@article{MartinezChicharroTorrejonOskinovaetal.2018,
  author    = {Martinez-Chicharro, M. and Torrejon, J. M. and Oskinova, Lida and Furst, F. and Postnov, K. and Rodes-Roca, J. J. and Hainich, Rainer and Bodaghee, A.},
  title     = {Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700-37},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {473},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnrasl/slx165},
  pages     = {L74 -- L78},
  year      = {2018},
  abstract  = {Based on new Chandra X-ray telescope data, we present empirical evidence of plasma Compton cooling during a flare in the non-pulsating massive X-ray binary 4U1700-37. This behaviour might be explained by quasi-spherical accretion on to a slowly rotating magnetized neutron star (NS). In quiescence, the NS in 4U1700-37 is surrounded by a hot radiatively cooling shell. Its presence is supported by the detection of mHz quasi-periodic oscillations likely produced by its convection cells. The high plasma temperature and the relatively low X-ray luminosity observed during the quiescence, point to a small emitting area similar to 1 km, compatible with a hotspot on an NS surface. The sudden transition from a radiative to a significantly more efficient Compton cooling regime triggers an episode of enhanced accretion resulting in a flare. During the flare, the plasma temperature drops quickly. The predicted luminosity for such transitions, similar to 3 x 10(35) erg s(-1), is very close to the luminosity of 4U1700-37 during quiescence. The transition may be caused by the accretion of a clump in the stellar wind of the donor star. Thus, a magnetized NS nature of the compact object is strongly favoured.},
  language  = {en}
}
@article{SanjurjoFerrrinTorrejonPostnovetal.2017,
  author    = {Sanjurjo-Ferrrin, G. and Torrejon, J. M. and Postnov, K. and Oskinova, Lida and Rodes-Roca, J. J. and Bernabeu, Guillermo},
  title     = {XMM-Newton spectroscopy of the accreting magnetar candidate 4U0114+65},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {606},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201630119},
  pages     = {4039 -- 4042},
  year      = {2017},
  abstract  = {Methods. We analysed the energy-resolved light curve and the time-resolved X-ray spectra provided by the EPIC cameras on board XMM-Newton. We also analysed the first high-resolution spectrum of this source provided by the Reflection Grating Spectrometer. Results. An X-ray pulse of 9350 +/- 160 s was measured. Comparison with previous measurements confirms the secular spin up of this source. We successfully fit the pulse-phase-resolved spectra with Comptonisation models. These models imply a very small (r similar to 3 km) and hot (kT similar to 2-3 keV) emitting region and therefore point to a hot spot over the neutron star (NS) surface as the most reliable explanation for the X-ray pulse. The long NS spin period, the spin-up rate, and persistent X-ray emission can be explained within the theory of quasi-spherical settling accretion, which may indicate that the magnetic field is in the magnetar range. Thus, 4U0114+65 could be a wind-accreting magnetar. We also observed two episodes of low luminosity. The first was only observed in the low-energy light curve and can be explained as an absorption by a large over-dense structure in the wind of the B1 supergiant donor. The second episode, which was deeper and affected all energies, may be due to temporal cessation of accretion onto one magnetic pole caused by non-spherical matter capture from the structured stellar wind. The light curve displays two types of dips that are clearly seen during the high-flux intervals. The short dips, with durations of tens of seconds, are produced through absorption by wind clumps. The long dips, in turn, seem to be associated with the rarefied interclump medium. From the analysis of the X-ray spectra, we found evidence of emission lines in the X-ray photoionised wind of the B1Ia donor. The Fe K alpha line was found to be highly variable and much weaker than in other X-ray binaries with supergiant donors. The degree of wind clumping, measured through the covering fraction, was found to be much lower than in supergiant donor stars with earlier spectral types. Conclusions. The XMM-Newton spectroscopy provided further support for the magnetar nature of the neutron star in 4U0114+65. The light curve presents dips that can be associated with clumps and the interclump medium in the stellar wind of the mass donor.},
  language  = {en}
}
@article{TorrejonReigFuerstetal.2018,
  author    = {Torrejon, J. M. and Reig, Pablo and F{\"u}rst, F. and Martinez-Chicharro, M. and Postnov, K. and Oskinova, Lida},
  title     = {NuSTAR rules out a cyclotron line in the accreting magnetar candidate 4U2206+54},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {479},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {3},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/sty1628},
  pages     = {3366 -- 3372},
  year      = {2018},
  abstract  = {Based on our new NuSTAR X-ray telescope data, we rule out any cyclotron line up to 60 keV in the spectra of the high-mass X-ray binary 4U2206+54. In particular, we do not find any evidence of the previously claimed line around 30 keV, independently of the source flux, along the spin pulse. The spin period has increased significantly, since the last observation, up to 5750 +/- 10 s, confirming the rapid spin-down rate (nu)over dot = -1.8 x 10(-14) Hz s(-1). This behaviour might be explained by the presence of a strongly magnetized neutron star (B-s > several times 10(13) G) accreting from the slow wind of its main-sequence O9.5 companion.},
  language  = {en}
}
@article{PostnovOskinovaTorrejon2017,
  author    = {Postnov, K. and Oskinova, Lida and Torrejon, J. M.},
  title     = {A propelling neutron star in the enigmatic Be-star gamma Cassiopeia},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {465},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnrasl/slw223},
  pages     = {L119 -- L123},
  year      = {2017},
  abstract  = {gamma Cassiopeia (gamma Cas), is known to be a binary system consisting of a Be-type star and a low-mass (M similar to 1M(circle dot)) companion of unknown nature orbiting in the Be-disc plane. Here, we apply the quasi-spherical accretion theory on to a compact magnetized star and show that if the low-mass companion of gamma Cas is a fast spinning neutron star, the key observational signatures of. Cas are remarkably well reproduced. Direct accretion on to this fast rotating neutron star is impeded by the propeller mechanism. In this case, around the neutron star magnetosphere a hot shell is formed which emits thermal X-rays in qualitative and quantitative agreement with observed properties of the X-ray emission from gamma Cas. We suggest that gamma Cas and its analogues constitute a new subclass of Be-type X-ray binaries hosting rapidly rotating neutron stars formed in supernova explosions with small kicks. The subsequent evolutionary stage of gamma Cas and its analogues should be the X Per-type binaries comprising low-luminosity slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission from gamma Cas, and also establishes evolutionary connections between various types of rotating magnetized neutron stars in Be-binaries.},
  language  = {en}
}