\sim 1.1 \times 10^{36} erg/s$ in the energy range 3-30 keV, assuming a distance of $\sim 9 kpc$.Comment: 5 pages, 6 fgures, 2 tables, to appear in MNRA

Discovery of two new persistent Be/X-ray pulsar systems

We present RXTE observations of two recently identified massive X-ray binaries. RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are confirmed as accreting Be/X-ray systems following the discovery of X-ray pulsations, with barycentric pulse periods of 202.5$\pm$0.5 s and 860$\pm$2 s respectively. The X-ray spectral analysis shows that the energy spectra of the pulsars can be represented by a power-law, modified at low energy by an absorption component and at high energy by a cut-off. Very weak Fe lines may be present. Both sources appear to display a low cut-off energy when compared to typical X-ray pulsars, low X-ray variability (factor of < 10), and no dependence of the X-ray spectrum with energy. Given the similarity of these X-ray properties with those of the other persistent BeXRB pulsars, 4U0352+309/X Per and RX J0146.9+6121/LS I +61 235, we suggest that RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 are also members of this subclass.Comment: 8 pages 7 figures, accepted for publication in MNRA

Aqueduct Country and River Basin Rankings: A Weighted Aggregation of Spatially Distinct Hydrological Indicators

More and more countries around the world face high levels of water stress, but measuring and communicating that stress consistently is challenging. This paper ranks countries and river basins worldwide based on their exposure water-related risks. Specifically, it provides national and basin-level scores derived from more localized water-risk scores from the Aqueduct Water Risk Atlas. Rankings are available for 181 countries, the world's 100-largest river basins by area, and the planet's 100-most populous river basins for five different measures of water supply and demand

RXTE-PCA observations of 1A 1118--61: timing and spectral studies during an outburst

We report detailed timing and spectral analysis of RXTE-PCA data obtained from observations during the outburst of a transient X-ray pulsar 1A 1118--61 in January 2009. The pulse profile showed significant evolution during the outburst and also significant energy dependence - a double peaked profile upto 10 keV and a single peak at higher energy. We have also detected quasi-periodic oscillations (QPO) at 0.07--0.09 Hz. The rms value of the QPO is 5.2% and it shows a significant energy dependence with highest rms of 7% at 9 keV. The QPO frequency changed from 0.09 Hz to 0.07 Hz within 10 days. The magnetic field strength calculated using the QPO frequency and the X-ray luminosity is in agreement with the magnetic field strength measured from the energy of the cyclotron absorption feature detected in this source. The 3-30 keV energy spectrum over the 2009 outburst of 1A 1118--61 can be well fitted with a partial covering power-law model with a high energy cutoff and an iron fluorescence line emission. The pulse phase resolved spectral analysis shows that the partial covering and high energy cutoff model parameters have significant changes with the pulse phase.Comment: Accepted for publication in MNRA

Discovery of a QPO in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability

Our goal is to investigate the X-ray timing and spectral variability of the high-mass X-ray binary 1A 1118-615 during a type-II outburst. We performed a detailed color, spectral and timing analysis of a giant outburst from 1A 1118-615, using RXTE data. Results. We report the discovery of a variable quasi-periodic oscillation (QPO) in the power spectral density of 1A 1118-615, with a centroid frequency of ~0.08 Hz. The centroid frequency of the QPO correlates with the X-ray flux, as expected according to the most accredited models for QPO production. For energies above ~4 keV, the QPO rms variability decreases as the energy increases. Pulse profiles display energy dependence, with a two-peak profile at lower energies, and a single peak at higher energies. From spectral analysis, we confirm the presence of a cyclotron absorption feature at ~60 keV, the highest value measured for an X-ray pulsar. We find that the spectral parameters (photon index, cutoff energy, iron fluorescence line strength) display a marked dependence with flux. We detect two different levels of neutral hydrogen column density, possibly due to the Be companion activity. We report for the first time a correlation between the timing and spectral parameters in an X-ray pulsar. All the correlations found between spectral/timing parameters and X-ray flux are present up to a flux of ~6x10^-9 erg cm^-2 s^-1, when a saturation level is reached. We propose that the saturation observed corresponds to the minimum extent of the neutron star magnetosphere. We estimate the magnetic field of the neutron star from two independent ways, using results from spectral (cyclotron line energy) and timing (QPO frequency) analysis, obtaining consistent values, of ~7-8x10^12 G. Results from the comprehensive spectral and timing analysis are discussed in comparison with other X-ray pulsars.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy and Astrophysic

