Sustained hydrogen photoproduction by phosphorus-deprived Chlamydomonas reinhardtii cultures

This study demonstrates that, besides sulfur deprivation, sustained H-2 photoproduction in Chlamydomonas reinhardtii cultures can be generated by incubating algae under phosphorus-deprived (-P) conditions. However, phosphorus deficiency in algal cells could not be obtained by resuspension of algae in -P medium, evidently due to a significant reserve of phosphorus in cells. In this study, phosphorus deficiency was accomplished by inoculating the washed algae into the -P medium at low initial cell densities (below 2 mg Chl l(-1)). After the initial growth period, where cells utilize intracellular phosphorus, algae established anaerobic environment followed by the period of H-2 photoproduction. The maximum H-2 output (similar to 70 ml l(-1)) was obtained in cultures with the initial Chl content similar to 1 mg l(-1). Cultures with Chl above 2 mg l(-1) did not produce H-2 gas. The physiological response of algal cultures to phosphorus deprivation demonstrated significant similarities with the response of algae to sulfur depletion. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Hydrogen production by sulfur-deprived Chlamydomonas reinhardtii under photoautotrophic conditions

Thus far, all experiments leading to H-2 production by sulfur-deprived cultures of microalga have been done with photoheterotrophic cultures in the presence of acetate, which increases the cost of the H-2 produced. This study demonstrates that sustained H-2 photoproduction by a sulfur-deprived green alga, Chlamydomonas reinhardtii, is possible under strictly photoautotrophic conditions in the absence of acetate or any other organic substrate in the medium. To accomplish this, we used cultures pre-grown with 2% CO2 under low light conditions (25 mu E m(-2) s(-1)) and also supplemented with CO2 during S-deprivation, along with a special light regime. Maximum H-2 production (56.4 +/- 16.7 ml l(-1) culture, equal to 56.4 x 10(-3) m(3) m(-3) culture) was observed with photoautotrophic cultures: (a) supplied with carbon dioxide for the first 24 h of sulfur deprivation, (b) exposed during the O-2-producing stage to high light (110 - 120 mu E m(-2) s(-1)), and (c) then exposed to low light (20 - 25 mu E m(-2) s(-1)) during the O-2-consumption and H-2-production stages. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved

Prolongation of H2 photoproduction by immobilized, sulfur-limited Chlamydomonas reinhardtii cultures

Two approaches to prolong the duration of hydrogen production by immobilized, sulfur-limited Chlamydomonas reinhardtii cells are examined. The results demonstrate that continuous H(2) photoproduction can occur for at least 90 days under constant flow of TAP medium containing micromolar sulfate concentrations. Furthermore, it is also possible to prolong the duration of H(2) production by cycling immobilized cells between minus and plus sulfate conditions. (C) 2008 Elsevier B.V. All rights reserved

Maximizing the hydrogen photoproduction yields in Chlamydomonas reinhardtii cultures: The effect of the H-2 partial pressure

Photoproduction of H-2 gas has been examined in sulfur/phosphorus-deprived Chalmydomonas reinhardtii cultures, placed in photobioreactors (PhBRs) with different gas phase to liquid phase ratios (V-g.p/V-l.p). The results demonstrate that an increase in the ratio stimulates H-2 photoproduction activity in both algal suspension cultures and in algae entrapped in thin alginate films. In suspension cultures, a 4x increase (from similar to 0.5 to similar to 2) in V-g.p/V-l.p in a 2x increase (from 10.8 to 23.1 mmol l(-1) or 264-565 ml l(-1)) in the total yield of H-2 gas. Remarkably, 565 ml of H-2 gas per liter of the suspension culture is the highest yield ever reported for a wild-type strain in a time period of less than 190 h. In immobilized algae, where diffusion of H-2 from the medium to the PhBR gas phase is not affected by mixing, the maximum rate and yield of H-2 photoproduction occur in PhBRs with V-g.p/V-l.p above 7 or in a PhBR with smaller headspace, if the H-2 is effectively removed from the medium by continuous flushing of the headspace with argon. These experiments in combination with studies of the direct inhibitory effect of high H-2 concentrations in the PhBR headspace on H-2 photoproduction activity in algal cultures clearly show that H-2 photoproduction in algae depends significantly on the partial pressure of H-2 (not O-2 as previously thought) in the PhBR gas phase. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

