1 A STUDY OF LONG-PERIOD GAMMA-RAYS BURSTS Knicole Colón May 2008 ABSTRACT Long-duration gamma-ray bursts (GRBs) are known to have associated afterglows in the X-ray, optical, and radio regimes. Models exploring the progenitors of the bursts and their afterglows are constantly being reviewed and updated as more observations are made. Thus, the first part of this review aims to provide an updated overview of long GRBs, as well as a discussion of their afterglows and possible progenitors. The second part of this review focuses primarily on optical afterglows, as there was a recent naked eye GRB that is of great interest and importance to understanding long GRB afterglows. 1. INTRODUCTION Gamma-ray bursts are the most energetic events that occur in nature, as well as being some of the most complicated. They have been studied intensively in recent years, especially with the launch of the Swift satellite. While both long (t > 2 sec) and short (t < 2 sec) GRBs exist, this review focuses primarily on long GRBs. In general, long GRBs are known to have associated X-ray, optical, and radio afterglows. There have also been definite associations made between long GRBs and some core-collapse Type Ic supernovae. These observations provide information about possible progenitors of these GRBs. The first part of this review outlines the general properties of long GRBs and provides a summary of possible progenitors. Optical afterglows are then examined in more detail in the second part, with the naked-eye GRB B being the focus. For a more extensive discussion of GRBs, a recommended review is that of Zhang (2007). (PART I) 2. GENERAL PROPERTIES Long GRBs are the most powerful explosions in the universe. They are defined as having burst durations that last for more than two seconds, with the longest known having a duration of approximately 2000 seconds. Their standard total energy is typically greater than ergs, with the most energetic being measured at ~10 54 ergs (GRB B, which is discussed in further detail below). They occur isotropically, with a frequency of ~2 per day in all the sky (Gehrels 2008) Afterglows Besides the initial gamma-ray burst, there are associated X-ray, optical, and radio afterglows (AGs) which are known to occur shortly after the initial burst. Not all three types of AGs are detected for every GRB, and there is no definite explanation for this. Canonical models exist for at least the X-ray and optical AGs (Zhang 2007; Panaitescu & Vestrand 2008). The AG light curves are discussed in more detail below, with the optical AGs being of primary focus in the second part of this review.
2 2.2. Host Galaxies Long GRBs are predominantly found to occur in late-type, mostly irregular, dwarf galaxies. The average redshift of the hosts is <z> = 2.3, based on Swift observations (pre-swift data had a slightly lower average redshift of 1.2). Long GRBs are generally located near the center of star forming regions in their host galaxies, but not typically near the center of the galaxy itself (Gehrels 2008; also see Lapi et al. 2008) Related Phenomena X-ray flashes (XRFs) are events that are very similar to long GRBs, but they extend to a softer, fainter regime. They could either be extrinsically different, or intrinsically different, though the exact connection is rather unclear. Zhang (2007) has an in-depth discussion of the GRB-XRF connection. Another related astronomical event involves supernovae. While the progenitors of most long GRBs have not been determined, a few have been found to be associated with luminous core-collapse Type Ic supernovae. Wolf-Rayet stars are believed to produce these Type Ic SNe (Fryer et al. 2007; Kaneko et al. 2007; Nomoto et al. 2007; Yoon et al. 2008). This process is discussed in detail below. 3. COMPARISON WITH SHORT DURATION GRBs There is a clear distinction between long GRBs and short GRBs, as discussed in Gehrels & Cannizzo (2007). Short GRBs clearly have energies less than ergs, and durations of less than seconds, while long GRBs reside between and seconds and have energies between and ergs. Recent observations have shown the emergence of a possible new class of GRBs, labeled as underluminous long GRBs. These are somewhat unique in that they produce energies comparable to short GRBs, yet last as long or longer than the longest GRBs. Interestingly, three of these underluminous GRBs are GRBs that have definite supernovae associated with them. Statistically, not much can be said about this new class, but future observations should help constrain the nature of these bursts as well as their progenitors (Gehrels & Cannizzo 2007). 4. OBSERVATIONS Swift, launched in 2004, is providing a whole new generation of GRB studies. Comparisons of pre-swift and Swift data have yielded that the observations are comparable, with no significant differences between the data quality. Swift carries three instruments: the Burst Alert Telescope (BAT), X-Ray Telescope (XRT), and UV-Optical Telescope (UVOT). At the time of this review, 317 GRBs have been detected by Swift, with 93 GRBs having both X-ray and UV/optical detections. Radio afterglows have been detected for 20 of the GRBs (note that these numbers change every day, as new GRBs are observed at a rate of one or two a day). BAT light curves and statistical information can be obtained via the Swift website (http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html).
