Calibration of X-ray absorption in our Galaxy Willingale, R; Starling, R. L. C; Beardmore, A. P ...
Monthly notices of the Royal Astronomical Society,
05/2013, Letnik:
431, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Prediction of the soft X-ray absorption along lines of sight through our Galaxy is crucial for understanding the spectra of extragalactic sources, but requires a good estimate of the foreground ...column density of photoelectric absorbing species. Assuming uniform elemental abundances this reduces to having a good estimate of the total hydrogen column density,
. The atomic component,
, is reliably provided using the mapped 21 cm radio emission but estimating the molecular hydrogen column density,
, expected for any particular direction, is difficult. The X-ray afterglows of Gamma Ray Bursts (GRBs) are ideal sources to probe X-ray absorption in our Galaxy because they are extragalactic, numerous, bright, have simple spectra and occur randomly across the entire sky. We describe an empirical method, utilizing 493 afterglows detected by the Swift X-ray Telescope, to determine N
Htot through the Milky Way which provides an improved estimate of the X-ray absorption in our Galaxy and thereby leads to more reliable measurements of the intrinsic X-ray absorption and, potentially, other spectral parameters, for extragalactic X-ray sources. We derive a simple function, dependent on the product of the atomic hydrogen column density,
, and dust extinction, E(B − V), which describes the variation of the molecular hydrogen column density,
, of our Galaxy, over the sky. Using the resulting N
Htot we show that the dust-to-hydrogen ratio is correlated with the carbon monoxide emission and use this ratio to estimate the fraction of material which forms interstellar dust grains. Our resulting recipe represents a significant revision in Galactic absorption compared to previous standard methods, particularly at low Galactic latitudes.
We present the 2SXPS (Swift-XRT Point Source) catalog, containing 206,335 point sources detected by the Swift X-ray Telescope (XRT) in the 0.3-10 keV energy range. This catalog represents a ...significant improvement over 1SXPS, with double the sky coverage (now 3790 deg2), and several significant developments in source detection and classification. In particular, we present for the first time techniques to model the effect of stray light-significantly reducing the number of spurious sources detected. These techniques will be very important for future, large effective area X-ray missions, such as the forthcoming Athena X-ray observatory. We also present a new model of the XRT point-spread function and a method for correctly localizing and characterizing piled-up sources. We provide light curves-in four energy bands, two hardness ratios, and two binning timescales-for every source, and from these deduce that over 80,000 of the sources in 2SXPS are variable in at least one band or hardness ratio. The catalog data can be queried or downloaded via a web interface at https://www.swift.ac.uk/2SXPS, via HEASARC, or in Vizier (IX/58).
We present an analysis of 123 gamma-ray bursts (GRBs) with known redshifts possessing an afterglow plateau phase. We reveal that
$L_{\rm a}\hbox{-}T^{*}_{\rm a}$
correlation between the X-ray ...luminosity L
a at the end of the plateau phase and the plateau duration,
$T^*_{\rm a}$
, in the GRB rest frame has a power-law slope different, within more than 2σ, from the slope of the prompt
$L_{{\rm f}}\hbox{-}T^{*}_{{\rm f}}$
correlation between the isotropic pulse peak luminosity, L
f, and the pulse duration,
$T^{*}_{{\rm f}}$
, from the time since the GRB ejection. Analogously, we show differences between the prompt and plateau phases in the energy duration distributions with the afterglow emitted energy being on average 10 per cent of the prompt emission. Moreover, the distribution of prompt pulse versus afterglow spectral indexes does not show any correlation. In the further analysis we demonstrate that the L
peak–L
a distribution, where L
peak is the peak luminosity from the start of the burst, is characterized with a considerably higher Spearman correlation coefficient, ρ = 0.79, than the one involving the averaged prompt luminosity, L
prompt–L
a, for the same GRB sample, yielding ρ = 0.60. Since some of this correlation could result from the redshift dependences of the luminosities, namely from their cosmological evolution we use the Efron–Petrosian method to reveal the intrinsic nature of this correlation. We find that a substantial part of the correlation is intrinsic. We apply a partial correlation coefficient to the new de-evolved luminosities showing that the intrinsic correlation exists.
