A Wide‐Area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths

Law, Casey J.; Yusef‐Zadeh, F.; Cotton, W. D.

doi:10.1086/588218

Cited by 68 publications

(50 citation statements)

References 62 publications

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“…Our XRT position of AX J1742.6-2901 coincides with 2RXP J174241.8-290215 and lies 2.5 from a radio source listed in the 6-and 20-cm survey of the Galactic centre (Law et al 2008). This is well within our 3.4 XRT positional uncertainty (see Table 2).…”

Section: Ax J17426-2901 and Ax J17208-3710supporting

confidence: 80%

Swiftfollow-up observations of unclassified ASCA sources

Degenaar

Starling

Evans

et al. 2012

A&A

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We present Swift follow-up observations of a sample of 35 unclassified faint X-ray sources drawn from the ASCA Galactic centre and Galactic plane surveys. Our short, pointed XRT observations allow detections down to a limiting 0.3-10 keV flux of F X ∼ 10 −13 erg cm −2 s −1 , which translates into a luminosity of L X ∼ 10 33 erg s −1 for an assumed distance of D = 8 kpc. The brightest source in our sample reaches a maximum 0.3-10 keV luminosity of L X ∼ 2 × 10 36 (D/8 kpc) 2 erg s −1 during our Swift observations. We detect 16 (46%) of the ASCA sources with the XRT, while 19 were not detected during our program. Since we are probing the faint end of the ASCA source populations, we expect a large fraction of the non-detections to be due to the Eddington bias. This is strengthened by the fact that we find the observed XRT count rates to be predominantly lower than expected based on the reported ASCA intensities. Nevertheless, investigation of the ASCA properties and any possible long-term X-ray variability leads us to conclude that the nondetections likely include two spurious ASCA detections and three objects that are highly variable or transient X-ray sources. For the 16 XRT-detected sources we obtain positional accuracies of ∼2−4 , which significantly improves upon their ASCA uncertainties of ∼1 . We use the X-ray spectra and variability to characterise these objects. Most appear to be faint, persistent X-ray emitters that have highly absorbed spectra. Based on their X-ray properties we identify three accreting compact objects: one confirmed (AX J1754.2-2754) and one candidate (AX J1538.3-5541) X-ray binary, and one possible magnetically accreting white dwarf (AX J1620.1-5002). Furthermore, we use the improved positions of XRT-detected sources to search for counterparts in simultaneously obtained Swift/UVOT images and possible associations with catalogued sources at various wavelengths. This reveals three possible main sequence stars amongst our sample. The other sources remain unclassified, but our improved XRT positions provide good prospects for dedicated follow-up observations that have the potential to elucidate the nature of these faint, unclassified ASCA sources.

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Section: Ax J17426-2901 and Ax J17208-3710supporting

confidence: 80%

Swiftfollow-up observations of unclassified ASCA sources

Degenaar

Starling

Evans

et al. 2012

A&A

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“…This feature is similar in size (50 pc) to the non-thermal radio filaments seen prominently in the direction perpendicular to the Galactic plane 2 (Yusef-Zadeh & Morris 1987). However, these radio filaments were observed by Law et al (2008) to have flux densities of ∼20−200 mJy at 20 cm wavelength which, given the distance ratio of ∼400 would appear as ∼1 µJy in M82. The observed flux density at 1.6 GHz is more than a thousand times the luminosity expected if this was a non-thermal filament similar to those in the centre of the Milky way.…”

Section: A Peculiar Linear Featurementioning

confidence: 98%

Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz

Varenius

Conway

Martí-Vidal

et al. 2015

A&A

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Context. The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent. Aims. The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via highresolution radio imaging at low frequencies. Methods. The international LOFAR telescope was used to observe M82 at 110−126 MHz and 146−162 MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines. Results. The 154 MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3 ) and sensitivity (σ = 0.15 mJy/beam) at low frequencies (<327 MHz). We detected 16 objects at 154 MHz, six of these also at 118 MHz. Seven objects detected at 154 MHz have not been catalogued previously. For the nine objects previously detected, we obtained spectral indices and emission measures by fitting models to spectra (combining LOFAR with literature data). Four weaker but resolved features are also found: a linear (50 pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk.

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“…The large field of view (FOV) of the ATA allows us to cover a relatively large region on the sky, and our frequent visits provide thorough sampling of variability on a range of timescales. The compact configuration of the ATA also makes our observations sensitive to static extended structures in the GP, such as nonthermal radio filaments (Yusef-Zadeh et al 1984;Law et al 2008), H II regions (Brogan et al 2003;Nord et al 2006), and supernova remnants (Gray 1994).…”

Section: Introductionmentioning

confidence: 99%

Asgard: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results

Williams

Bower

Croft

et al. 2012

ApJ

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Searches for slow radio transients and variables have generally focused on extragalactic populations, and the basic parameters of Galactic populations remain poorly characterized. We present a large 3 GHz survey performed with the Allen Telescope Array (ATA) that aims to improve this situation: ASGARD, the ATA Survey of Galactic Radio Dynamism. ASGARD observations spanned 2 years with weekly visits to 23 deg 2 in two fields in the Galactic Plane, totaling 900 hr of integration time on science fields and making it significantly larger than previous efforts. The typical blind unresolved source detection limit was 10 mJy. We describe the observations and data analysis techniques in detail, demonstrating our ability to create accurate wide-field images while effectively modeling and subtracting large-scale radio emission, allowing standard transient-and-variability analysis techniques to be used. We present early results from the analysis of two pointings: one centered on the microquasar Cygnus X-3 and one overlapping the Kepler field of view ( = 76 • , b = +13.5 • ). Our results include images, catalog statistics, completeness functions, variability measurements, and a transient search.Out of 134 sources detected in these pointings, the only compellingly variable one is Cygnus X-3, and no transients are detected. We estimate number counts for potential Galactic radio transients and compare our current limits to previous work and our projection for the fully-analyzed ASGARD dataset.

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A Wide‐Area VLA Continuum Survey near the Galactic Center at 6 and 20 cm Wavelengths

Cited by 68 publications

References 62 publications

Swiftfollow-up observations of unclassified ASCA sources

Swiftfollow-up observations of unclassified ASCA sources

Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz

Asgard: A Large Survey for Slow Galactic Radio Transients. I. Overview and First Results

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