3 5. LIGHT CURVES 5.1. Gamma-Rays The gamma-ray light curves detected by BAT are given in units of photon count versus the time since the instrument was triggered to start observations. In many cases a time designated T90 is used, which is the duration time in which the instrument detected 5% to 95% of the counts. These curves are vastly different for each burst, thus making it evident that any canonical model for GRBs is still uncertain (see for more information) X-Ray Afterglow Gehrels (2008) examines the X-ray afterglow of three different GRBs (GRB , GRB B, and GRB ) to reveal the major differences that can be seen in their light curves. Respectively, these afterglows show a steep-to-shallow transition, a large X-ray flare, and a relatively gradually declining afterglow. Refer to Gehrels (2008) for further discussion Canonical X-Ray Light Curve Extensive studies of the X-ray afterglows have revealed a canonical light curve that contains the following components: (0) a prompt emission phase, (I) a steep decay that is smoothly connected to the prompt emission, (II) a shallow decay phase with a temporal break, (III) a normal decay phase, and (IV) a post jet break phase. X-ray flares (V) are another component that can exist at any point in the light curve. Zhang (2007) includes a representation of this light curve. One thing to note is that not every GRB has all five of these components. Zhang (2007) has an extensive discussion of this topic Optical Afterglow As previously mentioned, an optical afterglow is commonly present in observations of GRBs. In general, these AGs can last for hours to days after the initial burst (Panaitescu & Vestrand 2008). An extensive discussion of optical afterglows is included below Radio Afterglow Radio afterglows are the least observed AGs, and they appear to be relatively featureless, with one peak and then a steady decay (see Pihlström et al. 2007; Willingale et al. 2004). 6. PROGENITORS OF LONG DURATION GRBs Gamma-ray bursts produce a great deal of information, between the initial gamma-ray bursts themselves, and then the different afterglows that can be detected. Along with knowledge about the host galaxies of these bursts, a number of questions can be attempted to be answered. First, what can the AGs reveal about the progenitors of long GRBs? And, what else can be learned by examining the GRB-SNe relation in detail? 6.1. Single Stars (Collapsar/Fire Ball Model) It is now confirmed that some long GRBs are definitely associated with core-collapse Type Ic SNe (see below for a further discussion of this connection). The central engine of these SNe/GRBs are assumed to be massive Wolf-Rayet (WR) stars, which are stars that have a minimum mass of ~20 solar masses and very hot, strong stellar winds. The winds expel so much gas that an atmosphere almost as large as the star itself surrounds
4 the star. Due to the massive nature, WR stars have a relatively short lifetime. Thus, upon their death, they collapse into either a stellar-size black hole (BH) or a rapidly spinning, highly magnetized neutron star (NS). Infalling material from the collapse forms a torus around the central compact object (either the BH or NS). Subsequent accretion of material in the torus is what fuels the jet of relativistic particles (gamma-rays). Then, internal shocks within this jet cause the actual bursts of gamma-rays (note that an XRF may be produced by this process as well). External shocks with residual wind material account for the creation of the afterglows, with a reverse shock causing the prompt X-ray and optical AGs, a jet shock occurring on the interstellar medium, and a forward shock producing further X-ray, optical, and radio AGs. This entire model is known as the Collapsar or Fire Ball model, and it is assumed to be the standard model for long duration GRBs (see Davies et al. 2007; Fryer et al. 2007; Yoon et al.2008). There is also an alternate theory known as the Cannon Ball model that will only be discussed as it relates to GRB B (see Dado et al. 2008) GRB-SNe Connection As of this review, there have been four direct observations of SNe that are associated both temporally and spatially with long duration GRBs (these include GRB /SN 1998bw, GRB /SN 2003dh, GRB /SN 2003lw, and GRB /SN 2006aj). All four are confirmed as Type Ic, which is because they exhibit the following common properties of Type Ic SNe: no/weak H, He, Si II lines and broad spectral lines. The associations come about because of rebrightenings in the late stages of the optical AGs of the GRBs. At a late enough stage, the optical component of the GRB will decay to a magnitude