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  • 1. Maitra, Chandreyee
    et al.
    Raichur, Harsha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Pradhan, Pragati
    Paul, Biswajit
    Understanding the nature of the intriguing source X Persei: a deep look with a Suzaku observation2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 1, p. 713-722Article in journal (Refereed)
    Abstract [en]

    We present detailed broad-band timing and spectral analysis of the persistent, low luminosity and slowly spinning pulsar X Persei using a deep Suzaku observation of the source. The spectrum is unusually hard with a cyclotron resonance scattering feature (CRSF), the presence of which has been debated. By comparing the spectral models relevant for accretion-powered pulsars, we have obtained the best constraint on the broad-band spectral model of X Persei obtained so far. The CRSF is not confirmed in the average spectrum. We have also identified the presence of different intensity levels in the source with distinct changes in the pulse profile and the energy spectrum indicating changes in the accretion geometry. We further find evidence of a CRSF in the highest intensity levels at similar to 40 keV, indicating a magnetic field strength of 3.4 x 10(12) G.

  • 2.
    Singh, Nishant K.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Raichur, Harsha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    HIGH-WAVENUMBER SOLAR f-MODE STRENGTHENING PRIOR TO ACTIVE REGION FORMATION2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 832, no 2, article id 120Article in journal (Refereed)
    Abstract [en]

    We report a systematic strengthening of the local solar surface or fundamental f- mode one to two days prior to the emergence of an active region (AR) in the same (corotating) location. Except for a possibly related increase in the kurtosis of the magnetic field, no indication can be seen in the magnetograms at that time. Our study is motivated by earlier numerical findings of Singh et al., which showed that, in the presence of a nonuniform magnetic field that is concentrated a few scale heights below the surface, the f- mode fans out in the diagnostic kw diagram at high wavenumbers. Here we explore this possibility using data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory and show for six isolated ARs, 11130, 11158, 11242, 11105, 11072, and 11768, that at large latitudinal wavenumbers (corresponding to horizontal scales of around 3000 km), the f- mode displays strengthening about two days prior to AR formation and thus provides a new precursor for AR formation. Furthermore, we study two ARs, 12051 and 11678, apart from a magnetically quiet patch lying next to AR. 12529, to demonstrate the challenges in extracting such a precursor signal when a newly forming AR emerges in a patch that lies in close proximity to. one or several already existing ARs, which are expected to pollute neighboring patches. We then discuss plausible procedures for extracting precursor signals from regions with crowded environments. The idea that the f- mode is perturbed days before any visible magnetic activity occurs at the surface can be important in constraining dynamo models aimed at understanding the global magnetic activity of the Sun.

  • 3. Varun,
    et al.
    Maitra, Chandreyee
    Pradhan, Pragati
    Raichur, Harsha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Paul, Biswajit
    Probing the Cyclotron line characteristics of 4U 1538-522 using AstroSat-LAXPC2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 484, no 1, p. l1-L6Article in journal (Refereed)
    Abstract [en]

    We report the first report on cyclotron line studies with the LAXPC instrument onboard AstroSat of the high-mass X-ray binary pulsar 4U 1538-52. During the observation of source that spanned about 1 d with a net exposure of 50 ks, the source X-ray flux remained constant. Pulse profile is double peaked in low-energy range and has a single peak in high-energy range, the transition taking place around the cyclotron line energy of the source. Cyclotron scattering feature (CRSF) is detected at similar to 22 keV with a very high significance in phase averaged spectrum, It is one of the highest signal to noise ratio detections of CRSF for this source. We performed detailed pulse-phase-resolved spectral analysis with 10 independent phase bins. We report the results of pulse-phase-resolved spectroscopy of the continuum and CRSF parameters. The cyclotron line parameters show pulse phase dependence over the entire phase with a CRSF energy variation of similar to 13 per cent, which is in agreement with previous studies, We also confirm the increase ill the centroid energy of the CRSF observed between the 1996-2004 (RXTE) and the 2012 (Suzaku) observations, reinforcing that the increase was a long-term change.

  • 4. Varun,
    et al.
    Pradhan, Pragati
    Maitra, Chandreyee
    Raichur, Harsha
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Paul, Biswajit
    Pulse Phase Variation of the Cyclotron Line in HMXB 4U 1907+09 with AstroSat LAXPC2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 880, no 1, article id 61Article in journal (Refereed)
    Abstract [en]

    We present timing and spectral analysis of data from an observation of the high mass X-ray binary pulsar 4U 1907+09 with the Large Area X-ray Proportional Counter instrument onboard AstroSat. The light curve consisted of a flare at the beginning of the observation, followed by persistent emission. The pulsar continues to spin down, and the pulse profile is found to be double-peaked up to 16 keV with the peaks separated by a phase of similar to 0.45. Significant energy dependence of the pulse profile is seen with diminishing amplitude of the secondary peak above 16 keV, and increasing amplitude of the main peak up to 40 keV with a sharp decline after that. We confirm earlier detections of the cyclotron resonance scattering feature (CRSF) in 4U 1907+09 at similar to 18.5 +/- 0.2 keV in the phase-averaged spectrum with a high detection significance. An intensity resolved spectral analysis of the initial flare in the light curve shows that the CRSF parameters do not change with a change in luminosity by a factor of 2.6. We also performed pulse phase-resolved spectral analysis with 10 independent phase bins. The energy and the strength of the cyclotron line show pulse phase dependence that is in agreement with previous measurements. Two features from the current observation (different energy dependence of the two pulse peaks and a strong CRSF only around the secondary peak) both indicate a deviation from a dipole geometry of the magnetic field of the neutron star, or complex beaming pattern from the two poles.

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