arXiv subjects

N. Mirabal

Publications and source records attributed to N. Mirabal.

At least 55 records · Page 3Linked to original sources

arXiv2009

A Point-Source Catalog from Swift Observations of Unidentified Fermi Sources

We present X-ray point source catalogs obtained from archival Swift observations that have partially or fully covered the 95% confidence error contour of unidentified Fermi sources. In total, 21 out of 37 unidentified Fermi sources have been observed by the X-Ray Telescope (XRT) on board the Swift observatory. Basic properties such as position, positional errors, count rates, and hardness ratios are derived for a total of 18 X-ray point sources found in the sample. From these detections, we discuss potential counterparts to 0FGL J0910.2-5044, 0FGL J1045.6-5937, 0FGL J1115.8-6108, 0FGL J1231.5-1410, 0FGL J1326.6-5302, 0FGL J1604.0-4904, and 0FGL J1805.3-2138. The catalog will assist observers planning future programs to identify 0FGL sources without obvious counterparts.

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arXiv2009

0FGL J1830.3+0617: A Fermi Blazar Near the Galactic Plane

We present a multiwavelength study of the unidentified Fermi gamma-ray source 0FGL J1830.3+0617, which exhibits variability above 200 MeV on timescales of days to weeks. Within the Fermi 95% confidence error contour lies B1827+0617, a radio source with spectral index alpha = 0.09 between 1.4 and 4.85 GHz. The flat spectral index and flux density of 443 mJy at 4.85 GHz are consistent with the bulk of Fermi sources associated with blazars. It is also detected in the 0.3-10 keV band by Swift. Optical imaging in 2009 May identifies B1827+0617 at R ~ 16.9, and shows that it is at least 2 magnitudes brighter than on the Palomar Sky Survey plates. Contemporaneous optical spectroscopy acquired during this high state finds a weak emission line that we attribute to Mg II at redshift z = 0.75, supporting a flat spectrum radio quasar (FSRQ) classification. The variability characteristics and radio properties together indicate that 0FGL J1830.3+0617 at Galactic latitude b = 7.5 degrees is a blazar. Blazar identifications of three additional low-latitude Fermi sources, 0FGL J0643.2+0858, 0FGL J1326.6-5302, and 0FGL J1328.8-5604, are also suggested.

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arXiv2008

Discovery of a Very Bright, Nearby Gravitational Microlensing Event

We report the serendipitous detection of a very bright, very nearby microlensing event. In late October 2006, an otherwise unremarkable A0 star at a distance ~1 kpc (GSC 3656-1328) brightened achromatically by a factor of nearly 40 over the span of several days and then decayed in an apparently symmetrical way. We present a light curve of the event based on optical photometry from the Center for Backyard Astrophysics and the All Sky Automated Survey, as well as near-infrared photometry from the Peters Automated Infrared Imaging Telescope. This light curve is well-fit by a generic microlensing model. We also report optical spectra, and Swift X-ray and UV observations that are consistent with the microlensing interpretation. We discuss and reject alternative explanations for this variability. The lens star is probably a low-mass star or brown dwarf, with a relatively high proper motion of >20 mas/yr, and may be visible using precise optical/infrared imaging taken several years from now. A modest, all-sky survey telescope could detect ~10 such events per year, which would enable searches for very low-mass planetary companions to relatively nearby stars.

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arXiv2007

Echo Emission From Dust Scattering and X-Ray Afterglows of Gamma-Ray Bursts

We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs). We find that the echo emission can provide an alternative way of understanding X-ray shallow decays and jet breaks. In particular, a shallow decay followed by a "normal" decay and a further rapid decay of X-ray afterglows can be together explained as being due to the echo from prompt X-ray emission scattered by dust grains in a massive wind bubble around a GRB progenitor. We also introduce an extra temporal break in the X-ray echo emission. By fitting the afterglow light curves, we can measure the locations of the massive wind bubbles, which will bring us closer to finding the mass loss rate, wind velocity, and the age of the progenitors prior to the GRB explosions.