that is low enough to be able to see other emission components, such as light from the host galaxy or in this case, from a corresponding SN. This rebrightening is often called a supernova bump. Since most of the host galaxies have intense star formation regions, this can be a potential result of having star formation expedited via SNe shocks. Thus if there is intense star formation, an SN may have recently occurred. A GRB located in a galaxy with a high SFR thus may be associated with an SN (see Della Valle 2008; Gehrels & Cannizzo 2007; Kaneko et al. 2007; Nomoto et al. 2007). The variety of properties between the four known GRB-SNe companions is quite interesting. GRB /SN 1998bw is a relatively subenergetic GRB, but its SN was extremely energetic, prompting the labeling of a new class known as hypernovae. Further discussions can be found in Nomoto et al and Gehrels & Cannizzo Four general classifications now exist for GRBs and SNe, as explored by Nomoto et al. (2007). An object can be a GRB-SN/HN, an XRF-SN, a non-sn GRB, or a normal SN. They postulate that a GRB-SN/HN is the result of the central engine collapsing into an energetic black hole, an XRF-SN is the result of a collapse into a neutron star, and a non- SN GRB is related to the collapse of a less energetic black hole. This conclusion is based on their models, which find that the mass of the progenitors, the ejected mass, and the energy account for the differences between the various scenarios/connections. For more information, refer to Nomoto et al. (2007). Other models exist detailing this GRB-SNe connection. Yoon et al. (2008) produced one other of interest. What they modeled is the collapse of a rotating massive star, at different metallicities, dependent on the initial mass of the star and the initial fraction of
5 the Keplerian value of the equatorial rotational velocity. Also taken into account is whether the stars evolve quasi-chemically homogenously or into simply the classical core-envelope structure, as well as core mass, core spin and stellar radius. What they find is that at different metallicities, different objects are observed. At all metallicities, it is possible to produce Type II SN (with and without a black hole). At higher metallicities (z ~ or greater), regions exist where black holes are produced in connection with Type Ib and/or Type Ic SNe. At the highest metallicity modeled (z = 0.004) the core spin of the WR star is insufficient to allow the production of GRBs. As the metallicity of the progenitor decreases, GRBs are produced in increasingly larger distributions of rotational velocities and initial masses of the progenitor. Note that Yoon et al. (2008) also found a region where pair-instability SNe may occur (at the lowest metallicity, z = ). Refer to the discussion in Yoon et al for more information Binary Systems Despite some solid connections between Type Ic SNe and long GRBs, it is not fully accepted that GRBs are generated solely by the core-collapse of a single massive star. It has been proposed in several papers that the evolution of massive binaries into a compact system with a rapidly-rotating core-collapse SN (with initial individual masses being greater than 20 solar masses) can produce a long GRB. A general model used by Davies et al. (2007) initially involves one massive star evolving first, with a possible transfer of material to the secondary. The primary produces either a NS or BH and explodes as a core-collapse SN. After this, the secondary evolves, fills its Roche lobe, and transfers material to the companion (the compact object), producing a common envelope. The primary (compact object) and the He core of the secondary then spiral together, in the process ejecting the surrounding envelope. A very compact binary is produced this way. Finally, tidal locking results in a rapidly-rotating secondary (He star), which is rapid enough so that upon explosion as a core-collapse SN, a torus is formed around this secondary compact object. The infalling material that forms the torus then generates a GRB via the standard Fire Ball model. Therefore, after the SN/GRB event, a binary compact system containing a NS-NS, NS-BH, or BH-BH can remain (Davies et al. 2007). Some specific cases of binary systems were explored by Fryer et al. (2007). They discussed the following possible progenitors: a classic binary (the secondary star ejects the H envelope of the primary via mass transfer, then the primary continues to evolve into a collapsar), tidal binary (similar to Davies et al. (2007) model, this model works when the secondary collapses first, then the resulting compact