Single Emission Mechanism Research Articles

Congruity of the Crab Pulsar’s γ-ray spectrum with the spectral distribution of tightly focused caustics (Corrigendum)

The spectrum derived here for the most tightly-focused component of the radiation generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star has a distribution function that fits the entire gamma-ray spectrum of the Crab pulsar on its own. This is the first time that the undivided breadth of this spectrum, from $10^2$ to $10^6$ MeV, is not only described by a single distribution function but is also explained by means of a single emission mechanism.

Radio spectra of pulsars fitted with the spectral distribution function of the emission from their current sheet

ABSTRACT In their catalogue of pulsars’ radio spectra, Swainston et al. distinguish between five different forms of these spectra: those that can be fitted with (i) a simple power law, (ii) a broken power law, (iii) a low-frequency turn-over, (iv) a high-frequency turn-over or (v) a double turn-over spectrum. Here, we choose two examples from each of these categories and fit them with the spectral distribution function of the caustics that are generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star. In contrast to the prevailing view that the curved features of pulsars’ radio spectra arise from the absorption of the observed radiation in high-density environments, our results imply that these features are intrinsic to the emission mechanism. We find that all observed features of pulsar spectra (including those that are normally fitted with simple or broken power laws) can be described by a single spectral distribution function and regarded as manifestations of a single emission mechanism. From the results of an earlier analysis of the emission from a pulsar’s current sheet and the values of the fit parameters for each spectrum, we also determine the physical characteristics of the central neutron star of each considered example and its magnetosphere.

Congruity of the Crab Pulsar’s γ-ray spectrum with the spectral distribution of tightly focused caustics

The spectrum derived here for the most tightly focused component of the radiation generated by the super-luminally moving current sheet in the magnetosphere of a non-aligned neutron star has a distribution function that fits the entire γ-ray spectrum of the Crab Pulsar on its own. This is the first time that the undivided breadth of this spectrum, from 102 to 106 MeV, is not only described by a single distribution function but is also explained by means of a single emission mechanism. To illustrate that the derived function describes the spectral distribution of the high-energy emission from any non-aligned neutron star, we analyse, in addition, the spectra of two other γ-ray pulsars for which sufficiently large datasets are available: PSR J0101−6422 and PSR J1709−4429. From the connection between the parameters of the fitted spectra and the physical characteristics of their sources, we moreover infer certain attributes of the magnetospheres of the analysed pulsars: the angle between the magnetic and spin axes of their central neutron stars, the scale factor of the electric current density that is associated with their current sheet, and the privileged latitudinal direction (relative to the spin axis) in which they are observed.

Progressive steepening of the SNR RX J1713.7–3946 X-ray spectrum from XMM–Newton to INTEGRAL

ABSTRACT In this work, we present the first detailed analysis of the supernova remnant RX J1713.7–3946 in the hard X-ray energy range with the Imager on Board the INTEGRAL Satellite (IBIS) coded-mask telescope onboard the INTEGRAL observatory. The shell-type morphology of the entire remnant is mapped in hard X-rays for the first time and significantly detected up to 50 keV. The IBIS sky image of RX J1713.7–3946, accumulated over 14 yr of operations, demonstrates two extended hard X-ray sources. These sources are spatially consistent with north-west and south-west rims of RX J1713.7–3946 and are also clearly visible at energies below 10 keV with XMM–Newton. This points to a single emission mechanism operating in soft and hard X-rays. The INTEGRAL 17–120 keV spectrum of RX J1713.7–3946 is characterized by a power-law continuum with the photon index of Γ ≈ 3 that is significantly softer than Γ ≈ 2 determined by XMM–Newton in the 1–10 keV energy range, suggesting a progressive steepening of the spectrum with the energy.

Probing the jets of blazars using the temporal symmetry of their multiwavelength outbursts

We compare the rise and decay timescales of $\sim$200 long-term ($\sim$weeks-months) GeV and R-band outbursts and $\sim$25 short-term ($\sim$hr-day) GeV flares in a sample of 10 blazars using light curves from the Fermi-LAT and the Yale/SMARTS monitoring project. We find that most of the long-term outbursts are symmetric, indicating that the observed variability is dominated by the crossing timescale of a disturbance, e.g., a shock. A larger fraction of short-term flares are asymmetric with an approximately equal fraction of longer and shorter decay than rise timescale. We employ the MUlti-ZOne Radiation Feedback (MUZORF) model to interpret the above results. We find that the outbursts with slow rise times indicate a gradual acceleration of the particles to GeV energy. A change in the bulk Lorentz factor of the plasma or the width of the shocked region can lead to an increase of the cooling time causing a faster rise than decay time. Parameters such as the luminosity or the distance of the broad line region (BLR) affects the cooling time strongly if a single emission mechanism, e.g., external Compton scattering of BLR photons is considered but may not if other mechanisms, e.g., synchrotron self-compton and external Compton scattering of the torus photon are included. This work carries out a systematic study of the symmetry of flares, which can be used to estimate relevant geometric and physical parameters of blazar jets in the context of the MUZORF model.

