Radio Emitters Research Articles

Low radio frequency signatures of ram pressure stripping in Virgo spiral NGC 4254

We report the detection of extended low radio frequency continuum emission beyond the optical disc of the spiral galaxy NGC 4254 using the Giant Metrewave Radio Telescope. NGC 4254, which has an almost face-on orientation, is located in the outskirts of the Virgo cluster. Since such extended emission is uncommon in low-inclination galaxies, we believe it is a signature of magnetized plasma pushed out of the disc by ram pressure of the intracluster medium as NGC 4254 falls into the Virgo cluster. The detailed spectral index distribution across NGC 4254 shows that the steepest spectrum α < -1 (S ∝ ν α ) arises in the gas beyond the optical disc. This lends support to the ram pressure scenario by indicating that the extended emission is not from the disc gas but from matter which has been stripped by ram pressure. The steeper spectrum of the extended emission is reminiscent of haloes in edge-on galaxies. The sharp fall in intensity and enhanced polarization in the south of the galaxy, in addition to enhanced star formation reported by others, provide evidence towards the efficacy of ram pressure on this galaxy. Hi 21-cm observations show that the gas in the north lags in rotation and hence is likely the atomic gas which is carried along with the wind. NGC 4254 is a particularly strong radio emitter with a power of 7 x 10 22 W Hz -1 at 240 MHz. We find that the integrated spectrum of the galaxy flattens at lower frequencies and is well explained by an injection spectrum with α 0 = -0.45 ± 0.12. We end by comparing published simulation results with our data and conclude that ram pressure stripping is likely to be a significant contributor to evolution of galaxies residing in X-ray poor groups and cluster outskirts.

SwiftXRT Observation of 34 NewINTEGRALIBIS AGNs: Discovery of Compton‐Thick and Other Peculiar Sources

For a significant number of the sources detected at high energies (>10 keV) by the INTEGRAL/IBIS and Swift/BAT instruments there is either a lack information about them in the 2-10 keV range or they are totally unidentified. Herein, we report on a sample of 34 IBIS AGN or AGN candidate objects for which there is X-ray data in the Swift/XRT archive. Thanks to these X-ray follow up observations, the identification of the gamma ray emitters has been possible and the spectral shape in terms of photon index and absorption has been evaluated for the first time for the majority of our sample sources. The sample, enlarged to include 4 more AGN already discussed in the literature, has been used to provide photon index and column density distribution. We obtain a mean value of 1.88 with a dispersion of 0.12, i.e. typical of an AGN sample. Sixteen objects (47%) have column densities in excess of 10^{22} cm^{-2} and, as expected, a large fraction of the absorbed sources are within the Sey 2 sample. We have provided a new diagnostic tool (NH versus F(2-10)keV/F(20-100)keV softness ratio) to isolate peculiar objects; we find at least one absorbed Sey 1 galaxy, 3 Compton thick AGN candidates; and one secure example of a true type 2 AGN. Within the sample of 10 still unidentified objects, 3 are almost certainly AGN of type 2; 3 to 4 have spectral slopes typical of AGN; and two are located high on the galactic plane and are strong enough radio emitters so that can be considered good AGN candidates.

Non-thermal emission processes in massive binaries

In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons, the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars. Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet binary—will be considered in details. Finally, strategies for future developments in this field will be discussed.

Non-thermal radio emission from O-type stars

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5-8 V + O5-8 V) and an O8 supergiant. It is also a well known non-thermal radio emitter. We observed the radio emission of HD 167971 with the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). By combining these data with VLA archive observations we constructed a radio lightcurve covering a 20-yr time-range. We searched for, but failed to find, the 3.3-day spectroscopic period of the binary in the radio data. This could be due to the absence of intrinsic synchrotron radiation in the colliding-wind region between the two components of the eclipsing binary, or due to the large amount of free-free absorption that blocks the synchrotron radiation. We are able to explain many of the observed characteristics of the radio data if the non-thermal emission is produced in a colliding-wind region between the supergiant and the combined winds of the binary. Furthermore, if the system is gravitationally bound, the orbital motion occurs over a period of ~20 years or longer, as suggested by the long-term variability in the radio data. We argue that the variability is due to the free-free absorption that changes with orbital phase or may also in part be due to changes in separation, should the orbit be eccentric.