Identification of the optical counterparts of high-mass X-ray binaries through optical photometry and spectroscopy

We present the results of our search for optical counterparts to high-mass X-ray transient sources discovered by various X-ray missions. We obtained CCD images of the X-ray fields through BVR and Halpha filters in order to identify early-type stars in the R-Halpha versus B-V colour-colour diagram. We also obtained medium-resolution spectroscopy of the candidates in order to confirm the presence of H\alpha emission and perform spectral classification. We report on the discovery of the optical counterparts to two X-ray sources: XTE J1858+034 and IGR J01363+6610, and the follow-up observations of another two, newly identified by our group: SAX J2103.5+4545 and GRO J2058+42. For another source, IGR J00370+6122, we present the first detailed optical spectral analysis. The optical photometry and spectroscopy reveal B-type companions in all five sources; GRO J2058+42, SAX J2103.5+4545 and IGR J01363+6610 are positively identified with Be/X-ray binaries, IGR J00370+6122 with a supergiant X-ray binary, while the nature of XTE J1858+034 is uncertain. We also study the relationship between the optical and X-ray emission during quiescent states.Comment: 10 pages, to be published in A&

XMM-Newton observation of the persistent Be/NS X-ray binary pulsar RX J1037.5-5647 in a low luminosity state

The spectra of several X-ray binary pulsars display a clear soft excess, which in most cases can be described with a blackbody model, above the main power-law component. While in the high-luminosity sources it is usually characterized by low temperature (kT 100 km), in the two persistent and low-luminosity pulsars 4U 0352+309 and RX J0146.9+6121 this component has a high temperature (kT > 1 keV) and a smaller radius (R < 0.5 km), consistent with the estimated size of the neutron-star polar cap. Here we report on the timing and spectral analysis of RX J1037.5-5647, another low-luminosity persistent Be binary pulsar, based on the first XMM-Newton observation of this source. We have found a best-fit period P = 853.4(+/-0.2) s, that implies an average pulsar spin-up dP/dt ~ -2E-8 s/s in the latest decade. The estimated source luminosity is Lx ~ 10^34 erg/s, a value comparable to that of the other persistent Be binary pulsars and about one order of magnitude lower than in most of the previous measurements. The source spectrum can be described with a power law plus blackbody model, with kTbb = 1.26(+0.16/-0.09) keV and Rbb = 128(+13/-21) m, suggesting a polar-cap origin of this component. These results strengthen the hypothesis that, in addition to low luminosities and long periods, this class of sources is characterized also by common spectral propertiesComment: 9 pages, 8 figures, 2 tables. Accepted for publication by Astronomy and Astrophysic

Near-infrared/optical identification of five low-luminosity X-ray pulsators

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 MNRA

Does GRS 1915+105 exhibit "canonical" black-hole states?

We have analysed RXTE data of the superluminal source GRS 1915+105 in order to investigate if, despite its extreme variability, it also exhibits the canonical source states that characterise other black-hole candidates. The phenomenology of GRS 1915+105 has been described in terms of three states (named A, B and C) based on their hardness ratios and position in the colour-colour diagram. We have investigated the connection between these states and the canonical behaviour and found that the shape of the power spectral continuum and the values of the best-fit model parameters to the noise components in all three states indicate that the source shows properties similar to the canonical very high state.Comment: 5 pages, 3 figures, accepted for publication in A&

Tidal Interaction in High Mass X-ray Binaries

Our aim is to investigate tidal interaction in High-Mass X-ray Binary stars in order to determine in which objects the rotation of the mass donors is synchronized or pseudosynchronized with the orbital motion of the compact companion. We calculate the pseudosynchronization period (P_ps) and compare it with the rotational period of the mass donors (P_rot). We find that (1) the Be/X-ray binaries are not synchronized, the mass donors rotate faster than the orbital period and the ratio P_ps/P_rot is 2-300; (2) the giant and supergiant systems are close to synchronization and for them the ratio P_ps/P_rot is 0.3-2.Comment: 6 pages, 1 figure, Accepted for publication in Astronomical Notes (AN