We report the non-detection of dispersed bursts between 4 - 8 GHz from 2.5 hours of observations of FRB20200120E at 6 GHz using the Robert C. Byrd Green Bank Telescope. Our fluence limits are several times lower than the average burst fluences reported at 600 and 1400 MHz. We conclude that these non-detections are either due to high-frequency bursts being weaker and/or scintillation-induced modulated. It is also likely that our observations were non-concurrent with any activity window of FRB20200120E.
The detections of both X-ray and radio emission from the cluster G1 in M31 have provided strong support for existing dynamical evidence for an intermediate mass black hole (IMBH) of mass 1.8 +/- 0.5 x 10^4 solar masses at the cluster center. However,
given the relatively low significance and astrometric accuracy of the radio detection, and the non-simultaneity of the X-ray and radio measurements, this identification required further confirmation. Here we present deep, high angular resolution, strictly simultaneous X-ray and radio observations of G1. While the X-ray emission (L_X = 1.74^{+0.53}_{-0.44} x 10^{36} (d/750 kpc)^2 erg/s in the 0.5-10 keV band) remained fully consistent with previous observations, we detected no radio emission from the cluster center down to a 3-sigma upper limit of 4.7 microJy/beam. Our favored explanation for the previous radio detection is flaring activity from a black hole low mass X-ray binary (LMXB). We performed a new regression of the Fundamental Plane of black hole activity, valid for determining black hole mass from radio and X-ray observations of sub-Eddington black holes, finding log M_{BH} = (1.638 +/- 0.070)log L_R - (1.136 +/- 0.077)log L_X - (6.863 +/- 0.790), with an empirically-determined uncertainty of 0.44 dex. This constrains the mass of the X-ray source in G1, if a black hole, to be <9700 solar masses at 95% confidence, suggesting that it is a persistent LMXB. This annuls what was previously the most convincing evidence from radiation for an IMBH in the Local Group, though the evidence for an IMBH in G1 from velocity dispersion measurements remains unaffected by these results.
The recent discovery of a fast radio burst (FRB) in a globular cluster of M81 points to more than one channels for the formation of objects that produce these powerful radio pulses. Association of an FRB to a globular cluster (or other old stellar sy
stems) suggests that strongly magnetized neutron stars, which are the most likely objects responsible for these bursts, are born not only when young massive stars undergo core-collapse, but also by mergers of old white dwarfs. We find that the fractional contribution to the total FRB rate by old stellar populations is at least a few percent, and the precise fraction can be constrained by FRB searches in the directions of nearby galaxies, both star-forming and elliptical ones. Using very general arguments, we show that the activity time of the M81-FRB is between 10^4 and 10^6 years under conservative assumptions, and more likely of order 10^5 years. The energetics of radio outbursts puts a lower limit on the magnetic field strength of 10^{13} G, and the spin period > 0.2 sec, thereby ruling out the source being a milli-second pulsar. The upper limit on the persistent X-ray luminosity (provided by Chandra), together with the high FRB luminosity and frequent repetitions, severely constrains (or rules out) the possibility that the M81-FRB is a scaled-up version of giant pulses from Galactic pulsars. Finally, the 50 ns variability time of the FRB lightcurve suggests that the emission is produced in a compact region inside the neutron star magnetosphere, as it cannot be accounted for when the emission is at distances > 10^{10} cm.
We present the results of simultaneous Suzaku and NuSTAR observations of the nearest Low-Luminosity Active Galactic Nucleus (LLAGN), M81*. The spectrum is well described by a cut-off power law plus narrow emission lines from Fe K$alpha$, Fe XXV and F
e XXVI. There is no evidence of Compton reflection from an optically thick disc, and we obtain the strongest constraint on the reflection fraction in M81* to date, with a best-fit value of $R = 0.0$ with an upper limit of $R < 0.1$. The Fe K$alpha$ line may be produced in optically thin, $N_H = 1 times 10^{23}$ cm$^{-2}$, gas located in the equatorial plane that could be the broad line region. The ionized iron lines may originate in the hot, inner accretion flow. The X-ray continuum shows significant variability on $sim 40$ ks timescales suggesting that the primary X-ray source is $sim 100$s of gravitational radii in size. If this X-ray source illuminates any putative optically thick disc, the weakness of reflection implies that such a disc lies outside a few $times 10^3$ gravitational radii. An optically thin accretion flow inside a truncated optically thick disc appears to be a common feature of LLAGN that are accreting at only a tiny fraction of the Eddington limit.
We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-r
ich ([Fe/H]=-0.60+/-0.10), alpha-enhanced ([Alpha/Fe]=0.20+/0.05), core-collapsed (core radius r_c=1.2 pc, tidal radius r_t = 76r_c), old (>13 Gyr) cluster. It has an ultraviolet excess equivalent of ~2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 10 million solar masses, the cluster is comparable in mass to M31-G1 and is four times more massive than Omega Cen. The values of r_c, absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.
Radio magnetars are exotic sources noted for their diverse spectro-temporal phenomenology and pulse profile variations over weeks to months. Unusual for radio magnetars, the Galactic Center (GC) magnetar $rm PSR~J1745-2900$ has been continually activ
e since its discovery in 2013. We monitored the GC magnetar at $rm 4-8~GHz$ for 6 hours in August$-$September 2019 using the Robert C. Byrd Green Bank Telescope. During our observations, the GC magnetar emitted a flat fluence spectrum over $rm 5-8~GHz$ to within $2sigma$ uncertainty. From our data, we estimate a $rm 6.4~GHz$ period-averaged flux density, $overline{S}_{6.4} approx (240 pm 5)~mu$Jy. Tracking the temporal evolution of $overline{S}_{6.4}$, we infer a gradual weakening of GC magnetar activity during $2016-2019$ relative to that between $2013-2015.5$. Typical single pulses detected in our study reveal marginally resolved sub-pulses with opposing spectral indices, a feature characteristic of radio magnetars but unseen in rotation-powered pulsars. However, unlike in fast radio bursts, these sub-pulses exhibit no perceptible radio frequency drifts. Throughout our observing span, $rm simeq 5~ms$ scattered pulses significantly jitter within two stable emission components of widths, $rm 220~ms$ and $rm 140~ms$, respectively, in the average pulse profile.
Vishal Gajjar
,Daniele Michilli
,Jakob T. Faber
.
(2021)
.
"Absence of bursts between 4-8 GHz from FRB20200120E located in an M81 Globular Cluster"
.
Vishal Gajjar
هل ترغب بارسال اشعارات عن اخر التحديثات في شمرا-اكاديميا