XMM���Newtonobservations of the massive colliding wind binary and non-thermal radio emitter Cyg���OB2���#8A [O6If + O5.5III(f)

We report on the results of four XMM‐Newton observations separated by about ten days from each other of Cyg OB2 #8A [O6If + O5.5III(f)]. This massive colliding wind binary is a very bright X-ray emitter — one of the first X-ray emitting O-stars discovered by the Einstein satellite — as well as a confirmed non-thermal radio emitter whose binarity was discovered quite recently. The X-ray spectrum between 0.5 and 10.0 keV is essentially thermal, and is best fitted with a three-component model with temperatures of about 3, 9 and 20 MK. The X-ray luminosity corrected for the interstellar absorption is rather large, i.e. about 10 34 erg s −1 . Compared to the ‘canonical’ LX/Lbol ratio of O-type stars, Cyg OB2 #8A was a factor of 19‐28 overluminous in X-rays during our observations. The EPIC spectra did not reveal any evidence for the presence of a non-thermal contribution in X-rays. This is not unexpected considering that the simultaneous detections of non-thermal radiation in the radio and soft X-ray (below 10.0 keV) domains is unlikely. Our data reveal a significant decrease in the X-ray flux from apastron to periastron with an amplitude of about 20 per cent. Combining our XMM‐ Newton results with those from previous ROSAT-PSPC and ASCA-SIS observations, we obtain a light curve suggesting a phase-locked X-ray variability. The maximum emission level occurs around phase 0.75, and the minimum is probably seen shortly after the periastron passage. Using hydrodynamic simulations of the wind‐wind collision, we find a high X-ray emission level close to phase 0.75, and a minimum at periastron as well. The high X-ray luminosity, the strong phase-locked variability and the spectral shape of the X-ray emission of Cyg OB2 #8A revealed by our investigation point undoubtedly to X-ray emission dominated by colliding

USING THE WESTERBORK RADIO OBSERVATORY TO DETECT UHE COSMIC PARTICLES INTERACTING ON THE MOON

Ultra-High-Energy (UHE) particles of cosmological origin (cosmic-rays and neutrinos), carry information on the most spectacular events known. These extremely energetic (energies larger than 1 ZeV= 1021 eV) cosmic-rays or neutrinos initiate in the lunar regolith a cascade of charged particles which acts as a radio pulse emitter. The instantaneous power produced can be detected here at the Earth, with a radio telescope operating at the optimal frequency window around 150 MHz. Using 12 telescopes of the Westerbork Synthesis Radio Telescope, WSRT, with a field of view covering the whole lunar surface, our calculations show that one should identify 10 UHE events within an observation time of 500 hours, assuming an extrapolated power law dependence of the highest ever measured cosmic-ray events, around an energy of 1020 eV. A null result will determine unambiguously the GKZ effect for the cosmic-ray flux and improve the present world upper limit on the neutrino flux above 1 ZeV, by three orders of magnitude, allowing for the first time to test the Waxman-Bahcall neutrino flux limit.

Isophotal shapes of elliptical/lenticular galaxies from the Sloan Digital Sky Survey

We revisit the shapes of isophotes for elliptical (E) and lenticular (S0) galaxies by studying 847 nearby early-type galaxies selected from the Sloan Digital Sky Survey (SDSS) Data Release 4 (DR4) with velocity dispersions above 200 km s −1 . The IRAF task ELLIPSE was used to derive the deviations of the isophotes from pure ellipses (Fourier coefficients a 3/a and a 4/a), position angles (PAs) and ellipticities as a function of radius. We show the statistical distributions of the a 3/a and a 4/a parameters as a function of velocity dispersion, magnitude and colour. The a 4/a parameter is correlated with the ellipticity and absolute Petrosian magnitude of galaxies. No significant correlation was found between the a 4/a parameter with colour and velocity dispersion. A cross-correlation between the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey and the SDSS data reveals a lack of powerful radio emitters in discy E/S0s, as previously found by Bender et al. We also find that boxy E/S0s favour denser environments while discy E/S0s favour more isolated environments. The median values of changes in the ellipticity and PA between one and one and a half Petrosian half-light radii in the isophotes are about −0.023 and 1. ◦ 61, respectively. The average change in the PA is much larger, about 4. 12, due to an extended tail. The change in ellipticity is weakly correlated with the ellipticity itself, with an increasing ellipticity for galaxies with higher ellipticity as the radius increases. The isophote parameters for the 847 galaxies are available online.