SMC X-3: the closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field

Aims. The magnetic field of accreting neutron stars determines their overall behavior including the maximum possible luminosity. Some models require an above-average magnetic field strength (greater than or similar to 10(13) G) in order to explain super-Eddington mass accretion rate in the recently discovered class of pulsating ultraluminous X-ray sources (ULX). The peak luminosity of SMCX-3 during its major outburst in 2016-2017 reached similar to 2.5x10(39) erg s(-1) comparable to that in ULXs thus making this source the nearest ULX-pulsar. Determination of the magnetic field of SMCX-3 is the main goal of this paper.Methods. SMCX-3 belongs to the class of transient X-ray pulsars with Be optical companions, and exhibited a giant outburst in July 2016-March 2017. The source has been observed over the entire outburst with the Swift/XRT and Fermi/GBM telescopes, as well as the NuSTAR observatory. Collected data allowed us to estimate the magnetic field strength of the neutron star in SMCX-3 using several independent methods.Results. Spin evolution of the source during and between the outbursts, and the luminosity of the transition to the so-called propeller regime in the range of (0.3-7) x 10(35) erg s(-1) imply a relatively weak dipole field of (1-5) x 10(12) G. On the other hand, there is also evidence for a much stronger field in the immediate vicinity of the neutron star surface. In particular, transition from super-to sub-critical accretion regime associated with the cease of the accretion column and very high peak luminosity favor a field that is an order of magnitude stronger. This discrepancy makes SMCX-3 a good candidate for possessing significant non-dipolar components of the field, and an intermediate source between classical X-ray pulsars and accreting magnetars which may constitute an appreciable fraction of ULX population

Cyclotron emission, absorption, and the two faces of X-ray pulsar A 0535+262

Deep NuSTAR observation of X-ray pulsar A 0535+262, performed at a very low luminosity of similar to 7 x 10(34) erg s(-1), revealed the presence of two spectral components. We argue that the high-energy component is associated with cyclotron emission from recombination of electrons collisionally excited to the upper Landau levels. The cyclotron line energy of E-cyc = 47.7 +/- 0.8 keV was measured at the luminosity of almost an order of magnitude lower than what was achieved before. The data firmly exclude a positive correlation of the cyclotron energy with the mass accretion rate in this source

Losing a minute every two years: SRG X-ray view of the rapidly accelerating X-ray pulsar SXP 1323

The source SXP 1323 is a peculiar high-mass X-ray binary located in the Small Magellanic Cloud. It is renowned for its rapid spin-up. We investigate for the first time broadband X-ray properties of SXP 1323 as observed by the Mikhail Pavlinsky ART-XC and eROSITA telescopes on board the Spectrum-Roentgen-Gamma observatory. Using ART-XC and eROSITA data, we produced a first broadband 1-20 keV X-ray spectrum and estimated the pulsed fraction box 8 keV. With the addition of archival XMM-Newton observations, we traced the evolution of the spin period of SXP 1323 over the last five years and found that after 2016, the source switched to a linear spin-up with a rate of -29.9 s yr-1. The broadband X-ray spectrum is typical for accreting X-ray pulsars. It has a steep power-law index (Γ = -0.15) and an exponential cutoff energy of 5.1 keV. No significant difference between spectra obtained in states with and without pulsations were found.</p

GROJ1750-27: A neutron star far behind the Galactic Center switching into the propeller regime

We report on analysis of properties of the X-ray binary pulsar GROJ1750-27 based on X-ray (Chandra, Swift, and Fermi/GBM), and near-infrared (VVV and UKIDSS surveys) observations. An accurate position of the source is determined for the first time and used to identify its infrared counterpart. Based on the VVV data we investigate the spectral energy distribution (SED) of the companion, taking into account a non-standard absorption law in the source direction. A comparison of this SED with those of known Be/X-ray binaries and early-type stars has allowed us to estimate a lower distance limit to the source at >12 kpc. An analysis of the observed spin-up torque during a giant outburst in 2015 provides an independent distance estimate of 14-22 kpc, and also allows to estimate the magnetic field on the surface of the neutron star at B similar or equal to (3.5-4.5) x 10(12) G. The latter value is in agreement with the possible transition to the propeller regime, a strong hint for which was revealed by Swift/XRT and Chandra. We conclude, that GROJ1750-27 is located far behind the Galactic Centre, which makes it one of the furthest Galactic X-ray binaries known

The X-ray properties of Be/X-ray pulsars in quiescence

Observations of accreting neutron stars (NSs) with strong magnetic fields can be used not only for studying the accretion flow interaction with the NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating NS (magnetosphere) with the infalling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: (i) relatively bright objects with a luminosity around similar to 10(34) erg s(-1) and (hard) power-law spectra, and (ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group (i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the infalling matter with the NS magnetic field and those describing heating and cooling in accreting NSs

NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

We report on the first focused hard X-ray view of the absorbed supergiant system IGR J18027-2016 performed with the Nuclear Spectroscopic Telescope Array observatory. The pulsations are clearly detected with a period of P-spin = 139.866(1)s and a pulse fraction of about 50-60 per cent at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time-scale of more than dozen years with an average spin-down rate of P similar or equal to 6 x 10(-10) s s(-1). This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a possible cyclotron absorption feature at energy similar to 23 keV. This energy corresponds to the magnetic field B similar or equal to 3 x 10(12) G at the surface of the neutron star, which is typical for X-ray pulsars