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Register a new userA Spectacular Radio Flare from XRF 050416a at 40 days and Implications for the Nature of X-ray Flashes
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We present detailed optical, near-infrared, and radio observations of the X-ray flash 050416a obtained with Palomar and Siding Springs Observatories as well as HST and the VLA, placing this event among the best-studied X-ray flashes to date. In addition, we present an optical spectrum from Keck LRIS from which we measure the redshift of the burst, z=0.6528. At this redshift the isotropic-equivalent prompt energy release was about 10^51 erg, and using a standard afterglow synchrotron model we find that the blastwave kinetic energy is a factor of 10 larger, E_K,iso ~ 10^52 erg. The lack of an observed jet break to t ~ 20 days indicates that the opening angle is larger than 7 deg and the total beaming-corrected relativistic energy is larger than 10^50 erg. We further show that the burst produced a strong radio flare at t ~ 40 days accompanied by an observed flattening in the X-ray band which we attribute to an abrupt circumburst density jump or an episode of energy injection (either from a refreshed shock or off-axis ejecta). Late-time observations with HST show evidence for an associated supernova with peak optical luminosity roughly comparable to that of SN 1998bw. Next, we show that the host galaxy of XRF 050416a is actively forming stars at a rate of at least 2 M_solar per year with a luminosity of L_B ~ 0.5L* and metallicity of Z ~ 0.2-0.8 Z_solar. Finally, we discuss the nature of XRF 050416a in the context of short-hard gamma-ray bursts and under the framework of off-axis and dirty fireball models for X-ray flashes.
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We have developed a toy model for internal shocks which has been used to generate a large number of synthetic GRBs in order to find in the parameter space the conditions which can lead to the formation of X-ray flashes. The key condition appears to be a small contrast of the Lorentz factor in the relativistic wind emitted by the central engine.
We discuss the origin of X-Ray Flashes (XRFs), a recently discovered class of Gamma-Ray Bursts (GRBs). Using a simplified model for internal shocks we check if XRFs can be intrinsically soft due to some specific values of the parameters describing the relativistic outflow emerging from the central engine. We generate a large number of synthetic events and find that XRFs are obtained when the contrast Gamma_max/Gamma_min of the Lorentz factor distribution is small while the average Lorentz factor Gamma is large. A few XRFs may be GRBs at large redshifts but we exclude this possibility for the bulk of the population. If outflows with a small contrast are commonly produced, even a large population of XRFs could be explained. If conversely the Lorentz factor distribution within the wind is broad, one should then rely on extrinsic causes, such as viewing angle effects or high redshift.
In this work, we present a multi-wavelength study of XRF 040912, aimed at measuring its distance scale and the intrinsic burst properties. We performed a detailed spectral and temporal analysis of both the prompt and the afterglow emission and we estimated the distance scale of the likely host galaxy. We then used the currently available sample of XRFs with known distance to discuss the connection between XRFs and classical Gamma-ray Bursts (GRBs). We found that the prompt emission properties unambiguously identify this burst as an XRF, with an observed peak energy of E_p=17+/-13 keV and a burst fluence ratio S(2-30keV)/S(30-400keV)>1. A non-fading optical source with R~24 mag and with an apparently extended morphology is spatially consistent with the X-ray afterglow, likely the host galaxy. XRF 040912 is a very dark burst since no afterglow optical counterpart is detected down to R>25 mag (3 sigma limiting magnitude) at 13.6 hours after the burst. The host galaxy spectrum detected from 3800A to 10000A, shows a single emission line at 9552A. The lack of any other strong emission lines blue-ward of the detected one and the absence of the Ly alpha cut-off down to 3800A are consistent with the hypothesis of the [OII] line at redshift z=1.563+/-0.001. The intrinsic spectral properties rank this XRF among the soft GRBs in the E_peak-E_iso diagram. Similar results were obtained for most XRFs at known redshift. Only XRF 060218 and XRF 020903 represent a good example of instrinsic XRF(i-XRF) and are possibly associated with a different progenitor population. This scenario may calls for a new definition of XRFs.
We present the first fully simultaneous fits to the NIR and X-ray spectral slope (and its evolution) during a very bright flare from Sgr A*, the supermassive black hole at the Milky Ways center. Our study arises from ambitious multi-wavelength monitoring campaigns with XMM-Newton, NuSTAR and SINFONI. The average multi-wavelength spectrum is well reproduced by a broken power-law with $Gamma_{NIR}=1.7pm0.1$ and $Gamma_X=2.27pm0.12$. The difference in spectral slopes ($DeltaGamma=0.57pm0.09$) strongly supports synchrotron emission with a cooling break. The flare starts first in the NIR with a flat and bright NIR spectrum, while X-ray radiation is detected only after about $10^3$ s, when a very steep X-ray spectrum ($DeltaGamma=1.8pm0.4$) is observed. These measurements are consistent with synchrotron emission with a cooling break and they suggest that the high energy cut-off in the electron distribution ($gamma_{max}$) induces an initial cut-off in the optical-UV band that evolves slowly into the X-ray band. The temporal and spectral evolution observed in all bright X-ray flares are also in line with a slow evolution of $gamma_{max}$. We also observe hints for a variation of the cooling break that might be induced by an evolution of the magnetic field (from $Bsim30pm8$ G to $Bsim4.8pm1.7$ G at the X-ray peak). Such drop of the magnetic field at the flare peak would be expected if the acceleration mechanism is tapping energy from the magnetic field, such as in magnetic reconnection. We conclude that synchrotron emission with a cooling break is a viable process for Sgr A*s flaring emission.
We report the results of multiwavelength observations of the superluminal X-ray transient GRO J1655-40 during and following the prominent hard X-ray outburst of March-April 1995. GRO J1655-40 was continuously monitored by BATSE on board CGRO, and repeatedly observed in the radio and optical bands from the ground. About a month after the onset of the hard X-ray outburst, GRO J1655-40 was observed twice by HST on April 25 and 27 1995, with the aim of detecting faint optical emission from ejected plasmoids. Despite the similarity of the hard X-ray emission in April 1995 with previous events in 1994, no radio or optical emission related to plasmoids was detected. We conclude that GRO J1655-40 is subject to a complex behavior showing: radio-loud hard X-ray outbursts with strong radio emission (of flux $f_r goe 100$~mJy) both from a `core source and from propagating plasmoids (as those in 1994), and radio-quiet hard X-ray outbursts with no detectable radio emission and plasmoid activity ($f_r loe 0.5$~mJy) (as those in 1995). Our results can constrain models of particle acceleration and radiation of relativistic plasmoids.
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