DeeperChandraFollow‐up of Cygnus TeV Source Perpetuates Mystery

A 50 ks Chandra observation of the unidentified TeV source in Cygnus reported by the HEGRA collaboration reveals no obvious diffuse X-ray counterpart. However, 240 pointlike X-ray sources are detected within or nearby the extended TeV J2032+4130 source region, of which at least 36 are massive stars and two may be radio emitters. That the HEGRA source is a composite, having as a counterpart the multiple pointlike X-ray sources we observe, cannot be ruled out. Indeed, the distribution of pointlike X-ray sources appears nonuniform and concentrated broadly within the extent of the TeV source region. We offer a hypothesis for the origin of the very high energy gamma-ray emission in Cyg OB2 based on the local acceleration of TeV-range cosmic rays and the differential distribution of OB versus less massive stars in this association.

The X-ray properties of young radio-loud AGN

We present XMM–Newton observations of a complete sample of five archetypal young radioloud active galactic nucleus (AGN), also known as Compact Symmetric Objects (CSO) or Gigahertz Peaked Spectrum (GPS) sources. They are among the brightest and best studied GPS/CSO sources in the sky, with radio powers in the range L5 GHz = 1043-44 erg s−1 and with four sources having measured kinematic ages of 570–3000 yr. All five sources are detected, and have 2-10 keV luminosities ranging from 0.5 to 4.8 × 1044 erg s−1. A detailed analysis was performed, comparing the X-ray luminosities and NH absorption column densities of the GPS/CSO galaxies with their optical and radio properties, and with those of the general population of radio galaxies. We find the following. (i) GPS/CSO galaxies show a wide range in absorption column densities with a distribution not different from that of the general population of radio galaxies.We therefore find no evidence that GPS/CSO galaxies could reside in a significantly more dense circumnuclear environment such that they could be ‘frustrated’ radio sources – hampered in their development. (ii) The ratio of radio to X-ray luminosity is significantly higher for GPS/CSO sources than for classical radio sources. This is consistent with an evolution scenario in which young, compact radio sources are more efficient radio emitters than large extended objects, at a constant accretion power. (iii) Taking the X-ray luminosity of radio sources as a measure of their ionization power, we find that GPS/CSO sources are significantly underluminous in their [O III]5007A line luminosity, including a weak trend with age. This is consistent with the fact that the Str¨omgren sphere should still be expanding in these young objects. If true, this would mean that here we are witnessing the birth of the narrow-line region of radio-loud AGN.

Quasi-Periodic Oscillations of Microwave Emission in Solar Active Regions

Abstract In this paper we discuss QPOs registered at microwaves using the Nobeyama Radioheliograph (NoRH). Our main conclusions concerning the QPO, found from NoRH observations, include: In addition to widely known 3-min oscillations and comparatively less investigated shorter fluctuations, we have registered over sunspots at microwave range also a diversity of more long-term periodic processes having typical periods of tens and hundreds of minutes. In some sunspots these periods are dominating. In a single active region, different plasma structures can simultaneously oscillate within a long-term range with different periods. Most of the different plasma structures of the solar atmosphere registered at the Nobeyama radio maps at a wavelength of 1.76 cm at all heliographic latitudes show the presence of the long-term QPO. In observations of the QPO at microwaves, we have dealt mostly with nonstationary processes. We have thus used wavelet analysis to estimate the quality of proper plasma resonators responsible for QPOs. In our interpretation, three types of plasma oscillators were proposed to be of importance: resonators that coincide with the emitting region; resonators that are outside, but close to the radio emitter; and resonators of the global solar nature. We conclude that NoRH has opened a new era in the study of different quasi-periodic oscillations and waves in the plasma structures of the solar atmosphere.