object affects the primary before it collapses), Brown merger (approximately equal mass stars merge during the second common-envelope phase (when the secondary has evolved off the main sequence) to form a single massive star with approximately no H/He envelope, which then collapses to produce a GRB), explosive ejection (similar to Brown merger, but instead of stars merging together the secondary accretes onto He core of primary, spinning up the core and also producing explosions in the core that eject the He shell and H envelope and leave a pure CO core this is consistent with current observations of SN associated with GRBs), He merger (one star evolves into a NS or BH and then merges with the companion (He-rich) star), He case C (similar to above, but the merger occurs after He burning), cluster (enhanced mergers that require cluster interactions, but this has not been looked at in detail yet). Of all these scenarios, Fryer et al. (2007) find the explosive ejection, He case C, and the cluster method to best pass the strong constraint and follow
6 the trend found in observations. Of course, much more work must be done to further constrain these results. See Fryer et al. (2007) for further discussions. 7. DISCUSSION & CONCLUSIONS Long duration GRBs thus pose several problems for both theoretical and observational astronomers. More observations are needed to constrain the canonical model of the GRB itself, as well as the X-ray, optical, and radio AG models. Similarly, the association between SNe and long GRBs needs to be explored further, especially since there are only four known associations thus far. The only definite conclusion is that Type Ic SNe are, in fact, associated with some long GRBs. As of now, it cannot be said without much debate whether single stars or binary systems are the progenitors of these SNe/long GRBs; while the answer may be that both scenarios are correct, many more observations and models are needed to confirm that. Solving this problem is rather difficult, since there are many factors to consider: metallicity, initial mass of progenitor, mass-loss rate, rotational velocity, angular momentum, host galaxy properties, properties of afterglows, etc. Thus, the only other conclusion to be made is that this is still a new and rapidly changing field, and more observations and data may either make it more complicated or answer several questions. This, of course, remains to be seen. For further information, Zhang (2007) provides a comprehensive review of these topics. One thing that can be done now is to take a closer look at one particular aspect of long GRBs their optical afterglows. (PART II) 8. OPTICAL AFTERGLOWS OF LONG GRBs While optical AGs were briefly discussed earlier, the second part of this review aims to provide a more comprehensive look at these AGs. In particular, a recently famous naked eye GRB occurred, and the properties of this will be looked at as well in order to see how the brightest burst to occur in nature fits into the scheme of all optical AGs. The optical afterglows of GRBs are synchrotron emission resulting from a relativistic, expanding gamma-ray jet colliding with the ambient medium. The collision between the gamma-ray jet and the surrounding medium creates shock fronts, with a reverse shock causing the prompt optical afterglow and a forward shock producing an extended optical component. The reverse shock propagates in the incoming ejecta, and can produce fastrising optical light curves either through increasing the number of radiating electrons before the shock crosses the ejecta shell, or through the emergence of the relativistically beamed emission from a structured outflow seen off-axis. The forward shock can produce similar light curves for similar reasons, though the shock itself energizes the ambient medium. The continuous transfer of energy to the swept-up medium, along with the events happening at the shock fronts yield power-law decaying light curves. Thus most optical light curves can be fit with either a single or broken power-law. Recent studies have also revealed two separate aspects of the optical component: a counterpart emission that tracks the prompt gamma-rays, and the afterglow emission, which starts during the prompt phase or (typically) shortly after and dims progressively for hours to days. As mentioned earlier, another aspect of these light curves is that they can be contaminated at later times by optical emission from the host galaxy or from an associated supernova (this is the SN bump feature) (see Dai et al. 2008; Della Valle 2008; Nardini et al. 2008; Panaitescu & Vestrand 2008).
