arXiv subjects

Grzegorz Madejski

Publications and source records attributed to Grzegorz Madejski.

26 records · Page 2Linked to original sources

arXiv2009

Suzaku Wide Band Analysis of the X-ray Variability of TeV Blazar Mrk 421 in 2006

We present the results of X-ray observations of the well-studied TeV blazar Mrk 421 with the Suzaku satellite in 2006 April 28. During the observation, Mrk 421 was undergoing a large flare and the X-ray flux was variable, decreasing by ~ 50 %, from 7.8x10^{-10} to 3.7x10^{-10} erg/s/cm^2 in about 6 hours, followed by an increase by ~ 35 %. Thanks to the broad bandpass coupled with high-sensitivity of Suzaku, we measured the evolution of the spectrum over the 0.4--60 keV band in data segments as short as ~1 ksec. The data show deviations from a simple power law model, but also a clear spectral variability. The time-resolved spectra are fitted by a synchrotron model, where the observed spectrum is due to a exponentially cutoff power law distribution of electrons radiating in uniform magnetic field; this model is preferred over a broken power law. As another scenario, we separate the spectrum into "steady" and "variable" components by subtracting the spectrum in the lowest-flux period from those of other data segments. In this context, the difference ("variable") spectra are all well described by a broken power law model with photon index Gamma ~ 1.6, breaking at energy epsilon_{brk} ~ 3 keV to another photon index Gamma ~ 2.1 above the break energy, differing from each other only by normalization, while the spectrum of the "steady" component is best described by the synchrotron model. We suggest the rapidly variable component is due to relatively localized shock (Fermi I) acceleration, while the slowly variable ("steady") component is due to the superposition of shocks located at larger distance along the jet, or due to other acceleration process, such as the stochastic acceleration on magnetic turbulence (Fermi II) in the more extended region.

astro-ph.HE↗

arXiv2008

Prevalence of High X-ray Obscuring Columns among AGN that Host H$_2$O Masers

Of 104 AGN known to exhibit H$_2$O maser emission, X-ray data that enable estimation of column densities, or lower limits, are available for 42. Contributing to this, we report analysis of new and archival X-ray data for 8 galaxies and collation of values for three more. Maser emission is indicative of large columns of cold gas, and in five of the eight new cases, maser spectra point toward origins in accretion disks viewed close-to edge-on (a.k.a. "disk-maser" systems). In these, we detect hard continuum and Fe K$α$ emission with equivalent widths on the order of 1 keV, which is consistent with Compton reflection, fluorescence by cold material, and obscuring columns $\ga 10^{24}$ cm$^{-2}$. Reviewing the full sample of 42, 95% exhibit N$_{\rm H} >10^{23}$ cm$^{-2}$ and 60% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. Half of these are now recognized to be disk masers (up from 13); in this sub-sample, which is likely to be more homogeneous vis-á-vis the origin of maser emission, 76% exhibit N$_{\rm H} >10^{24}$ cm$^{-2}$. The probability of a common parent distribution of columns for disk-masers and other AGN masers is <3%. Because ground-based surveys of AGN to detect new disk masers are relatively unbiased with respect to X-ray brightness and comparatively inexpensive, they may also be efficient guides for the sensitive pointed X-ray observations required to identify Compton-thick objects outside of shallow surveys.

astro-ph↗

arXiv2008

PoGOLite - A High Sensitivity Balloon-Borne Soft Gamma-ray Polarimeter

We describe a new balloon-borne instrument (PoGOLite) capable of detecting 10% polarisation from 200mCrab point-like sources between 25 and 80keV in one 6 hour flight. Polarisation measurements in the soft gamma-ray band are expected to provide a powerful probe into high-energy emission mechanisms as well as the distribution of magnetic fields, radiation fields and interstellar matter. At present, only exploratory polarisation measurements have been carried out in the soft gamma-ray band. Reduction of the large background produced by cosmic-ray particles has been the biggest challenge. PoGOLite uses Compton scattering and photo-absorption in an array of 217 well-type phoswich detector cells made of plastic and BGO scintillators surrounded by a BGO anticoincidence shield and a thick polyethylene neutron shield. The narrow FOV (1.25msr) obtained with well-type phoswich detector technology and the use of thick background shields enhance the detected S/N ratio. Event selections based on recorded phototube waveforms and Compton kinematics reduce the background to that expected for a 40-100mCrab source between 25 and 50keV. A 6 hour observation on the Crab will differentiate between the Polar Cap/Slot Gap, Outer Gap, and Caustic models with greater than 5 sigma; and also cleanly identify the Compton reflection component in the Cygnus X-1 hard state. The first flight is planned for 2010 and long-duration flights from Sweden to Northern Canada are foreseen thereafter.

