Pulsars in the Galactic centre promise to enable unparalleled tests of gravity theories and black hole physics and to serve as probes of the stellar formation history and evolution and the ...interstellar medium in the complex central region of the Milky Way. The community has surveyed the innermost region of the galaxy for decades without detecting a population of pulsars, which is puzzling. A strong scattering of the pulsed signals in this particular direction has been argued to be a potential reason for the non-detections. Scattering has a strong inverse dependence on observing frequency, therefore an effective way to alleviate its effect is to use higher frequencies in a survey for pulsars in the Galactic centre, in particular, close to the supermassive black hole Sagittarius A*. We present the first pulsar survey at short millimetre wavelengths, using several frequency bands between 84 and 156 GHz (λ = 3.5–1.92 mm), targeted to the Galactic centre. The observations were made with the Institut de Radioastronomie Millimétrique 30m Telescope in 28 epochs between 2016 December and 2018 May. This survey is the first that is essentially unaffected by scattering and therefore unbiased in population coverage, including fast-spinning pulsars that might be out of reach of lower-frequency Galactic centre surveys. We discovered no new pulsars and relate this result mainly to the decreased flux density of pulsars at high frequencies, combined with our current sensitivity. However, we demonstrate that surveys at these extremely high radio frequencies are capable of discovering new pulsars, analyse their sensitivity limits with respect to a simulated Galactic centre pulsar population, and discuss the main challenges and possible improvements for similar surveys in the future.
We report on simultaneous observations of the magnetar SGR J1745−2900 at frequencies ν = 2.54–225 GHz using the Nançay 94-m equivalent, Effelsberg 100-m, and IRAM 30-m radio telescopes. We detect SGR ...J1745−2900 up to 225 GHz, the highest radio frequency detection of pulsed emission from a neutron star to date. Strong single pulses are also observed from 4.85 up to 154 GHz. At the millimetre band we see significant flux density and spectral index variabilities on time scales of tens of minutes, plus variability between days at all frequencies. Additionally, SGR J1745−2900 was observed at a different epoch at frequencies ν = 296–472 GHz using the APEX 12-m radio telescope, with no detections. Over the period MJD 56859.83-56862.93 the fitted spectrum yields a spectral index of 〈α〉 = −0.4 ± 0.1 for a reference flux density 〈S
154〉 = 1.1 ± 0.2 mJy (with S
ν ∝ να), a flat spectrum alike those of the other radio-loud magnetars. These results show that strongly magnetized neutron stars can be effective radio emitters at frequencies notably higher to what was previously known and that pulsar searches in the Galactic Centre are possible in the millimetre band.
In 2015, Torne et al. reported detections of the magnetar SGR J1745-2900 up to 225 GHz (1.33 mm), which was the highest radio frequency detection of pulsar emission at that time. In this work, we ...present the results of new observations of the same magnetar with detections up to 291 GHz (1.03 mm), together with evidence of linear polarization in its millimetre emission. SGR J1745-2900 continues to show variability and is, on average, a factor ~4 brighter in the millimetre band than in our observations of 2014 July. The new measured spectrum is slightly inverted, with ... However, the spectrum does not seem to be well described by a single power law, which might be due to the intrinsic variability of the source, or perhaps a turn-up somewhere between 8.35 and 87 GHz. These results may help us to improve our still incomplete model of pulsar emission and, in addition, they further support the search for and study of pulsars located at the Galactic Centre using millimetre wavelengths. (ProQuest: ... denotes formulae/symbols omitted.)
Context. During the transition from the asymptotic giant branch (AGB) to planetary nebulae (PN), the circumstellar geometry and morphology change dramatically. Another characteristic of this ...transition is the high mass-loss rate, that can be partially explained by radiation pressure and a combination of various factors, such as the stellar pulsation, the dust grain condensation, and opacity in the upper atmosphere. The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope. Aims. Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216. More generally we intend to determine the magnetic field strength in the circumstellar envelope (CSE) of C-rich evolved stars, compare this field with previous studies for O-rich stars, and constrain the variation of the magnetic field with r the distance to the star’s centre. Methods. We use spectropolarimetric observations of the Stokes V parameter, collected with Xpol on the IRAM-30 m radiotelescope, observing the Zeeman effect in seven hyperfine components of the CN J = 1–0 line. We use the Crutcher et al. (1996, ApJ, 456, 217) method to estimate the magnetic field. For the first time, the instrumental contamination is investigated, through dedicated studies of the power patterns in Stokes V and I in detail. Results. For C-rich evolved stars, we derive a magnetic field strength (B) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN (PPN) AFGL618, and an upper value of 8 mG is found for the PN NGC 7027. These results are consistent with a decrease of B as 1/r in the environment of AGB objects, that is, with the presence of a toroidal field. But this is not the case for PPN and PN stars. Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time.