7 9. EARLY-TIME LIGHT CURVE The light curves of optical AGs can be examined at two different epochs: early-time and late-time. Early-time light curves are defined as being seconds after trigger, and during this interval it is observed that the behavior of the curves is very different in different bursts. Panaitescu and Vestrand (2008) completed a recent study of 28 GRB AGs that had a known redshift and had observations starting within a few minutes after trigger. They split the AGs into four types: fast-rising, slow-rising, decays, and plateaus. One thing to note is that the early curves which only show decays could have had peaks that occurred before the observations began, so the decays could actually be extremely fast-rising curves. The AGs that are fast-rising have very narrow distributions (even though they peak at different times), but the decays and plateaus have a much wider range of luminosity distributions. One explanation for this is that the angular structure of the relativistic outflow and/or variations in the location of the observer may account for the diversity seen between different early light curves (Panaitescu & Vestrand 2008). 10. LATE-TIME LIGHT CURVE Late-time light curves are those that occur after about 10 4 seconds after trigger. There are three main features that can be seen in late-time curves: jet breaks, flares and supernova bumps. Jet breaks are rather complicated events, but they are seen in a number of afterglows. Generally, they are the sudden increase of the fading rate of the afterglow due to the jet geometry, and they typically occur a few days after the initial GRB. A flare is a sudden, sharp increase in brightness, which then decays down to the regular decay slope. A supernova bump, as mentioned earlier, is a rebrightening in the light curve at later times due to the emission of an associated SN event. It is believed that jet breaks can be sometimes hidden in the light curve, either occurring under a flare or at some transition region in the curve. Dai et al. (2008) performed a study of several GRBs, looking in detail at their optical afterglow light curves. Flares, jet breaks, and a SN bump were found in some of their data. For more information, see Dai et al. (2008). 11. OVERALL BEHAVIOR An interesting paper written by Nardini et al. (2008) looked at the overall behavior of the optical afterglows of long GRBs. Using both pre-swift and Swift data, they examined the optical luminosity distribution 12 hours (in the rest-frame) after the initial burst. After including information about the visible extinction of the host galaxy where possible, what they found was a narrow and clustered bimodal distribution of the optical AG luminosity. There appears to be a clear separation between the luminous and sub-luminous families. While this could be an intrinsic property of long GRBs, Nardini et al. (2008) proposed the idea that about 60% of bursts are actually absorbed by a large amount (more than 1.5 mag) of gray dust. This is a very interesting result, but further studies must be completed before any conclusions are made. See Nardini et al for a complete discussion. 12. NAKED EYE GRB B Recently, there was a gamma-ray burst that had an optical afterglow that was visible to the naked eye! What made this burst special enough to allow this to happen? Specifically, where does the optical afterglow of GRB B fit in to the overall models described above? The rest of this review aims to fully explore these questions.