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arXiv2007

GRB 050826: A Subluminous Event at z = 0.296 Finds its Place in the Luminosity Distribution of Gamma-Ray Burst Afterglows

We present the optical identification and spectroscopy of the host galaxy of GRB 050826 at a redshift z = 0.296 +/- 0.001. Image subtraction among observations obtained on three consecutive nights, reveals a fading object 5 hours after the burst, confirming its identification as the optical afterglow of this event. Deep imaging shows that the optical afterglow is offset by 0.4 arcseconds (~1.76 kpc) from the center of its irregular host galaxy, which is typical for long-duration gamma-ray bursts. Combining these results with X-ray measurements acquired by the Swift XRT instrument, we find that GRB 050826 falls entirely within the subluminous, subenergetic group of long gamma-ray bursts at low redshift (z < 0.3). The results are discussed in the context of models that possibly account for this trend, including the nature of the central engine, the evolution of progenitor properties as a function of redshift, and incompleteness in current gamma-ray burst samples.

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arXiv2007

The nature of the X-Ray Flash of August 24 2005

We present comprehensive photometric R-band observations of the fading optical afterglow of the X-Ray Flash XRF050824, from 11 minutes to 104 days after the burst. The R-band lightcurve of the afterglow resembles the lightcurves of long duration Gamma-Ray Bursts (GRBs), i.e., a power-law albeit with a rather shallow slope of alpha=0.6. Our late R-band images reveal the host galaxy with a rest-frame B-band luminosity corresponding to roughly 0.5 Lstar. The star-formation rate as determined from the [O II] emission line luminosity is about 1.8 Msun per year. When accounting for the host contribution, the slope is alpha=0.65+-0.01 and a break in the lightcurve is also suggested. A potential lightcurve bump at 2 weeks can be interpreted as a supernova only if this is a supernova with a fast rise and a fast decay. However, the overall fit still show excess scatter in the lightcurve due to wiggles and bumps. The flat lightcurves in the optical and X-rays could be explained by a continuous energy injection scenario with an on-axis viewing angle and a wide jet opening angle (theta_j>10 deg). If the energy injections are episodic, this could potentially help explain the bumps and wiggles. Spectroscopy of the afterglow give a redshift of z=0.828+-0.005 from both absorption and emission lines. The spectral energy distribution (SED) of the afterglow has a power-law shape with slope beta=0.56+-0.04. This can be compared to the X-ray spectral index which is betaX=1.0+-0.1. The curvature of the SED constrain the dust reddening towards the burst to Av<0.5 mag. (abridged)

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arXiv2007

Optical and X-Ray Observations of GRB 060526: A Complex Afterglow Consistent with An Achromatic Jet Break

We obtained 98 R-band and 18 B, r', i' images of the optical afterglow of GRB 060526 (z=3.21) with the MDM 1.3m, 2.4m, and the PROMPT telescopes in Cerro Tololo over the 5 nights following the burst trigger. Combining these data with other optical observations reported in GCN and the Swift-XRT observations, we compare the optical and X-ray afterglow light curves of GRB 060526. Both the optical and X-ray afterglow light curves show rich features, such as flares and breaks. The densely sampled optical observations provide very good coverage at T>1.e4 sec. We observed a break at 2.4e5 sec in the optical afterglow light curve. Compared with the X-ray afterglow light curve, the break is consistent with an achromatic break supporting the beaming models of GRBs. However, the pre-break and post-break temporal decay slopes are difficult to explain in simple afterglow models. We estimated a jet angle of θ_j ~ 7 degrees and a prompt emission size of R_{prompt} ~ 2e14 cm. In addition, we detected several optical flares with amplitudes of Δm ~ 0.2, 0.6, and 0.2 mag. The X-ray afterglows detected by Swift have shown complicated decay patterns. Recently, many well-sampled optical afterglows also show decays with flares and multiple breaks. GRB 060526 provides an additional case of such a complex, well observed optical afterglow. The accumulated well-sampled afterglows indicate that most of the optical afterglows are complex.