Long gamma-ray bursts (GRBs) with a plateau phase in their X-ray afterglows obey a 3D relation, between the rest-frame time at the end of the plateau, Ta, its corresponding X-ray luminosity, La, and ...the peak luminosity in the prompt emission, Lpeak. This 3D relation identifies a GRB fundamental plane whose existence we here confirm. Here we include the most recent GRBs observed by Swift to define a "gold sample" (45 GRBs) and obtain an intrinsic scatter about the plane compatible within 1 with the previous result. We compare GRB categories, such as short GRBs with extended emission (SEE), X-ray flashes, GRBs associated with supernovae, a sample of only long-duration GRBs (132), selected from the total sample by excluding GRBs of the previous categories, and the gold sample, composed by GRBs with light curves with good data coverage and relatively flat plateaus. We find that the relation planes for each of these categories are not statistically different from the gold fundamental plane, with the exception of the SSE, which are hence identified as a physically distinct class. The gold fundamental plane has an intrinsic scatter smaller than any plane derived from the other sample categories. Thus, the distance of any particular GRB category from this plane becomes a key parameter. We computed the several category planes with Ta as a dependent parameter obtaining for each category smaller intrinsic scatters (reaching a reduction of 24% for the long GRBs). The fundamental plane is independent from several prompt and afterglow parameters.
Abstract
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi) shows long-lasting high-energy emission in many gamma-ray bursts (GRBs), similar to X-ray afterglows ...observed by the Neil Gehrels Swift Observatory (Swift). Some LAT light curves (LCs) show a late-time flattening reminiscent of X-ray plateaus. We explore the presence of plateaus in LAT temporally extended emission analyzing GRBs from the second Fermi-LAT GRB Catalog from 2008 to 2016 May with known redshifts, and check whether they follow closure relations corresponding to four distinct astrophysical environments predicted by the external forward shock model. We find that three LCs can be fit by the same phenomenological model used to fit X-ray plateaus and show tentative evidence for the existence of plateaus in their high-energy extended emission. The most favorable scenario is a slow-cooling regime, whereas the preferred density profile for each GRBs varies from a constant-density interstellar medium to an
r
−2
wind environment. We also compare the end time of the plateaus in
γ
-rays and X-rays using a statistical comparison with 222 Swift GRBs with plateaus and known redshifts from 2005 January to 2019 August. Within this comparison, the case of GRB 090510 shows an indication of chromaticity at the end time of the plateau. Finally, we update the 3D fundamental plane relation among the rest-frame end time of the plateau, its correspondent luminosity, and the peak prompt luminosity for 222 GRBs observed by Swift. We find that these three LAT GRBs follow this relation.
To find out the astrophysical processes responsible for gamma-ray burst (GRB), it is crucial to discover and understand the relations between their observational properties. This work was performed ...in the GRB rest frames using a sample of 62 long Swift GRBs with known redshifts. Following the earlier analysis of the correlation between afterglow luminosity (L*a) and break time (T*a), we extend it to correlations between the afterglow and the prompt emission GRB physical parameters. We find a tight physical scaling between the mentioned afterglow luminosity L*a and the prompt emission mean luminosity 〈L*p〉45≡E
iso/T*45. The distribution, with the Spearman correlation coefficient reaching 0.95 for the most accurately fitted subsample, scales approximately as L*a∝〈L*p〉0.7
45. We have also analysed correlations of L*a with several prompt emission parameters, including the isotropic energy E
iso and the peak energy in the νF
ν spectrum, E
peak. As a result, we obtain significant correlations also between these quantities, discovering that the highest correlated GRB subsample in the afterglow analysis leads also to the highest prompt-afterglow correlations. Such events can be considered to form a sample of standard GRBs for astrophysics and cosmology.
