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We report broad-band Hubble Space Telescope imaging of the field of soft gamma-ray repeater SGR 0418+5729 with ACS/WFC and WFC3/IR. Observing in two wide filters F606W and F110W, we find no counterpart within the positional error circle derived from Chandra observations, to limiting magnitudes mF606W>28.6, mF110W>27.4 (Vega system), equivalent to reddening-corrected luminosity limits LF606W<5e28, LF110W<6e28 erg s-1 for a distance d=2 kpc, at 3sig confidence. This, in turn, imposes lower limits on the contemporaneous X-ray/optical flux ratio of 1100 and X-ray/near-infra-red flux ratio of 1000. We derive an upper limit on the temperature and/or size of any fall-back disk around the magnetar. We also compare the detection limits with observations of other magnetars.
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We perform a detailed modelling of the post-outburst surface emission of the low magnetic field magnetar SGR 0418+5729. The dipolar magnetic field of this source, B=6x10^12 G estimated from its spin-down rate, is in the observed range of magnetic fields for normal pulsars. The source is further characterized by a high pulse fraction and a single-peak profile. Using synthetic temperature distribution profiles, and fully accounting for the general-relativistic effects of light deflection and gravitational redshift, we generate synthetic X-ray spectra and pulse profiles that we fit to the observations. We find that asymmetric and symmetric surface temperature distributions can reproduce equally well the observed pulse profiles and spectra of SGR 0418. Nonetheless, the modelling allows us to place constraints on the system geometry (i.e. the angles $psi$ and $xi$ that the rotation axis makes with the line of sight and the dipolar axis, respectively), as well as on the spot size and temperature contrast on the neutron star surface. After performing an analysis iterating between the pulse profile and spectra, as done in similar previous works, we further employed, for the first time in this context, a Markov-Chain Monte-Carlo approach to extract constraints on the model parameters from the pulse profiles and spectra, simultaneously. We find that, to reproduce the observed spectrum and flux modulation: (a) the angles must be restricted to $65deg < psi+xi < 125deg$ or $235deg < psi+xi <295deg$; (b) the temperature contrast between the poles and the equator must be at least a factor of $sim6$, and (c) the size of the hottest region ranges between 0.2-0.7 km (including uncertainties on the source distance). Last, we interpret our findings within the context of internal and external heating models.
We report on the long term X-ray monitoring of the outburst decay of the low magnetic field magnetar SGR 0418+5729, using all the available X-ray data obtained with RXTE, SWIFT, Chandra, and XMM-Newton observations, from the discovery of the source in June 2009, up to August 2012. The timing analysis allowed us to obtain the first measurement of the period derivative of SGR 0418+5729: dot{P}=4(1)x10^{-15} s/s, significant at ~3.5 sigma confidence level. This leads to a surface dipolar magnetic field of B_dip ~6x 10^{12} G. This measurement confirms SGR 0418+5729 as the lowest magnetic field magnetar. Following the flux and spectral evolution from the beginning of the outburst up to ~1200 days, we observe a gradual cooling of the tiny hot spot responsible for the X-ray emission, from a temperature of ~0.9 to 0.3 keV. Simultaneously, the X-ray flux decreased by about 3 orders of magnitude: from about 1.4x10^{-11} to 1.2x10^{-14} erg/s/cm^2 . Deep radio, millimeter, optical and gamma-ray observations did not detect the source counterpart, implying stringent limits on its multi-band emission, as well as constraints on the presence of a fossil disk. By modeling the magneto-thermal secular evolution of SGR 0418+5729, we infer a realistic age of ~550 kyr, and a dipolar magnetic field at birth of ~10^{14} G. The outburst characteristics suggest the presence of a thin twisted bundle with a small heated spot at its base. The bundle untwisted in the first few months following the outburst, while the hot spot decreases in temperature and size. We estimate the outburst rate of low magnetic field magnetars to be about one per year per galaxy, and we briefly discuss the consequences of such result in several other astrophysical contexts.