Arecibo Pulsar Survey Using ALFA. I. Survey Strategy and First Discoveries

We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth, and fast-dump digital spectrometers. The search targets low Galactic latitudes, |b| 5°, in the accessible longitude ranges 32° l 77° and 168° l 214°. The instrumentation, data processing, initial survey observations, sensitivity, and database management are described. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 μs. Analysis of the data with their full time and frequency resolutions is ongoing. Here we report the results of a preliminary, low-resolution analysis for which the data were decimated to speed up the processing. We have detected 29 previously known pulsars and discovered 11 new ones. One of these, PSR J1928+1746, with a period of 69 ms and a relatively low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another, PSR J1906+07, is a nonrecycled pulsar in a relativistic binary with an orbital period of 3.98 hr. In parallel with the periodicity analysis, we also search the data for isolated dispersed pulses. This technique has resulted in the discovery of PSR J0628+09, an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that ~1000 new pulsars will be found in our ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.

A Radio and X-Ray Study of Historical Supernovae in M83

We report the results of 15 years of radio observations of the six historical supernovae (SNe) in M83 using the Very Large Array. We note the near linear decline in radio emission from SN 1957D, a type II SN, which remains a non-thermal radio emitter. The measured flux densities from SNe 1923A and 1950B have flattened as they begin to fade below detectable limits, also type II SNe. The luminosities for these three SNe are comparable with the radio luminosities of other decades-old SNe at similar epochs. SNe 1945B, 1968L, and 1983N were not detected in the most recent observations and these non-detections are consistent with previous studies. We report the X-ray non-detections of all six historical SNe using the Chandra X-ray Observatory, consistent with previous X-ray searches of other decades-old SNe, and low inferred mass loss rates of the progenitors.

An XMM-Newton observation of the multiple system HD 167971 (O5-8V + O5-8V + (O8I)) and the young open cluster NGC 6604

We discuss the results of two XMM-Newton observations of the open cluster NGC 6604 obtained in April and September 2002. We concentrate mainly on the multiple system HD 167971 (O5-8V + O5-8V + (O8I)). The soft part of the EPIC spectrum of this system is thermal with typical temperatures of about 2 × 10 6 to 9 × 10 6 K. The nature (thermal vs. non-thermal) of the hard part of the spectrum is not unambiguously revealed by our data. If the emission is thermal, the high temperature of the plasma (∼2.3 × 10 7 to 4.6 × 10 7 K) would be typical of what should be expected from a wind-wind interaction zone within a long period binary system. This emission could arise from an interaction between the combined winds of the O5-8V + O5-8V close binary system and that of the more distant O8I companion. Assuming instead that the hard part of the spectrum is non-thermal, the photon index would be rather steep (∼3). Moreover, a marginal variability between our two XMM-Newton pointings could be attributed to an eclipse of the O5-8V + O5-8V system. The overall X-ray luminosity points to a significant X-ray luminosity excess of about a factor 4 possibly due to colliding winds. Considering HD 167971 along with several recent X-ray and radio observations, we propose that the simultaneous observation of non-thermal radiation in the X-ray (below 10.0 keV) and radio domains appears rather unlikely. Our investigation of our XMM-Newton data of NGC 6604 reveals a rather sparse distribution of X-ray emitters. Including the two bright non-thermal radio emitters HD 168112 and HD 167971, we present a list of 31 X-ray sources along with the results of the cross-correlation with optical and infrared catalogues. A more complete spectral analysis is presented for the brightest X-ray sources. Some of the members of NGC 6604 present some characteristics suggesting they may be pre-main sequence star candidates.

The influence of stellar wind conditions on the detectability of planetary radio emissions

Magnetized giant exoplanets in close orbits around their host star are expected to be strong nonthermal radio emitters. The anticipated radio flux is strong enough to allow its detection on Earth using the next generation of instruments. However, the measured quantity will not be the planetary radio flux but the sum of planetary and stellar emission. We compare the expected stellar and planetary radio signal for stellar systems of different ages. Solar-like stellar wind parameters as well as conditions corresponding to the young solar system (i.e. with increased stellar wind density and velocity) are considered. For young stellar systems, conditions appear to be more favorable than for older stellar systems. It is shown that configurations exist where the separation of the planetary signal from the stellar emission seems feasible.

