Sarah Villanova Borges

The most accepted scenario to explain Anomalous X-ray pulsars and Soft Gamma-Ray Repeaters are the magnetar model. In this model, the quiescent emission and the outbursts and flare events are described by the decay of a huge dipole magnetic field above the quantum limit. Nonetheless, the model presents some limitations, such as the existence of low magnetic field SGR/AXPs, which raised the interest for alternative models. Some examples are the neutron star (NS) accreting scenario and the white dwarf (WD) pulsar model. In this poster, we present a new scenario evoking an accreting WD to explain the AXP 4U 0142+61. In our proposed model, the persistent emission comes from the WD photosphere, a disk, and a magnetic accretion column. This scenario is inspired by (i) the periodic flux modulation, which could be explained by an accretion column (in page 1, we present a figure with the periodic flux ), and (ii) the detected IR emission and silicate line emission, which indicate the presence of a disk. On page 2, we present a picture with the scenario's schematic model, similar to the following description. The dusty disk is optically thick, and its emission can be represented by a combination of blackbodies of different temperatures. Conversely, the gaseous disk is optically thin, and its emission can be neglected. The inner radius of the gaseous disk is equal to the magnetosphere radius. For that point on, the matter flows into the WD surface following the magnetic field lines and the debris disk ceases to exist. Close to the WD photosphere, the infalling flow of matter suffers a collisional shock, forming an extremely hot region, the so-called post-shock region that emits bremsstrahlung. About half of that energy reaches the WD surface, where it is absorbed and reemitted in lower energies, forming hot spots. Using this model, we obtain a good fit for the entire spectral energy distribution of 4U 0142+61. On Page 3, we present the figure of the dereddened and deabsorbed spectral energy distribution (SED) of 4U 10142+61, along with the best fit. The emission from the hard X-ray implies a near-Chandrasekhar WD, for which we assume a mass of 1.41 Msun and a radius of 1021 km. From the fit of the optical/IR emission, we obtain a WD effective temperature of 9.4 10^4 K, pointing to a young WD of a few Myr. The gaseous disk has inner and outer temperatures of 1991 and 285 K, consistent with disks seen around WDs. We can estimate a magnetic field of 10^7 G from the spin-down rate, compatible with the values in magnetic WDs. The most plausible origin is the merger of two less massive WDs. Finally, new telescopes could answer the question about the origin of this object. For instance, spectroscopy from James Webb Space Telescope could tell the disk's composition and, hence, give hints about the origin of 4U 0142+61. Moreover, each model predicts a distinction degree of polarization in X-rays, which will be distinguishable with the first generation X-ray polarimeters telescopes, such as the Imaging X-ray Polarimetry Explorer. Pictures with the representation of both telescopes follow this explanation.