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arXiv2006

Exploring Broadband GRB Behavior During gamma-ray Emission

The robotic ROTSE-III telescope network detected prompt optical emission contemporaneous with the gamma-ray emission of Swift events GRB051109A and GRB051111. Both datasets have continuous coverage at high signal-to-noise levels from the prompt phase onwards, thus the early observations are readily compared to the Swift XRT and BAT high energy detections. In both cases, the optical afterglow is established, declining steadily during the prompt emission. For GRB051111, there is evidence of an excess optical component during the prompt emission. The component is consistent with the flux spectrally extrapolated from the gamma-rays, using the gamma-ray spectral index. A compilation of spectral information from previous prompt detections shows that such a component is unusual. The existence of two prompt optical components - one connected to the high-energy emission, the other to separate afterglow flux, as indicated in GRB051111 - is not compatible with a simple ``external-external'' shock model for the GRB and its afterglow.

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arXiv2006

GRB 060218/SN 2006aj: A Gamma-Ray Burst and Prompt Supernova at z=0.0335

We report the imaging and spectroscopic localization of GRB 060218 to a low-metallicity dwarf starburst galaxy at z = 0.03345 +/- 0.00006. In addition to making it the second nearest gamma-ray burst known, optical spectroscopy reveals the earliest detection of weak, supernova-like Si II near 5720 Angstroms (0.1c), starting 1.95 days after the burst trigger. UBVRI photometry obtained between 1 and 26 days post-burst confirms the early rise of supernova light, and suggests a short time delay between the gamma-ray burst and the onset of SN 2006aj if the early appearance of a soft component in the X-ray spectrum is understood as a ``shock breakout''. Together, these results verify the long-hypothesized origin of soft gamma-ray bursts in the deaths of massive stars.

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arXiv2005

A Redshift Determination for XRF 020903: First Spectroscopic Observations of an X-Ray Flash

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash, XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At $z=0.251$, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar from values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.

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arXiv2005

Optical Lightcurve & Cooling Break of GRB 050502A

We present lightcurves of the afterglow of GRB050502A, including very early data at t-t_{GRB} < 60s. The lightcurve is composed of unfiltered ROTSE-IIIb optical observations from 44s to 6h post-burst, R-band MDM observations from 1.6 to 8.4h post-burst, and PAIRITEL J H K_s observations from 0.6 to 2.6h post-burst. The optical lightcurve is fit by a broken power law, where t^{alpha} steepens from alpha = -1.13 +- 0.02 to alpha = -1.44 +- 0.02 at \~5700s. This steepening is consistent with the evolution expected for the passage of the cooling frequency nu_c through the optical band. Even in our earliest observation at 44s post-burst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistent with either an ISM or a Wind fireball model, but slightly favor the ISM interpretation. The expected spectral index in the ISM interpretation is consistent within 1 sigma with the observed spectral index beta = -0.8 +- 0.1; the Wind interpretation would imply a slightly (~2 sigma) shallower spectral index than observed. A small amount of dust extinction at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of beta. In this picture, the early optical decay, with the peak at or below 4.7e14 Hz at 44s, requires very small electron and magnetic energy partitions from the fireball.

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arXiv2004

The X-ray Afterglow of Dark GRB 970815: A Common Origin for GRBs and XRFs?

GRB 970815 was a well-localized gamma-ray burst (GRB) detected by the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) for which no afterglow was identified despite follow-up ASCA and ROSAT pointings and optical imaging to limiting magnitude R>23. While an X-ray source, AX/RX J1606.8+8130, was detected just outside the ASM error box, it was never associated with the GRB because it was not clearly fading and because no optical afterglow was ever found. We recently obtained an upper limit for this source with Chandra that is at least factor of 100 fainter than the ASCA detection. We also made deep optical observations of the AX/RX J1606.8+8130 position, which is blank to limits V>25.2 and I>24.0. In view of these extreme limits we conclude that AX/RX J1606.8+8130 was indeed the afterglow of GRB 970815, which corresponds to an optically "dark" GRB. AX/RX J1606.8+8130 can therefore be ruled out as the counterpart of the persistent EGRET source 3EG J1621+8203. The early light curves from BATSE and the RXTE ASM show spectral softening between multiple peaks of prompt emission. We propose that GRB 970815 might be a case in which the properties of an X-ray flash (XRF) and a "normal" GRB coincide in a single event.

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arXiv2004

Can a simple instrument measure polarization in gamma-ray bursts?

