Person:

Berger, Edo

We spectroscopically study the variability of Hα emission in mid- to late-M dwarfs on timescales of ∼ 0.1 − 1 hr as a proxy for magnetic variability. About 80% of our sample exhibits statistically significant variability on the full range of timescales probed by the observations, and with amplitude ratios in the range of ∼ 1.2 − 4. No events with an order of magnitude increase in Hα luminosity were detected, indicating that their rate is . 0.05 hr−1 (95% confidence level). We find a clear increase in variability with later spectral type, despite an overall decrease in Hα “activity” (i.e., LHα/Lbol). For the ensemble of Hα variability events, we find a nearly order of magnitude increase in the number of events from timescales of about 10 to 30 min, followed by a roughly uniform distribution at longer durations. The event amplitudes follow an exponential distribution with a characteristic scale of Max(EW)/Min(EW) − 1 ≈ 0.7. This distribution predicts a low rate of ∼ 10−6 hr−1 for events with Max(EW)/Min(EW) & 10, but serendipitous detections of such events in the past suggests that they represent a different distribution. Finally, we find a possible decline in the amplitude of events with durations of & 0.5 hr, which may point to a typical energy release in Hα events for each spectral type (EHα ∼ LHα × t ∼ const). Longer observations of individual active objects are required to further investigate this possibility. Similarly, a larger sample may shed light on whether Hα variability correlates with properties such as age or rotation velocity.

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known to-date, with spectroscopically determined redshifts of z = 6 .7 and z = 8.1, respectively. The detection of bursts at this early epoch of the Universe significantly constrains the nature of GRBs and their progenitors. We perform population synthesis studies of the formation and evolution of early stars, and calculate the resulting formation rates of short and long-duration GRBs at high redshift. The peak of the GRB rate from Population II stars occurs at z ∼ 7 for a model with efficient/fast mixing of metals, while it is found at z∼ 3 for an inefficient/slow metallicity evolution model. We show that in the redshift range 6 ∼ < z ∼< 10 essentially all GRBs originate from Population II stars, regardless of metallicity evolution model. These stars (having small, but non-zero metallicity) are the most likely progenitors for both long GRBs (collapsars) and short GRBs (NS-NS or BH-NS mergers) at this epoch. Although the predicted intrinsic rates of long and short GRBs are similar at these high redshifts, observational selection effects lead to higher (factor of ∼ 10) observed rates for long GRBs. We conclude that the two recently observed highz GRB events are most likely long GRBs originating from Population II collapsars.

As part of our on-going investigation into the magnetic field properties of ultracool dwarfs, we present simultaneous radio, X-ray, and Hα observations of three M9.5-L2.5 dwarfs (BRI 0021-0214, LSR060230.4+391059, and 2MASS J052338.2−140302). We do not detect X-ray or radio emission from any of the three sources, despite previous detections of radio emission from BRI 0021 and 2M0523−14. Steady and variable Hα emission are detected from 2M0523−14 and BRI 0021, respectively, while no Hα emission is detected from LSR 0602+39. Overall, our survey of nine M8-L5 dwarfs doubles the number of ultracool dwarfs observed in X-rays, and triples the number of L dwarfs, providing in addition the deepest limits to date, log(LX /Lbol) . −5. With this larger sample we find the first clear evidence for a substantial reduction in X-ray activity, by about two orders of magnitude, from mid-M to mid-L dwarfs. We find that the decline in both X-rays and Hα roughly follows LX,Hα/Lbol ∝ 10−0.4×(SP−M6) for SP & M6. In the radio band, however, the luminosity remains relatively unchanged from M0 to L4, leading to a substantial increase in Lrad/Lbol. Our survey also provides the first comprehensive set of simultaneous radio/X-ray/Hα observations of ultracool dwarfs, and reveals a clear breakdown of the radio/X-ray correlation beyond spectral type M7, evolving smoothly from Lν,rad/LX ≈ 10−15.5 to∼ 10−11.5 Hz−1 over the narrow spectral type range M7-M9. This breakdown reflects the substantial reduction in X-ray activity beyond M7, but its physical origin remains unclear since, as evidenced by the uniform radio emission, there is no drop in the field dissipation and particle acceleration efficiency. Based on the results of our survey, we conclude that a further investigation of magnetic activity in ultracool dwarfs will benefit from a two-pronged approach: multi-rotation observations of nearby known active sources, and a snapshot survey of a large sample within ∼ 50 pc to uncover rare flaring objects.

