Abstract
We report on a timing and spectral analysis of a 50 ks NuSTAR observation of IGR J16320−4751 (= AX J1631.9−4752), a high-mass X-ray binary hosting a slowly rotating neutron star. In this ...observation from 2015, the spin period was 1308.8 ± 0.4 s giving a period derivative
P
̇
∼
2
×
10
−
8
s s
−1
when compared with the period measured in 2004. In addition, the pulsed fraction decreased as a function of energy, as opposed to the constant trend that was seen previously. This suggests a change in the accretion geometry of the system during the intervening 11 yr. The phase-averaged spectra were fit with the typical model for accreting pulsars: a power law with an exponential cutoff. This left positive residuals at 6.4 keV attributable to the known iron K
α
line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5
σ
. We found no significant differences in the spectral parameters across the spin period, other than the expected changes in flux and component normalizations. A flare lasting around 5 ks was captured during the first half of the observation, where the X-ray emission hardened and the local column density decreased. Finally, the binary orbital period was refined to 8.9912 ± 0.0078 days thanks to Swift/BAT monitoring data from 2005–2022.
Aims. We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and Swift/UVOT, photo-zs are derived for five GRBs for ...which spectroscopic redshifts are not available. Methods. We use UV/optical/NIR data and synthetic photometry based on afterglow observations and theory to derive the photometric redshifts of GRBs and their accuracy. Taking into account the afterglow synchrotron emission properties, we investigate the application of photometry to derive redshifts in a theoretical range between z ~ 1 to z ~ 12. Results. The measurement of photo-zs for GRB afterglows provides a quick, robust and reliable determination of the distance scale to the burst, particularly in those cases where spectroscopic observations in the optical/NIR range cannot be obtained. Given a sufficiently bright and mildly reddened afterglow, the relative photo-z accuracy η = Δz/(1 + z) is better than 10% between z = 1.5 and z ~ 7 and better than 5% between z = 2 and z = 6. We detail the approach on 5 sources without spectroscopic redshifts observed with UVOT on-board Swift and/or GROND. The distance scale to those same afterglows is measured to be \hbox{$z=4.31^{+0.14}_{-0.15}$}z=4.31-0.15+0.14 for GRB 080825B, \hbox{$z=2.13^{+0.14}_{-0.20}$}z=2.13-0.20+0.14 for GRB 080906, \hbox{$z=3.44^{+0.15}_{-0.32}$}z=3.44-0.32+0.15 for GRB 081228, \hbox{$z=2.03^{+0.16}_{-0.14}$}z=2.03-0.14+0.16 for GRB 081230 and \hbox{$z=1.28^{+0.16}_{-0.15}$}z=1.28-0.15+0.16 for GRB 090530. Conclusions. Due to the exceptional luminosity and simple continuum spectrum of GRB afterglows, photometric redshifts can be obtained to an accuracy as good as η ~ 0.03 over a large redshift range including robust (η ~ 0.1) measurements in the ultra-high redshift regime (z ≳ 7). Combining the response from UVOT with ground-based observatories and in particular GROND operating in the optical/NIR wavelength regime, reliable photo-zs can be obtained from z ~ 1.0 out to z ~ 10, and possibly even at higher redshifts in some favorable cases, provided that these GRBs exist, are localized quickly, have sufficiently bright afterglows and are not heavily obscured.
Context. The prominent blazar 3C 279 is known for its large-amplitude variability throughout the electromagnetic spectrum and its often γ-ray-dominated spectral energy distribution. However, ...the characterization of its broadband spectral variability still lacks a consistent picture, and the origin of its high-energy emission is still unclear. Aims. We intend to characterize the spectral energy distribution (SED) and spectral variability of 3C 279 in its optical high state. Methods. Prompted by an optical high state of 3C 279, we organized an extensive multiwavelength campaign with coverage from radio to hard X-ray energies. The core components of the campaign were INTEGRAL and Chandra ToO observations in January 2006, augmented by X-ray data from Swift and RXTE as well as radio through optical coverage. Results. The blazar was observed at a moderately high optical state. A well-covered multifrequency spectrum from radio to hard X-ray energies could be derived. During the flare, the radio spectrum was inverted, with a prominent spectral peak near 100 GHz, which propagated in time toward lower frequencies. The SED shows the typical two-bump shape, the signature of non-thermal emission from a relativistic jet. As a result of the long exposure times of INTEGRAL and Chandra, the high-energy spectrum (0.3−100 keV) was precisely measured, showing – for the first time – a possible downward curvature. A comparison of this SED from 2006 to the one observed in 2003, also centered on an INTEGRAL observation, but during an optical low-state, revealed the surprising fact that – despite a significant change of the high-frequency synchrotron emission (near-IR/optical/UV) – the low-energy end of the high-energy component (X-ray energies) remained virtually unchanged compared to 2003. Conclusions. Our results prove that the two emission components do not vary simultaneously. This provides strong constraints on the modeling of the overall emission of 3C 279. When interpreted with a steady-state leptonic model, the variability among the SEDs displaying almost identical X-ray spectra at low flux levels, but drastically different IR/optical/UV fluxes, can be reproduced by a change solely of the low-energy cutoff of the relativistic electron spectrum. In an internal shock model for blazar emission, such a change could be achieved through a varying relative Lorentz factor of colliding shells producing internal shocks in the jet, and/or the efficiency of generating turbulent magnetic fields (e.g., through the Weibel instability) needed for efficient energy transfer from protons to electrons behind the shock.