astro-ph↗

arXiv2008

A Deep 0.3-10 keV Spectrum of the H$_{2}$O Maser Galaxy IC 2560

We present a new XMM-Newton spectrum of the Seyfert 2 nucleus of IC 2560, which hosts H$_{2}$O maser emission from an inclined Keplerian accretion disk. The X-ray spectrum shows soft excess due to multi-temperature ionized plasma, a hard continuum and strong emission features, from Mg, Si, S, Ca, Fe and Ni, mainly due to fluorescence. It is consistent with reflection of the continuum from a mostly neutral medium and obscuration due to a high column density, $>$ 10$^{24}$ cm$^{-2}$. The amplitude of the reflected component may exceed 10% of the central unobscured luminosity. This is higher than the reflected fraction, of a few percent, observed in other Seyfert 2 sources like NGC 4945. We observe an emission line at 6.7 keV, possibly due to FeXXV, undetected in previous Chandra observations. The absorption column density associated with this line is less than 10$^{23}$ cm$^{-2}$, lower than the obscuration of the central source. We hypothesize that this highly ionized Fe line emission originates in warm gas, also responsible for a scattered component of continuum emission that may dominate the spectrum between 1 and 3 keV. We compare X-ray and maser emission characteristics of IC 2560 and other AGN that exhibit water maser emission originating in disk structures around central engines. The temperature for the region of the disk associated with maser action is consistent with the expected 400-1000K range. The clumpiness of disk structures (inferred from the maser distribution) may depend on the unobscured luminosities of the central engines.

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arXiv2007

Suzaku wide-band X-ray Spectroscopy of the Seyfert 2 AGN in NGC 4945

Suzaku observed a nearby Seyfert 2 galaxy NGC4945, which hosts one of the brightest active galactic nuclei above 20 keV. Combining data from the X-ray CCD camera (XIS) and the Hard X-ray Detector (HXD), the AGN intrinsic nuclear emission and its reprocessed signals were observed simultaneously. The intrinsic emission is highly obscured with an absorbing column of $\sim 5 \times 10^{24}$ cm$^{-2}$, and was detectable only above $\sim 10$ keV. The spectrum below 10 keV is dominated by reflection continuum and emission lines from neutral/ionized material. Along with a neutral iron K$α$ line, a neutral iron K$β$ and a neutral nickel K$α$ line were detected for the first time from this source. The neutral lines and the cold reflection continuum are consistent with both originating in the same location. The Compton down-scattered shoulder in the neutral Fe-K$α$ line is $\sim 10%$ in flux of the narrow core, which confirms that the line originates from reflection rather than transmission. The weakness of the Compton shoulder also indicates that the reflector is probably seen nearly edge-on. Flux of the intrinsic emission varied by a factor of $\sim 2$ within $\sim 20$ ks, which requires the obscuring material to be geometrically thin. Broadband spectral modeling showed that the solid angle of the neutral reflector is less than a few $\times 10^{-2} \times 2π$. All this evidence regarding the reprocessed signals suggests that a disk-like absorber/reflector is viewed from a near edge-on angle.

astro-ph↗

arXiv2005

INTEGRAL and RXTE Observations of Centaurus A

INTEGRAL and RXTE performed three simultaneous observations of the nearby radio galaxy Cen A in 2003 March, 2004 January, and 2004 February with the goals of investigating the geometry and emission processes via the spectral/temporal variability of the X-ray/low energy gamma ray flux, and intercalibration of the INTEGRAL instruments with respect to those on RXTE. When combined with earlier archival RXTE results, we find the power law continuum flux and the line-of-sight column depth varied independently, and the iron line flux was essentially unchanging. Taking X-ray spectral measurements from satellite missions since 1970 into account, we discover a variability in the column depth between 1.0x10^23 cm^-2 and 1.5x10^23 cm^-2, and suggest that variations in the edge of a warped accretion disk viewed nearly edge-on might be the cause.

astro-ph↗

arXiv2004

Gamma-ray Polarimetry with Compton Telescope

Compton telescope is a promising technology to achieve very high sensitivity in the soft gamma-ray band (0.1-10 MeV) by utilizing Compton kinematics. Compton kinematics also enables polarization measurement which will open new windows to study gamma-ray production mechanism in the universe. CdTe and Si semiconductor technologies are key technologies to realize the Compton telescope in which their high energy resolution is crucial for high angular resolution and background rejection capability. We have assembled a prototype module using a double-sided silicon strip detector and CdTe pixel detectors. In this paper, we present expected polarization performance of a proposed mission (NeXT/SGD). We also report results from polarization measurements using polarized synchrotron light and validation of EGS4 MC simulation.

astro-ph↗

arXiv2004

BL Lac X-ray Spectra: simpler than we thought

We report results from {\it XMM-Newton} observations of thirteen X-ray bright BL Lacertae objects, selected from the {\it Einstein} Slew Survey sample. The spectra are generally well fit by power-law models, with four objects having hard ($α<1; F_ν\propto ν^{-α}$) spectra that indicates synchrotron peaks at $>5$ keV. None of our spectra show line features, indicating that soft X-ray absorption ``notches'' must be rare amongst BL Lacs, rather than common or ubiquitous as had previously been asserted. We find significant curvature in most of the spectra. This curvature is almost certainly intrinsic, as it appears nearly constant from 0.5 to 6 keV, an observation which is inconsistent with the small columns seen in these sources.

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