Clues on high-energy emission mechanism from blazar 3C 454.3 during 2015 August flare

We perform a detailed spectral study of a recent flaring activity from the Flat Spectrum Radio Quasar (FSRQ), 3C\,454.3, observed simultaneously in optical, UV, X-ray and $\gamma$-ray energies during 16 to 28 August, 2015. The source reached its peak $\gamma$-ray flux of $(1.9\pm0.2)\times\,10^{-05} \; {\rm ph\,cm^{-2}\,s^{-1}}$ on 22 August. The time averaged broadband spectral energy distribution (SED) is obtained for three time periods, namely "flaring state"; covering the peak $\gamma$-ray flux, "post flaring state"; immediately following the peak flare and "quiescent state"; separated from the flaring event and following the post flaring state. The SED corresponding to the flaring state is investigated using different emission models involving synchrotron, synchrotron self Compton (SSC) and external Compton (EC) mechanisms. Our study suggests that the X-ray and $\gamma$-ray emission from 3C\,454.3 cannot be attributed to a single emission mechanism and instead, one needs to consider both SSC and EC mechanisms. Moreover, the target photon energy responsible for the EC process corresponds to an equivalent temperature of 564 K, suggesting that the flare location lies beyond the broad line emitting region of the FSRQ. SED fitting of the other two flux states further supports these inferences.

High energy emission processes in OJ 287 during 2009 flare

The broadband spectrum of a BL Lac object, OJ 287, from radio to $\gamma$-rays obtained during a major $\gamma$-ray flare detected by \emph{Fermi} in 2009 are studied to understand the high energy emission mechanism during this episode. Using a simple one-zone leptonic model, incorporating synchrotron and inverse Compton emission processes, we show that the explanation of high energy emission from X-rays to $\gamma$-rays, by considering a single emission mechanism, namely, synchrotron self-Compton (SSC) or external Compton (EC) requires unlikely physical conditions. However, a combination of both SSC and EC mechanisms can reproduce the observed high energy spectrum satisfactorily. Using these emission mechanisms we extract the physical parameters governing the source and its environment. Our study suggests that the emission region of OJ 287 is surrounded by a warm infrared (IR) emitting region of $\sim 250 \, K$. Assuming this region as a spherical cloud illuminated by an accretion disk, we obtain the location of the emission region to be $\sim 9 pc$. This supports the claim that the $\gamma$-ray emission from OJ 287 during the 2009 flare arises from a location far away from the central engine as deduced from millimeter-gamma ray correlation study and very long baseline array images.

GRB 080916C: ON THE RADIATION ORIGIN OF THE PROMPT EMISSION FROM keV/MeV TO GeV

Fermi observations of high-energy gamma-ray emission from GRB 080916C shows that its spectrum is consistent with the Band function from MeV to tens of GeV. Assuming one single emission mechanism dominates in the whole energy range, we show that this spectrum is consistent with synchrotron origin by shock-accelerated electrons. The simple electron inverse-Compton model and the hadronic model are found to be less viable. In the synchrotron scenario, the synchrotron self-Compton scattering is likely to be in the Klein-Nishina regime and therefore the resulting high-energy emission is subdominant, even though the magnetic field energy density is lower than that in relativistic electrons. The Klein-Nishina inverse-Compton cooling may also affect the low-energy electron number distribution and hence results in a low-energy synchrotron photon spectrum $n(\nu)\propto\nu^{-1}$ below the peak energy. Under the framework of the electron synchrotron interpretation, we constrain the shock microphysical parameters and derive a lower limit of the upstream magnetic fields. The detection of synchrotron emission extending to about 70 GeV in the source frame in GRB 080916C favors the Bohm diffusive shock acceleration if the bulk Lorentz factor of the relativistic outflow is not significantly greater than thousands.

GAMMA-RAY BURST AT THE EXTREME: “THE NAKED-EYE BURST” GRB 080319B

On 19 March 2008, the northern sky was the stage of a spectacular optical transient that for a few seconds remained visible to the naked eye. The transient was associated with GRB 080319B, a gamma-ray burst at a luminosity distance of about 6 Gpc (standard cosmology), making it the most luminous optical object ever recorded by human kind. We present comprehensive sky monitoring and multi-color optical follow-up observations of GRB 080319B collected by the RAPTOR telescope network covering the development of the explosion and the afterglow before, during, and after the burst. The extremely bright prompt optical emission revealed features that are normally not detectable. The optical and gamma-ray variability during the explosion are correlated, but the optical flux is much greater than can be reconciled with single emission mechanism and a flat gamma-ray spectrum. This extreme optical behavior is best understood as synchrotron self-Compton model (SSC). After a gradual onset of the gamma-ray emission, there is an abrupt rise of the prompt optical flux suggesting that variable self-absorption dominates the early optical light curve. Our simultaneous multi-color optical light curves following the flash show spectral evolution consistent with a rapidly decaying red component due to large angle emission and the emergence of a blue forward shock component from interaction with the surrounding environment. While providing little support for the reverse shock that dominates the early afterglow, these observations strengthen the case for the universal role of the SSC mechanism in generating gamma-ray bursts.

