Browsing by Author "Islam, Nazma"
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Item An in-depth analysis of the variable cyclotron lines in GX 301−2(2024-03-25) Zalot, Nicolas; Sokolova-Lapa, Ekaterina; Stierhof, Jakob; Ballhausen, Ralf; Zainab, Aafia; Pottschmidt, Katja; Fürst, Felix; Thalhammer, Philipp; Islam, Nazma; Diez, Camille M.; Kretschmar, Peter; Berger, Katrin; Rothschild, Richard; Malacaria, Christian; Pradhan, Pragati; Wilms, JörnContext. The High-Mass X-ray Binary (HMXB) system GX 301−2 is a persistent source with a well-known variable cyclotron line centered at 35 keV. Recently, a second cyclotron line at 50 keV has been reported with a presumably different behavior than the 35 keV line. Aims. We investigate the presence of the newly discovered cyclotron line in the phase-averaged and phase-resolved spectra at higher luminosities than before. We further aim to determine the pulse-phase variability of both lines. Methods. We analyze a NuSTAR observation of GX 301−2 covering the pre-periastron flare, where the source luminosity reached its peak of ~4 × 10³⁷ erg s⁻¹ in the 5-50 keV range. We analyze the phase-averaged spectra in the NuSTAR energy range from 3.5−79 keV for both the complete observation and three time segments of it. We further analyze the phase-resolved spectra and the pulse-phase variability of continuum and cyclotron line parameters. Results. We confirm that the description of the phase-averaged spectrum requires a second absorption feature at 51.5⁺¹.¹₋₁.₀ keV besides the established line at 35 keV. The statistical significance of this feature in the phase-averaged spectrum is >99.999%. We further find that the 50 keV cyclotron line is present in three of eight phase bins. Conclusions. Based on the results of our analysis, we confirm that the detected absorption feature is very likely to be a cyclotron line. We discuss a variety of physical scenarios which could explain the proposed anharmonicity, but also outline circumstances under which the lines are harmonically related. We further present the cyclotron line history of GX 301−2 and evaluate concordance among each other. We also discuss an alternative spectral model including cyclotron line emission wings.Item Breakthroughs in Cool Star Physics with the Line Emission Mapper X-ray Probe(2023-10-26) Drake, Jeremy J.; Gomez, Julián Alvarado; Argiroffi, Costanza; Flaccomio, Ettore; Garraffo, Cecilia; Grosso, Nicolas; Islam, Nazma; Karovska, Margarita; Kashyap, Vinay L.; Monsch, Kristina; Ness, Jan-Uwe; Sciortino, Salvatore; Wargelin, BradfordWe outline some of the highlights of the scientific case for the advancement of stellar high energy physics using the Line Emission Mapper X-ray Probe ({\it LEM}). The key to advancements with LEM lie in its large effective area -- up to 100 times that of the {\it Chandra} MEG -- and 1~eV spectral resolution. The large effective area opens up for the first time the ability to study time-dependent phenomena on their natural timescales at high resolution, such as flares and coronal mass ejections, and also opens the sky to much fainter targets than available to {\it Chandra} or {\it XMM-Newton}.Item Brightening of MAXI J0903-531 Seen with Fermi GBM and Swift BAT, and a Candidate Orbital Period(The Astronomer's Telegram, 2021-06-21) Corbet, Robin; Jenke, P.; Ballhausen, R.; Becker, P. A.; Coley, J. B.; Fuerst, F.; Islam, Nazma; Jaisawal, G. K.; Kretschmar, P.; Malacaria, C.; Pottschmidt, Katja; Rothschild, R. E.; Thalhammer, P.; Wilms, J.; Wilson-Hodge, C. A.; Wolff, M. T.Item Investigating the superorbital modulations in 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 with Swift XRT, BAT and NuSTAR observations(IOP Publishing, 2023-05-04) Islam, Nazma; Corbet, Robin; Coley, Joel B.; Pottschmidt, Katja; Fuerst, FelixA puzzling variety of superorbital modulations has been discovered in several supergiant high mass X-ray binaries (sgHMXBs). To investigate the mechanisms driving these superorbital modulations, we have analyzed long-term Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) observations of three sgHMXBs: 4U 1909 + 07, IGR J16418–4532, and IGR J16479–4514, and constructed their dynamic power spectra and superorbital intensity profiles. These Swift BAT observations are complemented by pointed Swift X-ray Telescope (XRT) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations performed near the predicted maximum and minimum phase of a single superorbital cycle for each of these sources. The BAT dynamic power spectra show changes in the strength of the superorbital modulation on timescales of years, with either the peak at the fundamental frequency and/or the second harmonic present at different times for all three sources. The pointed Swift XRT and NuSTAR observations show no significant differences between the pulse profiles and spectral parameters at the superorbital maximum and minimum phase. This is likely due to the fact the superorbital modulation had weakened significantly during the times when the NuSTAR observations were carried out for all three sources. The results from the Swift XRT, BAT, and NuSTAR analysis indicate