Aims.The galactic black hole candidate XTE J1817-330 was discovered in outburst by RXTE in January 2006. We present here the results of an XMM-Newton target of opportunity observation, performed on ...13 March 2006 (44 days after the maximum), and an INTEGRAL observation performed on 15–18 February 2006 (18 days after the maximum). Methods.The EPIC-pn camera on-board XMM-Newton was used in the fast read-out burst mode to avoid photon pile-up, while the RGS was used in spectroscopy high count rate mode. We fit both the XMM-Newton and the INTEGRAL spectra with a two-component model consisting of a thermal accretion disk (represented by diskbb or diskpn models) and a comptonizing hot corona (represented by a power-law or the compTT model). Results.The soft X-ray spectrum is dominated by an accretion disk component, with a maximum temperature decreasing from 0.96±0.04 keV at the time of the INTEGRAL observation to 0.70±0.01 keV on 13 March. The optical monitors on board INTEGRAL and XMM-Newton showed the source with magnitudes V:11.3–11.4, U:15.0–15.1 and UVW1:14.7–14.8. The soft X-ray spectrum, together with the optical and UV data, show a low hydrogen column density towards the source, and several absorption lines, most likely of interstellar origin, are detected in the RGS spectrum: OIKα, OIKβ, OII, OIII and OVII, which trace both cold and hot components of the interstellar medium. The soft X-ray spectrum indicates the presence of a black hole, with an estimate for the upper limit of the mass of 6.0$^{\rm +4.0}_{-2.5}$ $M_\odot$.
Context. If one wants to understand the physics of blazars, better simultaneous observations are important at all wavelengths, so it was fortunate that a ToO observation of the TeV-emitting blazar ...Mrk 421 with INTEGRAL could be triggered in June 2006 by an increase in the RXTE count rate to more than 30 mCrab. The source was then observed with all INTEGRAL instruments, with the exception of the spectrometer SPI, for a total exposure of 829 ks. During this time several outbursts were observed by IBIS and JEM-X. Multiwavelength observations were immediately triggered, and the source was observed at radio, optical, and X-ray wavelengths up to TeV energies. Aims. The data obtained during these observations were analysed with respect to time variability, time lags, correlated variability, and spectral evolution and then compiled in a nu F_{\nu} spectrum. Methods. The observations of the different instruments/telescopes were analysed with the usual correlation and time-analysis methods. The spectral analysis of the X-ray data was performed with XSPEC. Results. Four strong flares at X-rays were observed that were not seen at other wavelengths (partially because of missing data). From the fastest rise in the X-rays, an upper limit could be derived to the extension of the emission region. A time lag between high-energy and low-energy X- rays was observed, which allowed an estimation of the magnetic-field strength. The spectral analysis of the X-rays revealed a slight spectral hardening of the low-energy (3- similar to 43 keV) spectral index. The hardness- ratio analysis of the Swift-XRT (0.2-10 keV) data indicated a small correlation with the intensity; i.e., a hard-to-soft evolution was observed. At the energies of IBIS/ISGRI (20-150 keV), such correlations are less obvious. A multiwavelength spectrum was composed and the X-ray luminosities were calculated. Conclusions. The observed flaring activity of Mrk 421 is mainly visible at X-rays. It is found that the spectral change with intensity is small. But at least one flare showed a completely different spectral behaviour than the other flares, so one can conclude that each blob of relativistic-moving electrons has its own individual physical environment that leads to different emission characteristics. From a fit of a leptonic emission model to the data, one finds that the observed variability may be due to a varying efficiency of particle acceleration.