8 12.1. General Properties GRB B, named as such because it was the second GRB to occur on March 19, 2008, had a burst duration of approximately 60 seconds. The gamma-ray energy observed was about erg, and the visual magnitude was about 5.6 (thus dubbing this the naked eye GRB, since the human eye can see up to a magnitude of 6). The absolute (peak) magnitude was GRB B is located a redshift of 0.937, so it is relatively nearby (recall that the Swift sample has an average redshift of 2.3). One astounding fact is that if this burst occurred in our own galaxy, at a distance of about 10 kpc, it would be much brighter than the Sun! This GRB is thus the highest-fluence event, as well as the largest isotropic-equivalent energy release, ever recorded. When compared to the next most energetic GRBs and even the most luminous QSOs and SNe, it is clear that this GRB beats them all! Swift also obtained both X-ray and UV/optical images using the XRT and UVOT. A group known as Pi of the Sky happened to be observing in the field of view of the GRB at the time it occurred, so they have an animation available showing the actual burst in real time (see Also see Bloom et al. (2008) for a complete discussion of this GRB Early-Time Light Curve GRB B exhibited an (obviously) strong optical afterglow, which can be explored further via its light curve. Examining the early-time light curve reveals a fast-rising afterglow, which decays extremely rapidly (evident via dropping from 5 th to 21 st magnitude in less than one day!). There are also two short-timescale flares present in the optical curve. Overall, this afterglow is quite smooth compared to the afterglow of many other GRBs (which can show significant jaggedness). A more extensive analysis of the early light curve can be found in Bloom et al. (2008) Late-Time Light Curve The late-time light curve again reveals the smoothness of the afterglow. There are no flares evident here, nor an SN bump (though this remains to be seen, as the light curve continues to decay). There is also no jet break clearly seen; however, it is possible that either a jet break occurred extremely early (within the first 100 seconds of the burst), or it is hidden in the transition region seen around 10 3 seconds (where the decay slope becomes slightly less steep). The second scenario is implied if the (early) rapid decay is reverse-shock dominated, and therefore the jet is extremely collimated. Other than this, the light curve is rather unremarkable at late times. It is also quite similar to the three other ultra-luminous GRBs recorded (Bloom et al. 2008). 13. CANNON BALL MODEL As mentioned earlier, there is one primary progenitor model that most of the GRB community accepts that is, the Collapsar/Fire Ball model. However, there is an alternate model that GRB B appears to match. This is the Cannon Ball model (described in more detail in papers by Dado et al., specifically Dado et al. 2008). In the most general terms, this model claims that GRB B is an ordinary GRB that was produced by a jet of highly relativistic plasmoids ( cannon balls ) that were ejected in the core-collapse SN (rather than being due to shock fronts in the jet). The main reason Dado et al. give for the brightness of this burst is that it was viewed very near the axis of the cannon ball emission. Their claim is backed by mathematical equations, as well as CB model predictions of the correlation between the peak photon energy and isotropic
9 equivalent total gamma-ray energy, as well as the isotropic peak gamma-ray luminosity. They indicate that GRB B fits very well with the modeled correlations, with its early-time light curve being fit by a sum of three synchrotron radiation pulses and then a late temporal decline. They also postulate that this GRB, being an ordinary one, could have been generated by a typical GRB-SNe, e.g. SN 1998bw. However, it may have been generated by the most-luminous one detected yet, SN 2006gy. Either way, it remains to be seen which type, if any, generated this energetic GRB. Later observations of the optical afterglow should reveal emission from a SN (if one is associated), as well as emission from the host galaxy. Refer to Dado et al for a complete discussion. 14. DISCUSSION & CONCLUSIONS Observations of the optical afterglow light curve reveal that, besides being extraordinarily energetic, the afterglow is actually rather simple and relatively featureless. It is a fastrising curve, with a fast decay, and there are no obvious jet breaks observed. What then could allow a GRB like this to occur? Bloom et al. (2008) conclude that the extreme brightness is related to macroscopic parameters of the central engine (perhaps a magnetized neutron star), primarily the collimation angle of the jet, but maybe also the Mejecta, initial Lorentz factor, or circumburst medium could play a role. Extrema in shock parameters are most likely not the cause of the large energetics of this burst. Similarly, Dado et al. (2008) claim that the cannon ball model fits, with the cannon balls being seen almost on-axis. However, this model is not as widely accepted as the fire ball model by the GRB community. Whichever model best fits this GRB remains to be seen. Further observational data will also allow evidence for either a SN bump or information about the host galaxy to appear. Regardless of how GRB B was created, it is still amazing that this GRB was so energetic that even if it were placed at the epoch of reionization, it would still be observable! That is simply eye-catching!
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