We present a search for an electromagnetic counterpart of the gravitational wave source GW151226. Using the Pan-STARRS1 telescope we mapped out 290 square degrees in the optical i_ps filter starting 11.5hr after the LIGO information release and lasting for a further 28 days. The first observations started 49.5hr after the time of the GW151226 detection. We typically reached sensitivity limits of i_ps = 20.3-20.8 and covered 26.5% of the LIGO probability skymap. We supplemented this with ATLAS survey data, reaching 31% of the probability region to shallower depths of m~19. We found 49 extragalactic transients (that are not obviously AGN), including a faint transient in a galaxy at 7Mpc (a luminous blue variable outburst) plus a rapidly decaying M-dwarf flare. Spectral classification of 20 other transient events showed them all to be supernovae. We found an unusual transient, PS15dpn, with an explosion date temporally coincident with GW151226 which evolved into a type Ibn supernova. The redshift of the transient is secure at z=0.1747 +/- 0.0001 and we find it unlikely to be linked, since the luminosity distance has a negligible probability of being consistent with that of GW151226. In the 290 square degrees surveyed we therefore do not find a likely counterpart. However we show that our survey strategy would be sensitive to NS-NS mergers producing kilonovae at D < 100 Mpc which is promising for future LIGO/Virgo searches.
We report the results of a Dark Energy Camera (DECam) optical follow-up of the gravitational wave (GW) event GW151226, discovered by the Advanced LIGO detectors. Our observations cover 28.8 deg$^2$ of the localization region in the $i$ and $z$ bands (containing 3% of the BAYESTAR localization probability), starting 10 hours after the event was announced and spanning four epochs at $2-24$ days after the GW detection. We achieve $5sigma$ point-source limiting magnitudes of $iapprox21.7$ and $zapprox21.5$, with a scatter of $0.4$ mag, in our difference images. Given the two day delay, we search this area for a rapidly declining optical counterpart with $gtrsim 3sigma$ significance steady decline between the first and final observations. We recover four sources that pass our selection criteria, of which three are cataloged AGN. The fourth source is offset by $5.8$ arcsec from the center of a galaxy at a distance of 187 Mpc, exhibits a rapid decline by $0.5$ mag over $4$ days, and has a red color of $i-zapprox 0.3$ mag. These properties roughly match the expectations for a kilonova. However, this source was detected several times, starting $94$ days prior to GW151226, in the Pan-STARRS Survey for Transients (dubbed as PS15cdi) and is therefore unrelated to the GW event. Given its long-term behavior, PS15cdi is likely a Type IIP supernova that transitioned out of its plateau phase during our observations, mimicking a kilonova-like behavior. We comment on the implications of this detection for contamination in future optical follow-up observations.
On 2009 June 5, the Gamma-ray Burst Monitor (GBM) onboard the Fermi Gamma-ray Space Telescope triggered on two short, and relatively dim bursts with spectral properties similar to Soft Gamma Repeater (SGR) bursts. Independent localizations of the bursts by triangulation with the Konus-RF and with the Swift satellite, confirmed their origin from the same, previously unknown, source. The subsequent discovery of X-ray pulsations with the Rossi X-ray Timing Explorer (RXTE), confirmed the magnetar nature of the new source, SGR J0418+5729. We describe here the Fermi/GBM observations, the discovery and the localization of this new SGR, and our infrared and Chandra X-ray observations. We also present a detailed temporal and spectral study of the two GBM bursts. SGR J0418+5729 is the second source discovered in the same region of the sky in the last year, the other one being SGR J0501+4516. Both sources lie in the direction of the galactic anti-center and presumably at the nearby distance of ~2 kpc (assuming they reside in the Perseus arm of our galaxy). The near-threshold GBM detection of bursts from SGR J0418+5729 suggests that there may be more such dim SGRs throughout our galaxy, possibly exceeding the population of bright SGRs. Finally, using sample statistics, we conclude that the implications of the new SGR discovery on the number of observable active magnetars in our galaxy at any given time is <10, in agreement with our earlier estimates.
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