Aims. Magnetic fields play a primordial role in the star formation process. The Zeeman effect on the CN radical lines is one of the few methods of measuring magnetic fields in the dense gas of star ...formation regions. Methods. We report new observations of the Zeeman effect on seven hyperfine CN $N=1{-}0$ lines in the direction of 14 regions of star formation. Results. We have improved the sensitivity of previous detections, and obtained five new detections. Good upper limits are also achieved. The probability distribution of the line-of-sight field intensity, including non-detections, provides a median value of the total field $B_{\rm tot}= 0.56$ mG while the average density of the medium sampled is $n({\rm H_2})=4.5\times 10^5$cm-3. We show that the CN line probably samples regions similar to those traced by CS and that the magnetic field observed mostly pervades the dense cores. The dense cores are found to be critical to slightly supercritical with a mean mass-to-flux ratio $M/\Phi \sim 1$ to 4 with respect to critical. Their turbulent and magnetic energies are in approximate equipartition.
Abstract
We describe the POLAMI (Polarimetric Monitoring of AGN at Millimetre Wavelengths) programme for the monitoring of all four Stokes parameters of a sample of bright radio-loud active galactic ...nuclei with the IRAM 30-m telescope at 3.5 and 1.3 mm. The programme started in 2006 October and accumulated, until 2014 August, 2300 observations at 3.5 mm, achieving a median time sampling interval of 22 d for the sample of 37 sources. This first paper explains the source selection, mostly blazars, the observing strategy and data calibration and gives the details of the instrumental polarization corrections. The sensitivity (1σ) reached at 3.5 mm is 0.5 per cent (linear polarization degree), 4$_{.}^{\circ}$7 (polarization angle), and 0.23 per cent (circular polarization), while the corresponding values at 1.3 mm are 1.7 per cent, 9$_{.}^{\circ}$9 and 0.72 per cent, respectively. The data quality is demonstrated by the time sequences of our calibrators Mars and Uranus. For the quasar 3C 286, widely used as a linear polarization calibrator, we give improved estimates of its linear polarization, and show for the first time occasional detections of its weak circular polarization, which suggests a small level of variability of the source at millimeter wavelengths.
ABSTRACT
We report on daily monitoring of the Seyfert galaxy ngc 7469, around 95 and 143 GHz, with the iram (Institut de Radioastronomie Millimetrique) 30- m radio telescope, and with the Swift X-ray ...and UV/optical telescopes, over an overlapping period of 45 d. The source was observed on 36 d with iram, and the flux density in both mm bands was on average ∼10 mJy, but varied by $\pm 50{{\ \rm per\ cent}}$, and by up to a factor of 2 between days. The present iram variability parameters are consistent with earlier monitoring, which had only 18 data points. The X-ray light curve of ngc 7469 over the same period spans a factor of 5 in flux with small uncertainties. Similar variability in the mm band and in the X-rays lends support to the notion of both sources originating in the same physical component of the active galactic nucleus (AGN), likely the accretion disc corona. Simultaneous monitoring in eight UV/optical bands shows much less variability than the mm and X-rays, implying this light originates from a different AGN component, likely the accretion disc itself. We use a tentative 14-d lag of the X-ray light curve with respect to the 95 GHz light curve to speculate on coronal implications. More precise mm-band measurements of a sample of X-ray-variable AGN are needed, preferably also on time-scales of less than a day where X-rays vary dramatically, in order to properly test the physical connection between the two bands.
Abstract
We present the first detection of pulsations from a neutron star in the submillimeter range. The source is the magnetar XTE J1810−197, observed with the James Clerk Maxwell Telescope on 2020 ...February 27, 2020 July 9, and 2021 May 15. XTE J1810−197 is detected at 353 GHz (
λ
= 0.85 mm) in the three epochs, but not detected in the simultaneously observed band at 666 GHz (
λ
= 0.45 mm). We measure an averaged flux density at 353 GHz of 6.7 ± 1.0, 4.0 ± 0.6, and 1.3 ± 0.3 mJy, and set 3
σ
flux density upper limits at 666 GHz of 11.3, 4.7, and 4.3 mJy, at each of the three observing epochs, respectively. Combining close-in-time observations with the Effelsberg 100 m and IRAM 30 m telescopes covering noncontiguously from 6 to 225 GHz (5.0 cm >
λ
> 1.33 mm), we investigate the spectral shape and frequency range of a potential spectral turn-up predicted by some pulsar radio emission models. The results demonstrate that the beamed radio emission from neutron stars can extend into the submillimeter regime, but are inconclusive on the existence and location of a potential spectral turn-up within the covered frequency range. The observed properties of the submillimeter emission resemble those of longer wavelengths and support a coherent mechanism for the production of pulsations at 353 GHz.
Heterodyne observations of Neptune have provided a measurement of the CO(2–1) line profile with a total bandpass of almost 8 GHz and a resolution of 4 MHz. The lineshape indicates that the CO mole ...fraction in Neptune's atmosphere is not uniform, but increases by a factor of ~2 from the troposphere/lower stratosphere (0.5 ppm at $p>20$ mbar) to the upper stratosphere (1 ppm at $p<20$ mbar). This indicates the existence of both external and internal sources of CO. The equivalent flux associated with the external source is ~$1\times10^8$ cm-2 s-1. We propose that the stratospheric CO results from a large (2 km) cometary impact that occurred ~200 years ago, although there remains problems with this hypothesis.
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