We present an 8.5-hour simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32±0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent non-variable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating mis-aligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (i) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (ii) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (vsini ≈ 27 km s−1), and the binary orbital inclination (i ≈ 142◦), to derive a radius for the primary star of 0.078 ± 0.010 R⊙. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085±0.010 M⊙ it provides a constraint on the mass-radius relation below 0.1 M⊙. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant mis-alignment between the rotational and orbital axes.

We investigate the environment of the nearby (d ≈ 40 Mpc) broad-lined Type Ic supernova SN 2009bb. This event was observed to produce a relativistic outflow likely powered by a central accreting compact object. While such a phenomenon was previously observed only in long-duration gamma-ray bursts (LGRBs), no LGRB was detected in association with SN 2009bb. Using an optical spectrum of the SN 2009bb explosion site, we determine a variety of ISM properties for the host environment, including metallicity, young stellar population age, and star formation rate. We compare the SN explosion site properties to observations of LGRB and broad-lined SN Ic host environments on optical emission line ratio diagnostic diagrams. Based on these analyses, we find that the SN 2009bb explosion site has a metallicity between 1.7Z⊙ and 3.5Z⊙, in agreement with other broad-lined SN Ic host environments and at odds with the low-redshift LGRB host environments and recently proposed maximum metallicity limits for relativistic explosions. We consider the implications of these findings and the impact that SN 2009bb’s unusual explosive properties and environment have on our understanding of the key physical ingredient that enables some SNe to produce a relativistic outflow.

Context. The detection of GeV photons from gamma-ray bursts (GRBs) has important consequences for the interpretation and modelling of these most-energetic cosmological explosions. The full exploitation of the high-energy measurements relies, however, on the accurate knowledge of the distance to the events. Aims. Here we report on the discovery of the afterglow and subsequent redshift determination of GRB 080916C, the first GRB detected by the Fermi Gamma-Ray Space Telescope with high significance detection of photons at energies >0.1 GeV. Methods. Observations were done with 7-channel imager GROND at the 2.2m MPI/ESO telescope, the SIRIUS instrument at the Nagoya-SAAO 1.4 m telescope in South Africa, and the GMOS instrument at Gemini-S. Results. The afterglow photometric redshift of z = 4.35 ± 0.15, based on simultaneous 7-filter observations with the Gamma-Ray Optical and Near-infrared Detector (GROND), places GRB 080916C among the top 5 % most distant GRBs, and makes it the most energetic GRB known to date. The detection of GeV photons from such a distant event is rather surprising. The observed gamma-ray variability in the prompt emission together with the redshift suggests a lower limit for the Lorentz factor of the ultra-relativistic ejecta of Γ > 1090. This value rivals any previous measurements of Γ in GRBs and strengthens the extreme nature of GRB 080916C.

