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Results for “transient absorbance spectroscopy”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1977

The photochemical cycle of bacteriorhodopsin

The reaction cycle of bacteriorhodopsin in the purple membrane isolated from Halobacterium halobium has been studied by optical absorption spectroscopy using low-temperature and flash kinetic techniques. After absorption of light, bacteriorhodopsin passes through at least five distinct intermediates. The temperature and pH dependence of the absorbance changes suggests that branch points and/or reversible steps exist in this cycle. Flash spectroscopy in the presence of a pH-indicating dye shows that the transient release of a proton accompanies the photoreaction cycle. The proton release occurs from the exterior and the uptake is on the cytoplasmic side of the membrane, as required by the function of bacteriorhodopsin as a light-driven proton pump. Proton translocating steps connecting release and uptake are indicated by deuterium isotope effects on the kinetics of the cycle. The rapid decay of a light-induced linear dichroism shows that a chromophore orientation change occurs during the reaction cycle.

Lozier, R. H.↗
NASA NTRS2021

Photospheric Radius Expansion and a Double-peaked Type-I X-Ray Burst from GRS 1741.9-2853

We present an analysis of two type-I X-ray bursts observed by NuSTAR originating from the very faint transient neutron star (NS) low-mass X-ray binary GRS 1741.9–2853 during a period of outburst in 2020 May. We show that the persistent emission can be modeled as an absorbed, Comptonized blackbody in addition to Fe Kα emission, which can be attributed to relativistic disk reflection. We measure a persistent bolometric, unabsorbed luminosity of L(bol) = 7.03(+0.04, -0.05) x 10^(36) erg/s, assuming a distance of 7 kpc, corresponding to an Eddington ratio of 4.5%. This persistent luminosity combined with light-curve analysis leads us to infer that the bursts were the result of pure He burning rather than mixed H/He burning. Time-resolved spectroscopy reveals that the bolometric flux of the first burst exhibits a double-peaked structure, placing the source within a small population of accreting NSs that exhibit multiple-peaked type-I X-ray bursts. We find that the second, brighter burst shows evidence for photospheric radius expansion (PRE) and that at its peak, this PRE event had an unabsorbed bolometric flux of F(peak)= 2.94 (+0.28, -0.26) 10^(-8) erg/sq.cm s. This yields a new distance estimate of d = 9.0 ± 0.5 kpc, assuming that this corresponds to the Eddington limit for pure He burning on the surface of a canonical NS. Additionally, we performed a detailed timing analysis that failed to find evidence for quasi-periodic oscillations or burst oscillations, and we place an upper limit of 16% on the rms variability around 589 Hz, the frequency at which oscillations have previously been reported.

Sean N. Pike↗
NASA NTRS2001

Serendipitous Detections of XTE J1906+09 with the Rossi X-ray Timing Explorer

In 1996 during Rossi X-ray Timing Explorer (RXTE) observations of SCR 1900+14, the 89-second X-ray pulsar XTE J1906+09 was discovered. As a result of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A frequency grid search of these observations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. The non-detection of XTE J1906+09 in most observations indicates that it is a transient X-ray pulsar and therefore likely has a Be star companion, Pulse timing analysis of the second outburst revealed a sinusoidal signature in the pulse frequencies that is likely produced by periastron passage in a long period, eccentric, orbit. During the second outburst, the pulse shape changed with increasing intensity, Low intensity pulse profiles from the second outburst are consistent with profiles from the fainter first outburst. Energy dependent pulse shape variations were seen in both outbursts. Pulse phase spectroscopy was used to quantify these variations. The phase averaged spectrum, using the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the galactic ridge, was well fitted by an absorbed power law with a high energy cutoff with column density N(sub H) = (6 +/- 1) x 10(exp 22) cm(sup -2), photon index = 1.01 +/- 0.08, cutoff energy E(sub cut) = 11 +/- 1 keV, and folding energy E(sub fold) = 19 +/- 4 keV.

Wilson, Colleen A.↗
NASA NTRS2001

First Detection of a Foreground Damped Ly-Alpha Absorber Along a GRB Line of Sight?

We present a VLT spectrum of the optical afterglow of GRB 991216, taken 1.5 days after the burst, and HST (Hubble Space Telescope) imaging of the host galaxy, obtained four months later. The spectrum contains three metal absorption-line systems with redshifts z = 1.024, z = 0.803, and z = 0.771, where the highest redshift most likely reflects the distance to the host galaxy. For the z = 1.024 and z = 0.803 systems we tentatively detect MgI which suggests a dense environment at these redshifts. This and the strength of the z = 0.803 Fe lines indicate that this system very likely is a damped Ly-alpha absorber (DLA), which would be the first foreground DLA to be detected along a GRB afterglow sight line. The HST images are consistent with these findings: they show two blobs of light, one underneath the projected OT position, the presumed host galaxy, and the other 0.6" away, which is probably responsible for the absorption lines at z = 0.803. The lowest redshift system can be explained by either one of the two galaxies that are located roughly 2" away from the transient. Including these newly found systems, the total number of DLAS and Lyman limit systems along GRB afterglow sight lines is consistent with the number expected from QSO (quasi-stellar object) absorption line studies. We expect early spectroscopy of GRB afterglows to significantly increase the number of detected foreground absorption systems, and we discuss some advantages over QSO lines of sight.

Vreeswijk, P. M.↗
NASA NTRS2002

Serendipitous Detections of XTE J1906+09 with the Rossi X-Ray Timing Explorer

The 89 s X-ray pulsar XTE J1906+09 was discovered during Rossi X-Ray Timing Explorer (RXTE) observations of SGR 1900+14 in 1996. Because of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A search for pulsations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. Pulsed flux upper limits for the rest of the observations show that XTE J1906+09 is a transient X-ray pulsar and likely has a Be star companion. The RXTE all-sky monitor did not reveal XTE J1906+09. Pulse-timing analysis of the second outburst discovered a sinusoidal signature in the pulse frequencies that is likely produced by an orbital periastron passage. Fits to pulse phases using an orbital model and quadratic phase model have chi(exp 2) minima at orbital periods of 26-30 days for fixed mass functions of 5, 10, 15, and 20 solar masses. The pulse shape showed energy- and intensity-dependent variations. Pulse-phase spectroscopy quantified the energy-dependent variations. The phase-averaged spectrum used the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the Galactic ridge and was well fitted by an absorbed power law with a high-energy cutoff with column density N(sub H) = 6 +/- 1 x 10(exp 22)/sq cm, a photon index of 1.01 +/- 0.08, cutoff energy E(sub cut) = 11 +/- 1 keV, and e-folding energy E(sub fold) = 19 +/- 4 keV. Estimated 2-10 keV peak fluxes, corrected for contributions from the Galactic ridge and SGR 1900+14, are 6 x l0(exp -12) and 1.1 x 10(exp -10) ergs/sq cm/s for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may be part of an unusual class of Be/X-ray binaries that do not lie on the general spin period versus orbital period correlation with the majority of Be/X-ray binaries.

Wilson, Colleen A.↗