Context. The high-redshift (z = 3.1) blazar PKS 0537-286, belonging to the flat spectrum radio quasar blazar subclass, is one of the most luminous active galactic nuclei (AGN) in the Universe. ...Blazars are very suitable candidates for multiwavelength observations. Indeed, the relativistic beaming effect at work within the jet enhances their luminosity. This in turn allows the properties of the extragalactic jets, the powering central engine, and the surrounding environment to be derived. Aims. Our aim is to present the results of a multifrequency campaign from the near-IR to hard X-ray energies on PKS 0537-286 and give insight into the physical environment where the radiation processes take place. Methods. We observed the source at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based optical telescopes. We also analyzed two archival spectra taken with XMM-Newton in 1999 and 2005. A comparative analysis of the results is performed. Results. The X-ray continuum of the blazar, as sampled by XMM, is described by a power law of index $\Gamma = 1.2$, modified by variable absorption at the soft X-rays, as found in other high-redshift QSOs. Modest X-ray continuum variability is found in the Swift observations. The combined Swift/BAT and Swift/XRT spectrum is very hard ($\Gamma = 1.3$). This, together with the non simultaneous EGRET detection and the more recent non detection by Fermi-LAT, constrains the peak of the high-energy component robustly. The optical/UV data, heavily affected by intervening Ly α absorption, indicate the presence of a bright thermal accretion disk that decreased in luminosity between 2006 and 2008. We infer from this a reduction of the BLR radius. When taking this into account, the 2006 and 2008 SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering where the accretion-disk luminosity decreases between the 2 epochs by a factor 2, while the bulk Lorentz factor remains unchanged and the magnetic field changed only marginally.
1ES 1959+650 is one of the most remarkable high-peaked BL Lacertae objects (HBL). In 2002, it exhibited a TeV {gamma}-ray flare without a similar brightening of the synchrotron component at lower ...energies. This orphan TeV flare remained a mystery. We present the results of a multifrequency campaign, triggered by the INTEGRAL IBIS detection of 1ES 1959+650. Our data range from the optical to hard X-ray energies, thus covering the synchrotron and inverse-Compton components simultaneously. We observed the source with INTEGRAL, the Swift X-Ray Telescope, and the UV-Optical Telescope, and nearly simultaneously with a ground-based optical telescope. The steep spectral component at X-ray energies is most likely due to synchrotron emission, while at soft {gamma}-ray energies the hard spectral index may be interpreted as the onset of the high-energy component of the blazar spectral energy distribution (SED). This is the first clear measurement of a concave X-ray-soft {gamma}-ray spectrum for an HBL. The SED can be well modeled with a leptonic synchrotron self-Compton model. When the SED is fitted this model requires a very hard electron spectral index of q {approx} 1.85, possibly indicating the relevance of second-order Fermi acceleration.
The dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are some of the most dark matter (DM) dominated objects known. We report on γ-ray observations of Milky Way dSphs based on six years ...of Fermi Large Area Telescope data processed with the new Pass8 event-level analysis. None of the dSphs are significantly detected in γ rays, and we present upper limits on the DM annihilation cross section from a combined analysis of 15 dSphs. These constraints are among the strongest and most robust to date and lie below the canonical thermal relic cross section for DM of mass ≲100 GeV annihilating via quark and τ-lepton channels.
We present a search for spatial extension in high-latitude ( ) sources in recent Fermi point source catalogs. The result is the Fermi High-Latitude Extended Sources Catalog, which provides source ...extensions (or upper limits thereof) and likelihood profiles for a suite of tested source morphologies. We find 24 extended sources, 19 of which were not previously characterized as extended. These include sources that are potentially associated with supernova remnants and star-forming regions. We also found extended γ-ray emission in the vicinity of the Cen A radio lobes and-at GeV energies for the first time-spatially coincident with the radio emission of the SNR CTA 1, as well as from the Crab Nebula. We also searched for halos around active galactic nuclei, which are predicted from electromagnetic cascades induced by the e+e− pairs that are deflected in intergalactic magnetic fields. These pairs are produced when γ-rays interact with background radiation fields. We do not find evidence for extension in individual sources or in stacked source samples. This enables us to place limits on the flux of the extended source components, which are then used to constrain the intergalactic magnetic field to be stronger than 3 × 10−16 G for a coherence length λ 10 kpc, even when conservative assumptions on the source duty cycle are made. This improves previous limits by several orders of magnitude.
We report on the search for spectral irregularities induced by oscillations between photons and axion-like particles (ALPs) in the gamma-ray spectrum of NGC 1275, the central galaxy of the Perseus ...cluster. Using 6 years of Fermi Large Area Telescope data, we find no evidence for ALPs and exclude couplings above 5 times 10 (sup -12) per gigaelectronvolt for ALP masses less than or approximately equal to 0.5 apparent magnitude (m (sub a)) less than or approximately equal to 5 nanoelectronvolts at 95 percent confidence. The limits are competitive withthe sensitivity of planned laboratory experiments, and, together with other bounds, strongly constrain thepossibility that ALPs can reduce the gamma-ray opacity of the Universe.
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