We report the discovery of the near-infrared and optical afterglow of the short-duration gamma-ray burst GRB 070724A. The afterglow is detected in iJHKs observations starting 2.3 hr after the burst with Ks = 19.59± 0.16 mag and i = 23.79±0.07 mag, but is absent in images obtained 1.3 years later. Fading is also detected in the Ks-band between 2.8 and 3.7 hr at a 4σ significance level. The optical/near-IR spectral index, βO,NIR −2, is much redder than expected in the standard afterglow model, pointing to either significant dust extinction, A host V ≈ 2 mag, or a non-afterglow origin for the near-IR emission. The case for extinction is supported by a shallow optical to X-ray spectral index, consistent with the definition for “dark bursts”, and a normal nearIR to X-ray spectral index. Moreover, a comparison to the optical discovery magnitudes of all short GRBs with optical afterglows indicates that the near-IR counterpart of GRB 070724A is one of the brightest to date, while its observed optical emission is one of the faintest. In the context of a non-afterglow origin, the near-IR emission may be dominated by a mini-supernova, leading to an estimated ejected mass of M ∼ 10−4 M⊙ and a radioactive energy release efficiency of f ∼ 5×10−3 (for v ∼ 0.3c). However, the mini-SN model predicts a spectral peak in the UV rather than near-IR, suggesting that this is either not the correct interpretation or that the mini-SN models need to be revised. Finally, the afterglow coincides with a star forming galaxy at z = 0.457, previously identified as the host based on its coincidence with the X-ray afterglow position (∼ 2′′ radius). Our discovery of the optical/near-IR afterglow makes this association secure, and furthermore localizes the burst to the outskirts of the galaxy, with an offset of 4.8±0.1 kpc relative to the host center. At such a large offset, the possible large extinction points to a dusty environment local to the burst and rules out a halo or intergalactic origin.

We present milliarcsecond-resolution radio very long baseline interferometry (VLBI) observations of the ultracool dwarfs TVLM 513–46546 (M8.5) and 2MASS J00361617+1821104 (L3.5) in an attempt to detect sub-stellar companions via direct imaging or reflex motion. Both objects are known radio emitters with strong evidence for periodic emission on timescales of about 2 and 3 hours, respectively. Using the inner seven VLBA antennas, we detect unresolved emission from TVLM 513–46546 on a scale of 2.5 mas (∼ 50 stellar radii), leading to a direct limit on the radio emission brightness temperature of TB & 4 × 105 K. However, with the higher spatial resolution afforded by the full VLBA we find that the source appears to be marginally and asymmetrically resolved at a low S/N ratio, possibly indicating that TVLM 513–46546 is a binary with a projected separation of ∼ 1 mas (∼ 20 stellar radii). Using the 7-hr baseline of our observation we find no astrometric shift in the position of TVLM 513–46546, with a 3σ limit of about 0.6 mas. This is about 3 times larger than expected for an equal mass companion with a few-hour orbital period. Future monitoring of its position on a range of timescales will provide the required astrometric sensitivity to detect a planetary companion with a mass of ∼ 10 MJ in a & 15 d (& 0.06 AU) orbit, or with a mass of ∼ 2 MJ in an orbit of & 0.5 yr (& 0.3 AU).

We present multi-epoch high-resolution optical spectroscopy, UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of an intermediate luminosity optical transient recently discovered in the nearby galaxy NGC 300. We find that the transient (NGC 300 OT2008-1) has a peak absolute magnitude of Mbol ≈ −11.8 mag, intermediate between novae and supernovae, and similar to the recent events M85 OT2006-1 and SN 2008S. Our high-resolution spectra, the first for this event, are dominated by intermediate velocity (∼ 200 − 1000km s−1) hydrogen Balmer lines and Ca II emission and absorption lines that point to a complex circumstellar environment, reminiscent of the yellow hypergiant IRC+10420. In particular, we detect broad Ca II H&K absorption with an asymmetric red wing extending to ∼ 103 km s−1, indicative of gas infall onto a massive and relatively compact star (blue supergiant or Wolf-Rayet star); an extended red supergiant progenitor is unlikely. The origin of the inflowing gas may be a previous ejection from the progenitor or the wind of a massive binary companion. The low luminosity, intermediate velocities, and overall similarity to a known eruptive star indicate that the event did not result in a complete disruption of the progenitor. We identify the progenitor in archival Spitzer observations, with deep upper limits from Hubble data. The spectral energy distribution points to a dust-enshrouded star with a luminosity of about 6×104 L⊙, indicative of a ∼ 10 − 20 M⊙ progenitor (or binary system). This conclusion is in good agreement with our interpretation of the outburst and circumstellar properties. The lack of significant extinction in the transient spectrum indicates that the dust surrounding the progenitor was cleared by the outburst. We thus predict that the progenitor should be eventually visible with Hubble if the transient event marks an evolutionary transition to a dust-free state, or with Spitzer if the event marks a cyclical process of dust formation.