The recent claim of a high degree of linear polarization in the prompt gamma-ray burst (GRB) emission of GRB 021206 might have important implications for the underlying mechanism ultimately responsible for the GRB radiation. While the claim itself remains controversial, a full characterization of the GRB polarization has become a scientific imperative. A review of past and present polarimetry missions motivates a set of guidelines for future dedicated GRB polarization experiments. It is also argued that polarization in GRBs could be measured by a relatively simple instrument using readily available technology.

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arXiv2004

The detailed optical light curve of GRB 030329

(Abridged) We present densely sampled BVRI light curves of the optical transient associated with the gamma-ray burst GRB 030329, the result of a coordinated observing campaign conducted at five observatories. Augmented with published observations of this GRB, the compiled optical dataset contains 2687 photometric measurements, obtained between 78 minutes and 79 days after the burst. We show that the underlying supernova 2003dh evolved faster than, and was probably somewhat fainter than the type Ic SN 1998bw, associated with GRB 980425. We find that our data can be described by a broken power-law decay perturbed by a complex variable component. The early- and late-time decay slopes are determined to be ~1.1 and ~2, respectively. Assuming this single power-law model, we constrain the break to lie between ~3 and ~8 days after the burst. This simple, singly-broken power-law model, derived only from the analysis of our optical observations, may also account for available multi-band data, provided that the break happened ~8 days after the burst. The more complex double-jet model of Berger et al. provides a comparable fit to the optical, X-ray, mm and radio observations of this event. We detect a significant change in optical colors during the first day. Our color analysis is consistent with a cooling break frequency sweeping through the optical band during the first day. The light curves of GRB 030329 reveal a rich array of variations, superposed over the mean power-law decay. We find that the early variations are asymmetric, with a steep rise followed by a relatively slower (by a factor of about two) decline. The variations maintain a similar time scale during the first four days, and then get significantly longer.

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arXiv2003

A Search for Short Time-Scale Optical Variability in the GRB 030329 Afterglow

We present a densely sampled R-band light curve of the optical afterglow of GRB 030329 obtained during the period 15.5-23 hours after the burst. This dataset allows us to search for short time-scale fluctuations that must be present at some level due to inhomogeneities in the circumburst medium and/or if the afterglow is intrinsically variable.

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arXiv2003

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

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arXiv2003

Was the X-ray Afterglow of GRB 970815 Detected?

GRB 970815 was a well-localized gamma-ray burst (GRB) detected by the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) for which no afterglow was identified despite follow-up ASCA and ROSAT pointings and optical imaging to limiting magnitude R > 23. While an X-ray source, AX/RX J1606.8+8130,was detected just outside the ASM error box, it was never associated with the GRB because it was not clearly fading and because no optical afterglow was ever discovered. We recently made deep optical observations of the AX/RX J1606.8+8130 position, which is blank to a limit of V > 24.3 and I > 24.0, implying an X-ray-to-optical flux ratio f{X}/f{V} > 500. In view of this extreme limit, we analyze and reevaluate the ASCA and ROSAT data and conclude that the X-ray source AX/RX J1606.8+8130 was indeed the afterglow of GRB 970815, which corresponds to an optically ``dark'' GRB. Alternatively, if AX/RX J1608+8130 is discovered to be a persistent source, then it could be associated with EGRET source 3EG J1621+8203, whose error box includes this position.

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arXiv2003

GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z3=2.328 for its host galaxy, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers at z3A=2.323, z3B= 2.317, and z3C= 2.293 blueshifted by 450, 990, and 3,155 km/s respectively relative to the host galaxy Lyman-alpha emission. We argue that these correspond to a fragmented shell nebula that has been radiatively accelerated by the gamma-ray burst (GRB) afterglow at a distance greater than 0.3 pc from a Wolf-Rayet star progenitor. The chemical abundance ratios indicate that the nebula is overabundant in carbon and silicon. The high level of carbon and silicon is consistent with a swept-up shell nebula gradually enriched by a WCL progenitor wind over the lifetime of the nebula prior to the GRB onset. The detection of statistically significant fluctuations and color changes about the jet-like optical decay further supports this interpretation since fluctuations must be present at some level due to inhomogeneities in a clumpy stellar wind medium or if the progenitor has undergone massive ejection prior to the GRB onset. This evidence suggests that the mass-loss process in a Wolf-Rayet star might lead naturally to an iron-core collapse with sufficient angular momentum that could serve as a suitable GRB progenitor.

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