Identifying Variable γ-ray Sources Through Radio Observations

We present preliminary results of a campaign undertaken with different radio interferometers to observe a sample of the most variable unidentified EGRET sources. We expect to detect which of the possible counterparts of the γ-ray sources (any of the radio emitters in the field) varies in time with similar timescales as the γ-ray variation. If the γ-rays are produced in a jet-like source, as we have modelled theoretically, synchrotron emission is also expected at radio wavelengths. Such radio emission should appear variable in time and correlated with the γ-ray variability.

Non-thermal radio emission from O-type stars

We present a radio lightcurve of the O5.5 III(f+) star HD 168112, based on archive data from the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). The fluxes show considerable variability and a negative spectral index, thereby confirming that HD 168112 is a non-thermal radio emitter. The non-thermal radio emission is believed to be due to synchrotron radiation from relativistic electrons that have been Fermi accelerated in shocks. For HD 168112, it is not known whether these shocks are due to a wind-wind collision in a binary system or to the intrinsic instability of the stellar wind driving mechanism. Assuming HD 168112 to be a single star, our synchrotron model shows that the velocity jump of the shocks should be very high, or there should be a very large number of shocks in the wind. Neither of these is compatible with time-dependent hydrodynamical calculations of O star winds. If, on the other hand, we assume that HD 168112 is a binary, the high velocity jump is easily explained by ascribing it to the wind-wind collision. By further assuming the star to be an eccentric binary, we can explain the observed radio variability by the colliding-wind region moving in and out of the region where free-free absorption is important. The radio data presented here show that the binary has a period between one and two years. By combining the radio data with X-ray data, we find that the most likely period is about 1.4 yr.

Direct position determination in the presence of model errors—known waveforms

The performance of the direct position determination (DPD) approach in the presence of model errors is examined. DPD was recently introduced as a promising technique for localization of multiple radio frequency emitters with superior accuracy under low signal-to-noise ratio conditions. We analyze the performance of DPD in the presence of model errors caused by multipath, calibration errors, mutual coupling, etc. The analysis is general enough to encapsulate various sources of errors. Monte Carlo simulations are used to validate the analysis. We show that in many cases of interest DPD should be selected as the preferred method of localization.

Blasts from the radio heavens

There is no coherent explanation for newly observed salvos of radio waves emanating from a direction near the Galactic Centre. Are they from a new type of stellar object? The search is on for similar radio emitters.

Direct Position Determination of Multiple Radio Signals

The most common methods for position determination of radio signal emitters such as communications or radar transmitters are based on measuring a specified parameter such as angle of arrival (AOA) or time of arrival (TOA) of the signal. The measured parameters are then used to estimate the transmitter's location. Since the measurements are done at each base station independently, without using the constraint that the AOA/TOA estimates at different base stations should correspond to the same transmitter's location, this is a suboptimal location determination technique. Further, if the number of array elements at each base station is , and the signal waveforms are unknown, the number of cochannel simultaneous transmitters that can be localized by AOA is limited to . Also, most AOA algorithms fail when the sources are not well angularly separated. We propose a technique that uses exactly the same data as the common AOA methods but the position determination is direct. The proposed method can handle more than cochannel simultaneous signals. Although there are many stray parameters, only a two-dimensional search is required for a planar geometry. The technique provides a natural solution to the measurements sources association problem that is encountered in AOA-based location systems. In addition to new algorithms, we provide analytical performance analysis, Cramér-Rao bounds and Monte Carlo simulations. We demonstrate that the proposed approach frequently outperforms the traditional AOA methods for unknown as well as known signal waveforms.

A Spectroscopic study of the non-thermal radio emitter Cyg OB2 #8A: Discovery of a new binary system

We present the results of a spectroscopic campaign revealing that the non-thermal radio emitter Cyg OB2 #8A is an O6 + O5.5 binary system. We propose the very first orbital solution indicating a period of about 21.9 days. The system appears to be eccentric (0.24 +/- 0.04) and is likely seen under a rather low inclination angle. The mass ratio of the components is close to unity. The impact of the binarity of this star in the framework of our understanding of non-thermal radio emission from early-type stars is briefly discussed.