the possible presence of multiple corotating interaction regions (CIRs) in the stellar winds of the supergiant stars, although a structured stellar wind from the supergiant star due to tidal oscillations cannot be ruled out.Item The role of complex ionized absorber in the soft X-ray spectra of Intermediate Polars(2021-07-12) Islam, Nazma; Mukai, KojiIn magnetic Cataclysmic Variables (mCVs), X-ray radiation originates from the shock heated multi-temperature plasma in the post-shock region near the white dwarf surface. These X-rays are modified by a complex distribution of absorbers in the pre-shock region. The presence of photo-ionized lines and warm absorber features in the soft X-ray spectra of these mCVs suggests that these absorbers are ionized. We developed the ionized complex absorber model zxipab, which is represented by a power-law distribution of ionized absorbers in the pre-shock flow. Using the ionized absorber model zxipab along with a cooling flow model and a reflection component, we model the broadband Chandra/HETG and NuSTAR spectra of two IPs: NY Lup and V1223 Sgr. We find that this model describes well many of the H and He like emission lines from medium Z elements, which arises from the collisionally excited plasma. However the model fails to account for some of the He like triplets from medium Z elements, which points towards its photo-ionization origin. We do not find a compelling evidence for a blackbody component to model the soft excess seen in the residuals of the Chandra/HETG spectra, which could be due to the uncertainties in estimation of the interstellar absorption of these sources using Chandra/HETG data and/or excess fluxes seen in some photo-ionized emission lines which are not accounted by the cooling flow model. We describe the implications of this model with respect to the geometry of the pre-shock region in these two IPs.Item Support for a 244 day Period in XTE J1829-098 from Swift BAT, MAXI and RXTE PCA Observations(The Astronomer's Telegram, 2022-09-12) Corbet, Robin; Coley, Joel B.; Gendreau, Keith C.; Guillot, S.; Islam, Nazma; Jaisawal, Gaurava K.; Malacaria, C.; Ng, Mason; Pottschmidt, Katja; Pradhan, Pragati; Ray, Paul S.; Sanna, Andrea; Wilms, J.; Wolff, Michael T.Item Swift/XRT observations of superorbital modulations in wind-fed supergiant X-ray binaries(Oxford University Press, 2023-12-26) Romano, P; Bozzo, E; Islam, Nazma; Corbet, R H DWe present the first Swift/XRT long-term monitoring of 2S 0114+650, a wind-fed supergiant X-ray binary for which both orbital and superorbital periods are known (Pₒᵣb ∼ 11.6 d and Pₛᵤₚ ∼ 30.8 d). Our campaign, summing up to ∼79 ks, is the most intense and complete sampling of the X-ray light curve of this source with a sensitive pointed X-ray instrument, and covers 17 orbital, and 6 superorbital cycles. The combination of flexibility, sensitivity, and soft X-ray coverage of the X-ray telescope (XRT) allowed us to confirm previously reported spectral changes along the orbital cycle of the source and unveil the variability in its spectral parameters as a function of the superorbital phase. For completeness, we also report on a similar analysis carried out by exploiting XRT archival data on three additional wind-fed supergiant X-ray binaries IGR J16418−4532, IGR J16479−4514, and IGR J16493−4348. For these sources, the archival data provided coverage along several superorbital cycles but our analysis could not reveal any significant spectral variability.Item X-rays from RS Ophiuchi's 2021 eruption: shocks in and out of ionization equilibrium(2023-11-28) Islam, Nazma; Mukai, Koji; Sokoloski, Jennifer L.The recurrent nova RS Ophiuchi (RS Oph) underwent its most recent eruption on 8 August 2021 and became the first nova to produce both detectable GeV and TeV emission. We used extensive X-ray monitoring with the Neutron Star Interior Composition Explorer Mission (NICER) to model the X-ray spectrum and probe the shock conditions throughout the 2021 eruption. The rapidly evolving NICER spectra consisted of both line and continuum emission that could not be accounted for using a single-temperature collisional equilibrium plasma model with an absorber that fully covered the source. We successfully modelled the NICER spectrum as a non-equilibrium ionization collisional plasma with partial-covering absorption. The temperature of the the non-equilibrium plasma show a peak on Day 5 with a kT of approximately 24 keV. The increase in temperature during the first five days could have been due to increasing contribution to the X-ray emission from material behind fast polar shocks or a decrease is the amount of energy being drained from shocks into particle acceleration during that time period. The absorption showed a change from fully covering the source to having a covering fraction of roughly 0.4, suggesting a geometrical evolution of the shock region within the complex global distribution of the circumstellar material. These findings show the evidence of the ejecta interacting with some dense equatorial shell initially and with less dense material in the bipolar regions at later times during the eruption.