Where are the hot ion lines in classical T Tauri stars formed?

Context. Classical T Tauri stars (hereafter CTTS) show a plethora of in- and outflow signatures in a variety of wavelength bands. Aims. In order to constrain gas velocities and temperatures, we analyse the emission in the hot ion lines. Methods. We use all available archival FUSE spectra of CTTS to measure the widths, fluxes and shifts of the detected hot ion lines and complement these data with HST/GHRS and HST/STIS data. We present theoretical estimates of the temperatures reached in possible emission models such as jets, winds, disks and accretion funnels and look for correlations with X-ray lines and absorption properties. Results. We find line shifts in the range from -170 km s -1 to +100 km s -1 . Most linewidths exceed the stellar rotational broadening. Those CTTS with blue-shifted lines also show excess absorption in X-rays. CTTS can be distinguished from main sequence (hereafter MS) stars by their large ratio of the O VII to O VI luminosities. Conclusions. No single emission mechanism can be found for all objects. The properties of those stars with blue-shifted lines are compatible with an origin in a shock-heated dust-depleted outflow.

X‐Ray Emission from the Old Pulsar B0950+08

We present the timing and spectral analyses of the XMM-Newton data on the 17 Myr old, nearby radio pulsar B0950+08. This observation revealed pulsations of the X-ray flux of PSR B0950+08 at its radio period, P ’ 253 ms. The pulse shape and pulsed fraction are apparently different at lower and higher energies of the observed 0.2‐10 keVenergy range, which suggests that the radiation cannot be explained by a single emission mechanism. The X-ray spectrum of the pulsar can be fitted with a power-law model with a photon index ¼1:75 � 0:15 and an (isotropic) luminosity LX ¼ (9:8 � 0:2) ; 10 29 ergs s � 1 in the 0.2‐10 keV range. Better fits are obtained with two-component, power-law plus thermal models with ¼1:30 � 0:10 and LX ¼ (9:7 � 0:1) ;10 29 ergs s � 1 for the power-law component, which presumably originates from the pulsar’s magnetosphere. The thermal component, dominating at EP0:7 keV, can be interpreted as radiation from heated polar caps on the neutron star surface covered with a hydrogen atmosphere. The inferred effective temperature, radius, and bolometric luminosity of the polar caps are TPC � 1M K,RPC � 250 m, and LPC � 3 ;1029 ergs s � 1 . Optical through X-ray nonthermal spectrum of the pulsar can be described as a single power law with ¼1:3 1:4 for the two-component X-ray fit. The ratio of the nonthermal X-ray (1‐10 keV) luminosity to the nonthermal optical (4000‐9000 8) luminosity, � 360, is within the range of 10 2 103 observed for younger pulsars, which suggests that the magnetospheric X-ray and optical emissions are powered by the same mechanism in all pulsars, independent of age and spin-down power. Assuming a standard neutron star radius, the upper limit on the temperature of the bulk of the neutron star surface, inferred from the optical and X-ray data, is about 0.15 MK. We also analyze X-ray observations of several other old pulsars, B2224+65, J2043+2740, B0628� 28, B1813� 36, B1929+10, and B0823+26, and compare their properties with those of PSR B0950+08.

HST and UKIRT imaging observations of z 1 6C radio galaxies - I. The data

The results of Hubble Space Telescope and UKIRT imaging observations are presented for a sample of 11 6C radio galaxies with redshifts 0.85 < z < 1.5. The observations of the 6C sources reveal a variety of different features, similar to those observed around the higher power 3CR sources of similar redshifts. However, the extent and luminosity of the aligned emission appear less extreme in the case of the 6C radio galaxies. For both samples, the aligned emission clearly cannot be explained by a single emission mechanism; however, line emission and related nebular continuum emission often provide a significant contribution to the aligned emission.

Flat-spectrum radio sources: cosmic conspiracy or relativistic neutrons?

RADIO-LOUD active galactic nuclei (AGNs) tend to show extended emission in the form of jets and an unresolved central core, from which the jets presumably originate. The intensity spectrum of the core varies smoothly from 108.5 to 1016 Hz in frequency1, and is flat (constant with frequency) between 109 and 1012 Hz, implying that a single emission mechanism, probably synchrotron radiation, is responsible. Because synchrotron emission below ∼10l3Hz would be self-absorbed for a uniform source under conditions typical of AGNs, inhomogeneous models have been used to account for these spectra. These models, which require the magnetic field strength and the electron density and energy to vary according to strict power laws, reproduce the flat spectra by a number of self-absorbed components2–4, but they do not explain the origin of radiating electrons at ∼1018 cm from the AGN core, in view of the very short lifetime of the electrons in an AGN environment. Here we propose that energy is transported from the central source by relativistic neutrons, which travel freely over a large volume and decay into relativistic protons. The protons produce secondary electrons which generate the observed radiation. The photon spectra thus produced are largely model independent and flat.