The current status of observations and theoretical models of gamma-ray bursts and some other related transients, including ultra-long bursts and tidal disruption events, is reviewed. We consider the ...impact of multi-wavelength data on the formulation and development of theoretical models for the prompt and afterglow emission including the standard fireball model utilizing internal shocks and external shocks, photospheric emission, the role of the magnetic field and hadronic processes. In addition, we discuss some of the prospects for non-photonic multi-messenger detection and for future instrumentation, and comment on some of the outstanding issues in the field.
Long-duration gamma-ray bursts (GRBs) are thought to be produced by the core-collapse of a rapidly rotating massive star. This event generates a highly relativistic jet and prompt gamma-ray and X-ray ...emission arises from internal shocks in the jet or magnetized outflows. If the stellar core does not immediately collapse to a black hole, it may form an unstable, highly magnetized millisecond pulsar or magnetar. As it spins down, the magnetar would inject energy into the jet causing a distinctive bump in the GRB light curve where the emission becomes fairly constant followed by a steep decay when the magnetar collapses. We assume that the collapse of a massive star to a magnetar can launch the initial jet. By automatically fitting the X-ray light curves of all GRBs observed by the Swift satellite, we identified a subset of bursts which have a feature in their light curves which we call an internal plateau – unusually constant emission followed by a steep decay – which may be powered by a magnetar. We use the duration and luminosity of this internal plateau to place limits on the magnetar spin period and magnetic field strength, and find that they are consistent with the most extreme predicted values for magnetars.
We present the 1SXPS (Swift-XRT point source) catalog of 151, 524 X-ray point sources detected by the Swift-XRT in 8 yr of operation. The catalog covers 1905 deg super(2) distributed approximately ...uniformly on the sky. We analyze the data in two ways. First we consider all observations individually, for which we have a typical sensitivity of ~3 x 10 super(-13) erg cm super(-2) s super(-1) (0.3-10 keV). Then we co-add all data covering the same location on the sky: these images have a typical sensitivity of ~9 x 10 super(-14) erg cm super(-2) s super(-1) (0.3-10 keV). Our sky coverage is nearly 2.5 times that of 3XMM-DR4, although the catalog is a factor of ~1.5 less sensitive. The median position error is 5''.5 (90% confidence), including systematics. Our source detection method improves on that used in previous X-ray Telescope (XRT) catalogs and we report >68,000 new X-ray sources. The goals and observing strategy of the Swift satellite allow us to probe source variability on multiple timescales, and we find ~30,000 variable objects in our catalog. For every source we give positions, fluxes, time series (in four energy bands and two hardness ratios), estimates of the spectral properties, spectra and spectral fits for the brightest sources, and variability probabilities in multiple energy bands and timescales.
The majority of short gamma-ray bursts (SGRBs) are thought to originate from the merger of compact binary systems collapsing directly to form a black hole. However, it has been proposed that both ...SGRBs and long gamma-ray bursts (LGRBs) may, on rare occasions, form an unstable millisecond pulsar (magnetar) prior to final collapse. GRB 090515, detected by the Swift satellite was extremely short, with a T90 of 0.036 ± 0.016 s, and had a very low fluence of 2 × 10−8 erg cm−2 and faint optical afterglow. Despite this, the 0.3–10 keV flux in the first 200 s was the highest observed for an SGRB by the Swift X-ray Telescope (XRT). The X-ray light curve showed an unusual plateau and steep decay, becoming undetectable after ∼500 s. This behaviour is similar to that observed in some long bursts proposed to have magnetars contributing to their emission. In this paper, we present the Swift observations of GRB 090515 and compare it to other gamma-ray bursts (GRBs) in the Swift sample. Additionally, we present optical observations from Gemini, which detected an afterglow of magnitude 26.4 ± 0.1 at T+ 1.7 h after the burst. We discuss potential causes of the unusual 0.3–10 keV emission and suggest it might be energy injection from an unstable millisecond pulsar. Using the duration and flux of the plateau of GRB 090515, we place constraints on the millisecond pulsar spin period and magnetic field.
PDF
